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Sat Feb 6 14:25:13 2016

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QGIS Planet

Increasing the stability of processing algorithms

Processing just got a new testing framework to improve the long-term stability of this important plugin. And you can help to improve it, even if you are not a software developer! This is yet another piece in our never-stopping crusade to

Generate parcels areas from parcel boundaries

Hi! In this blog I describe how you can create proper parcels with polygon geometry in from polylines (parcel boundaries) and points (Parcel point with parcel attributes placed inside parcel boundaries). Since the 1st of januari 2016 a dataset named, BRK (Basis Registratie Kadaster) is available from PDOK. You can download these in GML format … Continue reading Generate parcels areas from parcel boundaries

QGIS Features I long for while using ArcGIS

(aka Features that ArcGIS Desktop users might not know that exists)

EN | PT

From time to time, I read articles comparing ArcGIS vs QGIS. Since many of those articles are created from an ArcGIS user point of view, they invariably lead to biased observations on QGIS lack of features. It’s time for a QGIS user perspective, so bare with me on this (a bit) long, totally and openly biased post.

“Hello, my name is Alexandre, and I have been using… QGIS

This is something I would say at an anonymous QGIS user therapy group. I’m willing to attend one of those because being recently and temporally forced to use ArcGIS Desktop again (don’t ask!), I really really really miss QGIS in many ways.

There was a time when I have used ArcGIS on the regular basis. I used it until version 9.3.1 and then decided to move away (toward the light) into QGIS (1.7.4 I think). At that time, I missed some (or even many?) ArcGIS features, but I was willing to accept it in exchange for the freedom of the Open Source philosophy. Since then, a lot have happened in the QGIS world, and I have been watching it closely. I would expect the same have happened in ArcGIS side, but, as far I can see, it didn’t.

I’m using top shelf ArcGIS Desktop Advanced and I’m struggling to do very basic tasks that simply are nonexistent in ArcGIS. So here’s my short list of QGIS functionalities that I’m longing for. For those of you that use ArcGIS exclusively, some of this features may catch you by surprise.

Warning: For those of you that use ArcGIS exclusively, some of this features may catch you by surprise.

Transparency control

“ArcGIS have transparency! It’s in the Display tab, in the layer’s properties dialog!”

Yes, but… you can only set transparency at the layer level. That is, either it’s all transparent, or it’s not…

In QGIS on the other end, you can set transparency at layer level, feature/symbol level, and color level. You can say that this is being overrated, but check the differences in the following images.

Transparency_layer_levelTransparency_feature_symbol_levelTransparency_color_level

Notice that in QGIS you can set transparency at color level everywhere (or almost everywhere) there is a color to select. This includes annotations (like the ones in the image above), labels and composers items. You can even set transparency in colors by using the RGBA function in an expression! How sweet can that be? :-)

Screenshot from 2016-01-27 14:12:34

Blending modes

This is one of QGIS’s pristine jewels. The ability to combine layers the way you would do in almost any design/photo editing software. At layer or at feature level, you can control how they will “interact” with other layers or features below. Besides the normal mode, QGIS offers 12 other blending modes:  Lighten, Screen, Dodge, Darken, Multiply, Burn, Overlay, Soft light, Hard light, Difference, and Subtract. Check this page to know more about the math behind each one and this image for some examples

It’s not easy to grasp how can this be of any use for cartography before you try it yourself. I had the chance to play around while trying to answer this question.

2wph4

A very common application for this functionality is when you want to add shadows to simulate the relief by putting a hill shade on top of other layers. In ArcGIS, you can only control the layer transparency, and the result is always a bit pale. But in QGIS, you can keep the strength of the original colors by using the multiply mode in the hill shade layer.

Screenshot from 2016-01-27 15:24:38
Hypsometry original colors
Screenshot from 2016-01-27 15:25:45
Hypsometry colors paled by transparent hill shade
Screenshot from 2016-01-27 15:24:45
Hypsometry original colors with the hill shade using QGIS multiply blending

You can also use blending modes in the print composer items, allowing you to combine them with other items and textures. This gives you the opportunity to make more “artistic” things without the need to go post-processing in a design software.

Colour Picker Menu

Controlling color is a very important deal for a cartographer and QGIS allow you to control it like the professional you are. You can select your colours using many different interfaces. Interfaces that you might recognize from software like Inkscape, Photoshop, Gimp and others alike.

Screenshot from 2016-01-20 22:04:03 Screenshot from 2016-01-20 22:03:48 Screenshot from 2016-01-20 22:04:54

My favorite one is the color picker. Using color picker, you can pick colors from anywhere on your screen, either from QGIS itself or outside. This is quite handy and productive when you are trying to use a color from your map, it’s legend, a COLOURlovers palette or a company logo.

anim
Picking a color from outside QGIS

You can also copy/paste colors between dialogs, save and import color palettes, and you can even name a color and use it in a variable. With all this available for free, it’s hard to swallow Windows color selector :(.

Capture Untitled

Vector symbols renderers “powertools”

In ArcGIS, you have a few fancy ways to symbol your vector layers. You got: Single symbol, Unique values, Unique values many fields… and so on. At the first glance, you may think that QGIS lacks some of them. Believe me, it doesn’t! In fact, QGIS offers much more flexibility when you need to symbol your layers.

For starters, it allows you to use fields or expressions on any of the symbols renderers, while ArcGIS only allows the use of fields. Powered by hundreds of functions and the ability to create your owns in python, what you can do with the expression builder has no limits. This means, for instance, that you can combine, select, recalculate, normalize an infinite number of fields to create your own “values” (not to mention that you can tweak your values labels, making it ideal to create the legend).

