This is a short tip for you if you use QGIS on Windows and ended up with an error like this:
Windows’ suggestion to reinstall probably won’t fix this issue. Instead, you want Windows to run qgis.bat instead of any .exe it is trying to use. Right-click a .qgs file and go to Open with | Choose default program:
In OSGeo4W, the .bat file is located in the bin folder:
Today’s post was motivated by a question on GIS.StackExchange, which is looking for an automated way to symbolize the amenities available at a location using a series of icons, like this:
Assuming the information is available in a format similar to this example attribute table
we can create a symbol, which adapts to the values in the icon columns using data-defined overrides:
The five potential symbol locations are aligned next to each other using offsets. We use the following expression to determine the correct SVG symbol:
WHEN "icon4" = 'dinner'
WHEN "icon4" = 'sleep'
WHEN "icon4" = 'ship'
WHEN "icon4" = 'house'
To hide icons if the icon value is NULL, the marker size is set to 0 using, for example:
WHEN "icon4" is not NULL
Finally, to ensure that the labels don’t cover the icons, we can use the cartographic label placement with the position priority set to ‘TR,TL,BL’, which restricts labels to the top right, top left, and bottom left position.
With these settings in place, we can zoom out and the labeling algorithm picks the most suitable position from the list of allowed positions:
Default polygon symbols come with a fill and a border color:
When they are used in a graduated renderer, the fill color is altered for each class:
What if you want to change the border color instead?
The simplest solution is to add an outline symbol layer to your polygon symbol:
The outline layer has only one color property and it will be altered by the graduated renderer.
If you now hit ok, the graduated renderer will alter both the simple fill’s fill color and the outline’s color. To stop the fill color from changing, select the simple fill and lock it using the small lock icon below the list of symbol layers:
Have you ever thought “gee I wish I could have a ASCII QGIS map viewer for console use. I’m so over these fancy UIs with their fancy graphics, fonts, and icons”.
You are still reading? OK good I thought I lost you.
Anyway. Here is a fun idea. A ASCII QGIS map viewer that renders your project files in a console window (with colour possible). Still with me?
This project was mainly just a bit of fun to play with the curses Python library and QGIS. What started off as a random idea on the train seems to have turned into full “usable” thing, if viewing a project and the legend is considered usable.
If you are still with me and itching to see it in action here it is. In all the ASCII glory
Last week not only QGIS 2.14.0 Essen was released but also a great book “QGIS Map Design” based on this new version of QGIS. A release of QGIS is special but this release concerns an announced LTR release. QGIS 2.8.7 is the current LTR version in which al bugs found between 2.8 and 2.14.0 have been … Continue reading New release of QGIS and book!
So some good news for QGIS users who also need/want to use MapInfo. QGIS via GDAL 2.0 can support MapInfo TAB file editing. In all older versions of GDAL there was only support for read and/or write but not both.
MapInfo TAB editing has been supported in GDAL 2 but up until this point QGIS has only be built against GDAL 1.xx. GDAL 2.x is now the default GDAL release in OSGeo4w.
2.0.2 is now the default GDAL in OSGeo4W and the nightlies (qgis-ltr-dev,
qgis-rel-dev and qgis-dev) already picked it up.
With the next release the regular packages (2.14 and 2.8) will also be updated
to use it
Even if you don’t want to make the switch to full QGIS you can now use both bits of software and edit in both.
QGIS will still only support a single geometry type per layer so if you open a mixed tab file you will get the geometry type selector. You can load the layer 3 times if you need the 3 different geometry types.
This dataset is a pretty detailed building model, where each building is made up of multiple features that represent parts of the building with different height. Of course, if we just load the dataset in default style, we cannot really appreciate the data:
Loaded building parts layer
All this changes if we use the new 2.5D renderer. With just a few basic settings, we can create 2.5D representations of the building parts:
QGIS 2.5D renderer settings
Compare the results to aerial images in Google Maps …
QGIS 2.5D renderer and view in Google Maps
… not bad at all!
Except for a few glitches concerning the small towers at the corners of the building, and some situations where it seems like the wrong building part is drawn in the front, the 2.5D look is quite impressive.
Now, how does this compare to Qgis2threejs, one of the popular plugins which uses web technologies to render 3D content?
One obvious disadvantage of Qgis2threejs is that we cannot define a dedicated roof color. Thus the whole block is drawn in the same color.
On the other hand, Qgis2threejs does not suffer from the rendering order issues that we observe in the QGIS 2.5D renderer and the small towers in the building corners are correctly displayed as well:
QGIS 2.5D renderer and Qgis2threejs output
Overall, the 2.5D renderer is a really fun and exciting new feature. Besides the obvious building usecase, I’m sure we will see a lot of thematic maps making use of this as well.
Give it a try!
In the next post, I’m planning a more in-depth look into how the 2.5D renderer works. Here’s a small teaser of what’s possible if you are not afraid to get your hands dirty:
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…
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
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.
“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.
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?
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.
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.
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.
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.
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 :(.
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).
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).
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.
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.
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 …
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!
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)
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.
Fortunately for ArcGIS users with a low budget, they can easily make a request for a layer using the internet browser :-P.
Or they can simply use QGIS to download it. But, in both cases, be aware that the layers won’t update themselves by magic.
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.
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.
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).
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).
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:
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!
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
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).
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:
The global scope, consisting of variables set in the QGIS options dialog, and other installation-wide properties
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)
A composer scope, with any variables set for the current composer, plus variables for @layout_pagewidth, @layout_pageheight, @layout_numpages, etc.
A 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:
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:
The global scope
The project scope
A map settings scope, with variables relating to how the map is being rendered. Eg @map_rotation, @map_scale, etc
A 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:
The global scope
The project scope
The composer scope
An atlas scope, if atlas is enabled. This contains variables like @atlas_pagename, @atlas_feature, @atlas_totalfeatures.
A composer item scope for the map item
A map settings scope (with scale and rotation determined by the map item’s settings)
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 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:
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!
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
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!
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
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
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’:
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:
And now when we hit Apply the font for all our labelled layers will be updated all at once:
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…
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…
Then, you can set a data defined override for a symbol or label colour to the expression “project_color(‘red alert!’)“:
When you go back and change the corresponding colour in the Project Properties dialog, every symbol bound to this colour will also be updated!
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)
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
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
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
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.
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:
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)