Screenshot from 2016-01-20 22:34:54
QGIS Graduated renderer using an expression to calculate population density

And then, in QGIS, you have the special (and kinda very specific) renderers, that make you say “wooooh”. Like the Inverted polygons that allow you to fill the the outside of polygons (nice to mask other features), the Point displacement to show points that are too close to each others, and the Heatmap that will transform, ON-THE-FLY, all your points in a layer into a nice heatmap without the need to convert them to raster (and that will update as soon as you, or anyone else, edits the point features).

Screenshot from 2016-01-20 22:58:44
Inverted Polygon Renderer masking the outside of an interest area

But I have left the best to the end. The “One rendered to Rule them all”, the Rule-based symbols. With the rule-based renderer, you can add several rules, group them in a tree structure, and set each one with a desired symbol. This gives QGIS users full control of their layer’s symbols, and, paired with expression builder and data-defined properties, opens the door to many wonderful applications.

rulesymbol_ng_line
Rule-based renderer

Atlas

One of my favorite (and missed) features in QGIS is the Map Composer’s Atlas. I know that ArcGIS has its own “Atlas”, the Data Driven Pages, but frankly, it’s simply not the same.

I believe you know the basic functionally that both software allow. You create a map layout and choose a vector layer as coverage, and it will create an output panned or zoomed for each of the layer’s feature. You can also add some labels that will change according to the layers attributes.

But in QGIS, things go a little bit further…

Basically, you can use coverage layer’s attributes and geometry anywhere you can use an expression. And, in QGIS, expressions are everywhere. That way, most layers and map composer items properties can be controlled by a single coverage layer.

With the right configuration, while iterating over the atlas coverage features, you can,  choose what feature to show and what features to hide, change a theme color for your map, rotate and resize your page acording to the feature sizechoose a specific logo to came along with your map, and much more. Once again, the sky is the limit.

mosaico_regioes_fixed
Auto-resized maps that fits the coverage features at specific scale using atlas

So, if you pair Atlas it with QGIS data-defined properties, rule-based symbols and expressions, ArcGIS Data Driven Pages are no match. You don’t think so? Try to answer this question then.

Tip: If you really want to leverage your map production, using Spatialite or Postgis databases you can create the perfect atlas coverage layers from views that fit your needs. No data redundancy and they are always updated.

Label and Style dialogs

This one is more of a User Experience thing than an actual feature, but you won’t imagine how refreshing it is to have all Style and Labels options in two single dialogs (with several tabs, of course).

Using the symbol menu in ArcGIS makes me feel like if I’m in the Inception movie, at some point in time, I no longer know where the hell am I. For example, to apply a dashed outline in a fill symbol I needed to open 5 different dialogs, and then go back clicking OK, OK, OK, OK …

Capture
ArcGIS “Inception” symbol settings

In QGIS, once in the properties menu, every setting is (almost) one click way. And you just need to press OK (or Apply ) once to see the result!

Screenshot from 2016-01-20 21:51:33
QGIS Style setting

As an extra, you can copy/paste styles from one layer to another, making styling several layers even faster. And now you say:

“Don’t be silly! In ArcGIS you can import symbols from other layers too.”

Symbols? yes. Labels? No! And if you had lots of work setting your fancy labels, having to do the exact same/similar thing in another layer, it will make you wanna cry… I did.

(I think I will leave the multiple styles per layer vs data frames comparison for another time)

WFS

“Say what?!!”

Yup, that’s it, ArcGIS Desktop lacks support for WFS OGC standard unless you buy an extra extension: The Data Interoperability Extention.

In a GIS world that, more and more, is evolving towards Open Data, Open Standards and OGC Web Services, this reveals a very mercantile approach by ESRI. If I were an ESRI customer, I would feel outraged. <sarcasm>Or maybe not… maybe I would thank the opportunity to yet invest some more money in it’s really advanced features…<\sarcasm>

In QGIS, like everything else, WFS is absolutely free (as in freedom, not free beer). All you need to do is add the WFS server’s URL, and you can add all the layers you want, with the absolute sure that they are as updated as you can get.

Screenshot from 2016-01-20 21:58:54

Fortunately for ArcGIS users with a low budget, they can easily make a request for a layer using the internet browser :-P.

http://giswebservices.massgis.state.ma.us/geoserver/wfs?request=GetFeature&service=wfs&version=1.0.0&typename=massgis:GISDATA.TOWNS_POLY&outputformat=SHAPE-ZIP

Or they can simply use QGIS to download it. But, in both cases, be aware that the layers won’t update themselves by magic.

Expression builder

I have already mentioned the use of expressions several times, but for those of you that do not know the expression Builder, I though I end my post with one of my all time favourite features in QGIS.

I do not know enough of ArcGIS expression builder to really criticize it. But, AFAIK, you can use it to create labels and to populate a field using the field calculator. I know that there are many functions that you can use (I have used just a few) but they are not visible to the common user (you probably need to consult the ArcGIS Desktop Help to know them all). And you can create your own functions in VBScript, Python, and JsScript.

Capture

On QGIS side, like I said before, the Expression Builder can be used almost everywhere, and this makes it very convenient for many different purposes. In terms of functions, you have hundreds of functions right there in the builder’s dialog, with the corresponding syntax help, and some examples. You also have the fields and values like in ArcGIS, and you even have a “recent expressions” group for re-using recent expressions with no the need to remember prior expression.

Capture

Besides, you can create your own functions using Python (no VBScript or JsScript). For this purpose, you have a separate tab with a function editor. The editor have code highlighting and save your functions in your user area, making it available for later use (even for other QGIS sessions).

Capture

Conclusion

These are certainly not the only QGIS features that I miss, and they are probably not the most limiting ones (for instance, not being able to fully work with Spatialite and Postgis databases will make, for sure, my life miserable in the near future), but they were the ones I noticed right away when I (re)open ArcGIS for the first time.

I also feel that, based on the QGIS current development momentum, with each QGIS Changelog, the list will grow very fast. And although I haven’t tested ArcGIS Pro, I don’t think ESRI will be able to keep the pace.

“Are there things I still miss from ArcGIS?” Sure. I miss CMYK color support while exporting maps, for instance. But not as much as I miss QGIS now. Besides, I know that those will be addressed sooner or later.

In the end, I kinda enjoyed the opportunity to go back to ArcGIS, as it reinforced the way I think about QGIS. It’s all about freedom! Not only the freedom to use the software (that I was already aware) but also the freedom to control software itself and it’s outputs. Maintaining the users friendliness for new users, a lot have been done to make power users life easier, and they feel very pleased with it (at least I am).

All this being said, the winner is… QGIS!!

The End

(of a very biased post)


A QGIS router for GIP.at

Monday, January 4th 2016, was the open data release date of the official Austrian street network dataset called GIP.at. As far as I know, the dataset is not totally complete yet but it should be in the upcoming months. I’ve blogged about GIP.at before in Open source IDF parser for QGIS and Open source IDF router for QGIS where I was implementing tools based on the data samples that were available then. Naturally, I was very curious if my parser and particularly the router could handle the whole country release …

Some code tweaking, patience for loading, and 9GB of RAM later, QGIS happily routes through Austria, for example from my work place to Salzburg – maybe for some skiing:

Screenshot 2016-01-06 17.11.27

The routing request itself takes something between 1 and 2 seconds. (I should still add a timer to it.)

So far, I’ve implemented shortest distance routing for pedestrians, bikes, and cars. Since the data also contains travel speeds, it should be quite straight-forward to also add shortest travel time routing.

The code is available on Github for you to try. I’d appreciate any feedback!


PDOK services plugin update

Mostly interesting for dutchies A short post that there is a new 0.10 version of the pdokservicesplugin (http://plugins.qgis.org/plugins/pdokservicesplugin/), Some layers have been removed and some are new. The total number of layers is now 6723 (coming from 5298). Most important changes: Removed: – brtachtergrondtijdelijk – brtachtergrondgrijstijdelijk – top10nl (nu: top10nl2) – Noordzee Kabels en Leidingen … Continue reading PDOK services plugin update

Talking QGIS with UNIGIS Salzburg

Last summer, I had the pleasure to talk with UNIGIS Salzburg about the QGIS project: how it works and what makes it great. Now, finally, the video is out on Youtube:

Amongst other things, we are discussing the UNIGIS module on QGIS, which I have been teaching for the past few months.


Exploring variables in QGIS pt 3: layer level variables

In part 3 of my exploration of variables in QGIS 2.12, I’m going to dig into how variables are scoped in QGIS and what layer level variables are available (you can read parts 1 and 2 for a general introduction to variables).

Some background

Before we get to the good stuff, a bit of background in how variables work behind-the-scenes is important. Whenever an expression is evaluated in QGIS the context of the expression is considered. The context is built up from a set of scopes, which are all stacked on top of each other in order from least-specific to most-specific. It’s easier to explain with an example. Let’s take an expression used to set the source of a picture in a map composer. When this expression is evaluated, the context will consist of:

  1. The global scope, consisting of variables set in the QGIS options dialog, and other installation-wide properties
  2. The project scope, which includes variables set in the Project Properties dialog and the auto-generated project variables like @project_path, @project_title (you can read more about this in part 2)
  3. composer scope, with any variables set for the current composer, plus variables for @layout_pagewidth, @layout_pageheight, @layout_numpages, etc.
  4. composer item scope for the picture, with item-specific variables including @item_id

The more specific scopes will override any existing clashing variables from less specific scopes. So a global @my_var variable will be overridden by an @my_var variable set for the composer:

overridden

Another example. Let’s consider now an expression set for a data-defined label size. When this expression is evaluated the context will depend on where the map is being rendered. If it’s in the main map canvas then the context will be:

  1. The global scope
  2. The project scope
  3. map settings scope, with variables relating to how the map is being rendered. Eg @map_rotation, @map_scale, etc
  4. layer scope. More on this later, but the layer scope includes layer-level variables plus preset variables for @layer_name and @layer_id

If instead the map is being rendered inside a map item in a map composer, the context will be:

  1. The global scope
  2. The project scope
  3. The composer scope
  4. An atlas scope, if atlas is enabled. This contains variables like @atlas_pagename, @atlas_feature, @atlas_totalfeatures.
  5. composer item scope for the map item
  6. map settings scope (with scale and rotation determined by the map item’s settings)
  7. The layer scope

Using layer level variables

Ok, enough with the details. The reason I’ve explained all this is to help explain when layer level variables come into play. Basically, they’ll be available whenever an expression is evaluated inside of a particular layer. This includes data defined symbology and labeling, field calculator, and diagrams. You can’t use a layer-level variable inside a composer label, because there’s no layer scope used when evaluating this. Make sense? Great! To set a layer level variable, you use the Variables section in the Layer Properties dialog:

Setting a layer variablee

Setting a layer variable

Any layer level variables you set will be saved inside your current project, i.e. layer variables are per-layer and per-project. You can also see in the above screenshot that as well as the layer level variables QGIS also lists the existing variables from the Project and Global scopes. This helps show exactly what variables are accessible by the layer and whether they’ve been overridden by any scopes. You can also see that there’s two automatic variables, @layer_id and @layer_name, which contain the unique layer ID and user-set layer name too.

Potential use cases for layer-level variables

In the screenshot above I’ve set two variables, @class1_threshold and @class2_threshold. I’m going to use these to sync up some manual class breaks between rule based symbology and rule based labeling. Here’s how I’ve set up the rule-based symbols for the layer:

Rule based symbology using layer level variables

Rule based symbology using layer level variables

In a similar way, I’ve also created matching rule-based labeling (another new feature in QGIS 2.12):

Matching rule-based labels

Matching rule-based labels

Here’s what my map looks like now, with label and symbol colors matched:

*Map for illustrative purposes only... not for cartographic/visual design excellence!

*Map for illustrative purposes only… not for cartographic/visual design excellence!

If I’d hard-coded the manual class breaks, it would be a pain to keep the labeling and symbology in sync. I’d have to make sure that the breaks are updated everywhere I’ve used them in both the symbology and labeling settings. Aside from being boring, tedious work, this would also prevent immediate before/after comparisons. Using variables instead means that I can update the break value in a single place (the variables panel) and have all my labeling and symbols immediately reflect this change when I hit apply!

Another recent use case I had was teaming layer-level variables along with Time Manager. I wanted my points to falloff in both transparency and size with age, and this involved data defined symbol settings scattered all throughout my layer symbology. By storing the decay fall-off rate in a variable, I could again tweak this falloff by changing the value in a single place and immediately see the result. It also helps with readability of the data defined expressions. Instead of trying to decipher a random, hard-coded value, it’s instead immediately obvious that this value relates to a decay fall-off rate. Much nicer!

I’m sure there’s going to be hundreds of novel uses of layer-level variables which I never planned for when adding this feature. I’d love to hear about them though – leave a comment if you’d like to share your ideas!

One last thing – the new “layer_property” function

This isn’t strictly related to variables, but another new feature which was introduced in QGIS 2.12 was a new “layer_property” expression function. This function allows you to retrieve any one of a bunch of properties relating to a specific map layer, including the layer CRS, metadata, source path, etc.

This function can be used anywhere in QGIS. For instance, it allows you to insert dynamic metadata about layers into a print composer layout. In the screenshot below I’ve used expressions like layer_property(‘patron’,’crs’) and layer_property(‘patron’,’source’) to insert the CRS and source path of the “patron” layer into the label. If either the CRS or the file path ever changes, this label will be automatically updated to reflect the new values.

Inserting dynamic layer properties into a composer label

Inserting dynamic layer properties into a composer label

 

So there you go – layer level variables and the layer_property function – here in QGIS 2.12 and making your workflow in QGIS easier. In the final part of this series, we’ll explore the magical @value variable. Trust me, I’ve saved the best for last!

Geometry generator symbology

December traditionally is an amazing time since the weather is usually quite forgiving to long working hours. Therefore the first parts of our recent crowdfunding project for 2.5D rendering have been merged into QGIS master and will be shipped with QGIS

How to label only selected features in QGIS 2.8 and up

In 2011, I wrote “How to Label Only Selected Features in QGIS” which ends with the wish that

Another “data defined setting” like “show this label (true/false)” would be more intuitive.

… and now we have it!

It’s called Show label and you can find it in the Rendering section of the labeling dialog.

The following screenshot shows a quick example of how to label only airports starting with A by setting the expression

"NAME" LIKE 'A%'

labelselected

This post was motivated by a question by Eduardo here on this blog. Hope it helps!


Exploring variables in QGIS pt 2: project management

Following on from part 1 in which I introduced how variables can be used in map composers, I’d like to now explore how using variables can make it easier to manage your QGIS projects. As a quick refresher, variables are a new feature in QGIS 2.12 which allow you to create preset values for use anywhere you can use an expression in QGIS.

Let’s imagine a typical map project. You load up QGIS, throw a bunch of layers on your map, and then get stuck into styling and labelling them ‘just right’. Over time the project gets more and more complex, with a stack of layers all styled using different rendering and labelling rules. You keep tweaking settings until you’re almost happy with the result, but eventually realise that you made the wrong choice of font for the labelling and now need to go through all your layers and labelling rules and update each in turn to the new typeface. Ouch.

Variables to the rescue! As you may recall from part 1, you can reuse variables anywhere in QGIS where you can enter an expression. This includes using them for data defined overrides in symbology and labelling. So, lets imagine that way back at the beginning of our project we created a project level variable called @main_label_font:

Creating a variable for label font

Creating a variable for label font

Now, we can re-use that variable in a data defined override for the label font setting. In fact, QGIS makes this even easier for you by showing a “variables” sub-menu allowing easy access to all the currently defined variables accessible to the layer:

Binding the label font to the @main_label_font variable

Binding the label font to the @main_label_font variable

 

When we hit Apply all our labels will be updated to use the font face defined by the @main_label_font variable, so in this case ‘Courier New’:

courier_new

In a similar way we can bind all the other layer’s label fonts to the same variable, so @main_label_font will be reused by all the layers in the project. Then, when we later realise that Courier New was a horrible choice for labelling the map, it’s just a matter of opening up the Project Properties dialog and updating the value of the @main_label_font variable:

delicious

And now when we hit Apply the font for all our labelled layers will be updated all at once:

new_labels

It’s not only a huge time saver, it also makes changes like this easier because you can try out different font faces by updating the variable and hitting apply and seeing the effect that the changes have all at once. Updating multiple layers manually tends to have the consequence that you forget what the map looked like before you started making the change, making direct comparisons harder.

Of course, you could have multiple variables for other fonts used by your project too, eg @secondary_label_font and @highlighted_feature_font. Plus, this approach isn’t limited to just setting the label font. You could utilise project level variables for consolidating font sizes, symbol line thickness, marker rotation, in fact, ANYTHING that has one of those handy little data defined override buttons next to it:

See all those nice little yellow buttons? All those controls can be bound to variables...

See all those nice little yellow buttons? All those controls can be bound to variables…

One last thing before I wrap up part 2 of this series. The same underlying changes which introduced variables to QGIS also allows us to begin introducing a whole stack of new, useful functions to the expression engine. One of these which also helps with project management is the new project_color function. Just like how we can use project level variables throughout a project, project_color lets you reuse a color throughout your project. First, you need to create a named colour in the Default Styles group under the Project Properties dialog:

Define a colour in the project's colour scheme...

Define a colour in the project’s colour scheme…

Then, you can set a data defined override for a symbol or label colour to the expression “project_color(‘red alert!’)“:

bind_color

When you go back and change the corresponding colour in the Project Properties dialog, every symbol bound to this colour will also be updated!

blue_alert

So, there you have it. With a little bit of forward planning and by taking advantage of the power of expression variables in QGIS 2.12 you can help make your mapping projects much easier to manage and update!

That’s all for now, but we’re still only just getting started with variables. Part 3, coming soon!.. (Update: Part 3 is available now)

 

Exploring variables in QGIS 2.12, part 1

It’s been quite some time since I last had a chance to blog and a lot has happened since then. Not least of which is that QGIS 2.12 has now been released with a ton of new features that I’ve neglected to write about! To try and get things moving along here again I’m planning on writing a short series exploring how variables work in QGIS 2.12 and the exciting possibilities they unlock. First, let’s look into how variables can be used with QGIS map composer…

So, let’s get started! A new concept introduced in QGIS 2.12 is the ability to set custom variables for use in QGIS’ expression engine. The easiest way to do this is through the “Project Properties” dialog, under the “Variables” section:

Default project variables

Default project variables

You’ll see in the screenshot above that a blank project includes a number of read-only preset variables, such as @project_path and @project_title. (All variables in QGIS are prefixed with an @ character to differentiate them from fields or functions). You can add your own variables to this list by clicking the + button, as shown below:

Adding new variables to a project

Adding new variables to a project

Here I’ve added some new variables, @project_version and @author. Now, any of these variables can be used anywhere that you can use expressions in QGIS, including the field calculator, data defined symbology, labelling, map composer text, etc. So, you could make a map composer template with a label that includes the @author, @project_version and @project_path variables:

Variables in a composer label

Variables in a composer label

Sure, you *could* also manually enter all these details directly into the label for the same result. But what happens when you have multiple composers in your project, and need to update the version number in all of them? Or you move your project to a new folder and need to make sure the path is updated accordingly? Manually updating multiple composers is a pain – make QGIS do the work for you and instead use variables! This would especially be helpful if you’re saving map composer templates for use across multiple projects or users. Using variables will ensure that the template is automatically updated with the right details for the current project.

Another neat thing about QGIS variables is that they can be inherited and overridden, just like CSS rules. Opening the options dialog will also show a Variables group for setting “Global” variables. These variables are always available for your QGIS installation, regardless of what project you’re working on at the time. If your workplace tends to reorganise a lot and constantly shuffle your department around, you could add a global variable for @work_department, so that changing the global variable value in one place will automatically filter through to any existing and future projects you have.

Global variables

Global variables

And like I mentioned earlier, these variables are inherited through various “contexts” within QGIS. If I reopen the Project Properties dialog, you’ll see that a project has access to all the global variables plus the variables set within that specific project. In addition, by adding a variable with the same name to the Project variables the value of the Global variable will be overridden:

Overridden variables

Overridden variables

There’s also a variable editor within each individual composer’s properties tab, so variables can also be set and overridden on a composer-by-composer basis within a project. It’s a really flexible and powerful approach which both simplifies workflows and also opens up lots of new possibilities.

Stay tuned for more on this topic – this topic has only just scratched the surface of how expression variables have changed QGIS! (You can also read part 2 and part 3)

In brief: 4th QGIS user group in Scotland

Another sold-out event with a programme packed with useful, interesting and delightful talks. Fifty seven (57!) folk blew in from all over Scotland through a freezing rain but hot coffee and pastries were waiting in the Informatics Forum at the University of Edinburgh.

First up was an overview the current status of the QGIS project by Saber from Lutra Consulting. It was good for people new to QGIS and open-source to see how the project is organised and run and the direction it is taking. Pete, also from Lutra Consulting, then gave a quick summary of the bits of core functionality they have been working on including the new ruled based labelling system.
The group then split into two for 90 minute workshop sessions on cartographic labelling and advanced Atlas usage – a tough choice! Chris, from Ordnance Survey, presented a detailed how-to on the new ruled based labelling tools using some OS open data, interspersed with some slides on guidelines to good cartographic practices and labelling tips. The slides and material for this workshop are available here: https://dl.dropboxusercontent.com/u/185489368/QGIS-Scotland2015.pdf

Heikki, from thinkWhere, lead us through the process of using Atlas in Print Composer to automate map production for a series of maps containing a main context map and an inset overview map. Nothing better than doing something once and then being able to repeat it at the click of a button! The slides and material for this workshop are available here: https://github.com/HeikkiVesanto/Scottish_QGIS_User_Workshop

A break for lunch and a good hour of catching up with users from across all sectors – local government, central government, academia, forestry, planning consultancies, developers, student life and education. QGIS is popular and is obviously a flexible tool that meets many demands.

After lunch, Neil, from thinkWhere, organised a quick-fire “quirky QGIS quiz” with random questions from all aspects of FOSS4G demanding quick thinking for true/false answers. At least half the audience grabbed prizes courtesy of thinkWhere and Ordnance Survey.

What followed was a series of lightning talks on different aspects of using QGIS. Amy, from Cawdor Forestry, gave a brief overview of the plugins available in the QGIS plugin repository and highlighted some of her personal favourites. Paul, from Scottish Water, showcased some of the complex workflows created using SAGA, QGIS and the Processing Toolbox to model hydrological process. Ross, from Inverclyde Council, demonstrated the use of the QGIS Road Graph plugin to generate walking routes to school across a custom road and path network. Seb, from West Dunbartonshire Council, showed us how QGIS had put them in a happy place and showed how a “hearts and minds” campaign championing QGIS had changed the way they worked. Steve, fae Embra, gave some information on how to give back to the QGIS project through submitting Processing scripts and plugins to the repository. Don’t reinvent the wheel! Ross, from Angus Council, gave a quick demonstration of setting up a local plugin repository that could be used to share custom plugins or control access to plugins in an internet-less environment.

By this time, tea and cake was required and it gave everyone an opportunity to mix and ask questions of the speakers.

The last session was as series of longer talks started by Gemma, from Ordnance Survey, explaining how open-source software is used extensively at Ordnance Survey to underpin a lot of the cartographic processes and workflows. They use QGIS 2.8 LTR for stability and consistency across the business and a selection of plugins from both OS developers and the community. QGIS is used to generate all the cartographic styles sheets for the OS vector products: https://github.com/OrdnanceSurvey/OS-VectorMap-District-stylesheets

Tom, from EDINA, explained the processing of creating and delivering a QGIS training course to University staff and students. The first class sold out in next to no time and there is demand for more. Makes sense really, doesn’t it?

Steve, from GeoGeo, wrapped up the day with the kind of mapping we’d all like to be doing – high resolution elevation models, time series analysis of shadows, viewsheds across the Edinburgh skyline and analysis of rooftops for potential solar panels with sub 1m resolution LiDAR datasets. He uses a mix of QGIS, SAGA, Blender and other FOSS to inspiring effect. Check his Flickr stream: https://www.flickr.com/photos/stevefaeembra/

The day finished with Pete (Lutra Consulting) releasing into the open a new Search plugin for QGIS called Discovery (http://www.lutraconsulting.co.uk/products/discovery/) based on the PostGIS Search plugin from Tim Martin (Ordnance Survey). A very useful addition to any QGIS installation.

The day was sponsored by EDINA, thinkWhere and Ordnance Survey.

Links to slides will be coming shortly.


How to create connectivity-based line caps

It’s been a while since my last blog post mostly because I’ve been busy with some more long form writing. Most notably, I’ve been writing a paper on the QGIS Projcessing framework in the open access ISPRS International Journal of Geo-Information together with Victor Olaya and I’m still in the process of writing a new book titled “QGIS Map Design” together with Gretchen Peterson which is scheduled for early 2016.

Today’s post has been on my todo list for a while now. It’s inspired by a talk at a recent cartography conference I attended:

(For a summary of the whole event, check the storify I compiled.)

The idea of this slide and several more was to show all the attention to detail which goes into designing a good road map. One aspect seemed particularly interesting to me since I had never considered it before: what do we communicate by our choice of line caps? The speaker argued that we need different caps for different situations, such as closed square caps at the end of a road and open flat caps when a road turns into a narrower path.

I’ve been playing with this idea to see how to reproduce the effect in QGIS …

So first of all, I created a small test dataset with different types of road classes. The dataset is pretty simple but the key to recreating the style is in the attributes for the road’s end node degree values (degree_fro and degree_to), the link’s road class as well as the class of the adjacent roads (class_to and class_from). The degree value simply states how many lines connect to a certain network node. So a dead end as a degree of 1, a t-shaped intersection has a degree of 3, and so on. The adjacent class columns are only filled if the a neighbor is of class minor since I don’t have a use for any other values in this example. Filling the degree and adjacent class columns is something that certainly could be automated but I haven’t looked into that yet.

roadattributes

 

The layer is then styled using rules. There is one rule for each road class value. Rendering order is used to ensure that bridges are drawn on top of all other lines.

roadrules

Now for the juicy part: the caps are defined using a data-defined expression. The goal of the expression is to detect where a road turns into a narrow path and use a flat cap there. In all other cases, square cap should be used.

roadrule

Like some of you noted on Twitter after I posted the first preview, there is one issue and that is that we can only set one cap style per line and it will affect both ends of the line in the same way. In practice though, I’m not sure this will actually cause any issues in the majority of cases.

I wonder if it would be possible to automate this style in a way such that it doesn’t require any precomputed attributes but instead uses some custom functions in the data-defined expressions which determine the correct style on the fly. Let me know if you try it!


A new QGIS plugin allows dynamic filtering of values in forms

 

 

This plugin has been partially funded (50%) by ARPA Piemonte.

Description

This is a core-enhancement QGIS plugin that makes the implementation of complex dynamic filters in QGIS attribute forms an easy task. For example, this widget can be used to implement drill-down forms, where the values available in one field depend on the values of other fields.

Download

The plugin is available on the official QGIS Python Plugin Repository and the source code is on GitHub QGIS Form Value Relation plugin repository

Implementation

The new “Form Value Relation” widget is essentially a clone of the core “Value Relation” widget with some important differences:

When the widget is created:

  • the whole unfiltered features of the related layer are loaded and cached
  • the form values of all the attributes are added to the context (see below)
  • the filtering against the expression happens every time the widget is refreshed
  • a signal is bound to the form changes and if the changed field is present in
    the filter expression, the features are filtered against the expression and
    the widget is refreshed

Using form values in the expression

A new expression function is available (in the “Custom” section):

CurrentFormValue('FIELD_NAME')

This function returns the current value of a field in the editor form.

Note

  1. This function can only be used inside forms and it’s particularly useful when used together with the custom widget `Form Value Relation`
  2. If the field does not exists the function returns an empty string.

Visual guide

 

Download the example project.

 

This is the new widget in action: changing the field FK_PROV, the ISTAT values are filtered according to the filter expression.

The new widget in action

The new widget drill-down in action

layer_config_fields

Choosing the new widget

Configuring the widget

Configuring the widget

Configuring the expression

Configuring the expression to read FK_PROV value from the form

News from QGIS HackFest in Las Palmas

First I wish to thank Pablo & friends for the amazing organization, unfortunately I couldn’t spend more than two full days there, but those two days have been memorable!

Here is a picture of one of the most interesting discussions (photo: courtesy of Pablo).

QGIS discussion at the developer meeting in Las Palmas

An hack fest is an event for writing good code but what it’s really good at is to establish and cultivate relations with other coders, to exchange opinions and ideas and last but not least to have some fun and make new friends.

 

This time, we have had many interesting presentations and a couple of meetings where we spoke about technical aspects of the project management and infrastructure and about some important challenges, both in terms of code size and economical implications for who relies on it, that a growing project must face.

 

The latter was something I’ve also been considering for a while: now that pull requests (PR) for new features are coming down the pipeline, we must find a better way to manage their queue by giving a clear and transparent approval path and deadline. This management and approval process cannot rely entirely on volunteer work, the main reason being that most of the times the PR proposers have been paid for that PR and it’s not fair (nor reliable) that the (sometimes hard) job of doing a code review is not rewarded. On the other end, an investor cannot waste its time and money on a project without having a reasonable good chance to see its work eventually land into the core of QGIS.

 

Hugo (thanks for that!) organized a meeting to discuss this topic, that crosses personal business interests, ethical considerations and personal beliefs to a point that it’s not really easy to discuss in a calm and objective way, despite the premises, the discussion was very interesting and constructive and a QEP that tries to address at least some of this problems is open for discussion right now: https://github.com/qgis/QGIS-Enhancement-Proposals/issues/52

 

Another topic we’ve been discussing was how to manage python plugin dependencies, we’ve decided to start by adding a new metadata tag called external_deps that’s supposed to contain the PIP install string for the required packages, since PIP will be a builtin in python 3.4, that will probably solve most of our problems when we’ll integrate that into the plugin manager. At the moment the metadata is not documented nor required, but it’s there to allow for experiments.

 

We didn’t miss the occasion to talk about the ugly bug that affects fTools, not something I’m going to dig into in this post though.

 

Of course an hack fest is still a good opportunity for squashing bugs and implement new cool features, I’ve been busy mainly on the following topics:

  1. HiDPI screen support for web view widgets (help and plugin manager/installer)
  2. Form relations editing longstanding bugs
  3. New feature to optionally enter, edit and store Python form init code into the project (and DB), see the picture below
  4. Plugins website maintenance (added new metadata and fixed a few bugs, added an RPC call to export author email for admins)
New QGIS feature to store form init code

 

Thanks to all participants, to the organizers and to all QGIS sponsors and donors that made this possible!

 

Creating a Tissot’s Indicatrix in QGIS

The task of projecting, or unfolding the spherical Earth onto a flat map, is an age old problem in cartography. Projection almost always introduces distortion, most projections cannot preserve angles, areas and distances at the same time, they may be conformal (angle-preserving), equal-area (area-preserving) or equidistant (distance preserving) but not all at once. The only exception is a Globe, which preserves angles, areas and distances perfectly. Thus a projection is a compromise.

The choice of projection depends on a map’s use, scale and audience. Conformal projections, for example, are preferred for nautical charts or small scale maps because they locally preserve angles necessary for navigation and survey drawings. Equal-area projections are best suited for maps of broad continental region as they preserve the relative sizes of countries, seas and oceans and allow comparison between regions. Finally, there are hybrid projections that minimise the distortion by merging conformal and equal-area projections, these can be used to create visually pleasing maps of the entire Earth (for a guide to selecting a map projection see Fig. 9 in Jenny (2011), link below).

But how does one measure the degree and type of distortion in a map projection?

One elegant method was developed in the 1880s century by the French cartographer Monsieur Nicolas Auguste Tissot, the Tissot’s Indicatrix (or Tissot’s Ellipse). This mathematical contrivance consists of a grid of infinitely small circles that measures the degree and type of distortion caused by projection. While Monsieur Tissot’s approach is mathematical, involving infinity small circles, his technique can be approximated overlaying a regular grid of large circles and crosses to a map.

The Indicatrix Mapper plugin for QGIS by Ervin Wirth and Peter Kun creates a Tissot’s Indicatrix by adding a vector layer of circles and crosses in a gridded pattern on a map. The degree and type of distortion of the Tissot’s Indicatrix reveals the class of map projection as follows: –

  • If a projection is conformal, the area of circles and sizes of the crosses will change while the shapes of circles remain the same and intersection angle of the crosses will always meet at 90 degrees e.g. Mercator projection
  • If a projection is equal-area, the area of the circles will remain the same while the shape of the circles change and intersection angle of the crosses will not always meet at 90 degrees e.g. Mollweide projection or Hammer projection
  • If a projection preserves neither property, the area of the circles and their shape will change, and the intersection angle of the crosses will not always meet at 90 degrees e.g. Robinson

After adding the Indicatrix Mapper plugin to QGIS (menu Plugins – Manage and Install Plugins) first add a basemap using the OpenLayers plugin e.g. Bing Aerial layer, then click the Indicatrix Mapper icon and run the plugin using default settings. You can then select different projections (lower right in world icon QGIS) to see the effects of various protections on the Tissot Indicatrix. If the circles appear as squares after selecting a different projection, right click the Circles layer in the layers panel, then select the Rendering tab and deselect the Simplify geometry check box. Also, turn off the basemap layer when using different projections, unfortunately the OpenLayers plugin only supports Google Mercator projection (EPSG 3857). To create the basemap below, that can be displayed using different projections, I styled vector data downloaded from Natural Earth and OpenStreetMap.

Mercator

Mercator Projection – the area of the circles and size of the crosses increase towards the poles but their shape remains the same.

Mollweide

Mollweide Projection – the area of the circles remain the same but their shapes are distorted, the crosses do not always intersect at 90 degrees.

Robinson

Robinson Projection – both the area of the circles and intersection angle of the crosses circles vary.

It is important to note that a Tissot’s Indicatrix generated in QGIS is an approximation of mathematical ideal, we are not no longer dealing with infinity small circles. As a result, here will be some minor distortion visible towards the edge of a map independent of the projection used; notice that the circles in the Mercator projection nearest the poles are not quite symmetrical or the circles at the edge of the Mollweide projection do not appear to preserve area as they should. This anomalous distortion can be minimised by reducing the size and spacing of the circles and crosses created by the Indicatrix Mapper plugin. However, despite these limitations a Tissot’s Indicatrix elegantly reveals the distortion present. This is something to important to understand when when choosing a map projection.

References:

Jenny, B., 2012. Adaptive composite map projections [PDF]. Visualization and Computer Graphics, IEEE Transactions on, 18, 2575–2582.

QGIS Crowdfunding: 2.5D Rendering

2.5D rendering

Spatialite and lost styles

In this blog I will explain how to recover styles that are included in a SpatiaLite database but are lost after renaming or moving the SpatiaLite database to another location. It will show how to use the QGIS Field Calculator to change the contents of de field f_table_catalog in the table layer_styles so it directs … Continue reading Spatialite and lost styles

Dica para ajustar posição de símbolos em QGIS | Hack to adjust map symbols location in QGIS

De quando em vez aparecem-me zonas com demasiado símbolos no mesmo local, e pensei como seria fantástico se os pudesse arrastar para um local mais conveniente sem ter de alterar as suas geometrias, tal como é possível fazer com as etiquetas. Esse pensamento deu-me a ideia base para a dica que vou demonstrar.

Now and then I get too many map symbols (points) in the same place, and I thought how nice it would be if we could drag n’ drop them around without messing with their geometries position, just like we do with labels. That thought gave me an idea for a cool hack.

Escolha a sua camada de pontos e comece por criar dois novos campos chamados symbX e symbY (Tipo: Decimal; Tamanho: 20; precisão: 5). No separador “Estilo” das propriedades da camada, defina para cada nível do seu símbolo o seguinte: Escolher “unidade do mapa” como a unidade para as opções de afastamento; Usar a seguinte expressão na opção afastamento das propriedades definidas por dados.

Choose your point layer and start by creating two new fields called symbX and symbY (Type: Decimal number; Size: 20; Precision: 5). Now go the layer properties and in the Style tab edit your symbol. For each level of your symbol select “map units” as the offset units, and set the following expression in the offset data define properties option:


CASE WHEN symbX IS NOT NULL AND symbY IS NOT NULL THEN
    tostring($x - symbX) + ',' + tostring($y - symbY)
ELSE
    '0,0'
END

Screenshot from 2015-02-22 18:18:43

Tenha atenção que, se as coordenadas do seu mapa tiver valores negativos, será necessário uma pequena alteração ao código. E. g., se tiver valores negativos em X deverá usar-se  antes a expressão “tostring(symbX -$x)”.

Beware that if your coordinates have negative values you need to adapt the code. E.g., If you have negative values in X you should use “tostring(symbX -$x)” instead.

De forma temporária coloque etiquetas na sua camada usando um texto pequeno (eu usei o ‘+’ (sinal de mais) centrado e com um buffer branco) e defina as coordenadas X e Y dos propriedades definidadas por dados usando os campos symbX e symbY,

Now, temporarly  label your layer with a small convenient text (I used a centered ‘+’ (plus sign) with a white buffer) and set its coordinates to data defined using the symbX and symbY Fields.

Screenshot from 2015-02-22 22:42:07

A partir desse momento, quando usar a ferramenta de mover etiquetas, não só alterará a posição da etiqueta, mas também a do próprio símbolo! Fantástico, não?

From this point on, when you use the move label tool, not only the label position change but also the actual symbol! Pretty cool, isn’t it?

anim

Note que as geometria dos elementos não são alteradas durante o processo. Para além disso, lembre-se que neste caso também poderá adicionar linhas de guia para ligar os símbolos à posição original do ponto.

Notice that the features geometries are not changed during the process. Also, remember that in this case you can also add leading lines to connect the symbols to the original position of the points.


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