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Thu Jul 31 02:20:08 2014

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QGIS Planet

Visualizing direction-dependent values

When mapping flows or other values which relate to a certain direction, styling these layers gets interesting. I faced the same challenge when mapping direction-dependent error values. Neighboring cell pairs were connected by two lines, one in each direction, with an associated error value. This is what I came up with:

srtm_errors_1200px

Each line is drawn with an offset to the right. The size of the offset depends on the width of the line which in turn depends on the size of the error. You can see the data-defined style properties here:

directed_error_style

To indicate the direction, I added a marker line with one > marker at the center. This marker line also got assigned the same offset to match the colored line bellow. I’m quite happy with how these turned out and would love to hear about your approaches to this issue.

srtm_errors_detail

These figures are part of a recent publication with my AIT colleagues: A. Graser, J. Asamer, M. Dragaschnig: “How to Reduce Range Anxiety? The Impact of Digital Elevation Model Quality on Energy Estimates for Electric Vehicles” (2014).


OSM Toner style town labels explained

The point table of the Spatialite database created from OSM north-eastern Austria contains more than 500,000 points. This post shows how the style works which – when applied to the point layer – wil make sure that only towns and (when zoomed in) villages will be marked and labeled.

Screenshot 2014-07-12 12.30.21

In the attribute table, we can see that there are two tags which provide context for populated places: the place and the population tag. The place tag has it’s own column created by ogr2ogr when converting from OSM to Spatialite. The population tag on the other hand is listed in the other_tags column.

Screenshot 2014-07-12 13.00.15

for example

"opengeodb:lat"=>"47.5000237","opengeodb:lon"=>"16.0334769","population"=>"623"

Overview maps would be much too crowded if we simply labeled all cities and towns. Therefore, it is necessary to filter towns based on their population and only label the bigger ones. I used limits of 5,000 and 10,000 inhabitants depending on the scale.

Screenshot 2014-07-12 12.56.33

At the core of these rules is an expression which extracts the population value from the other_tags attribute: The strpos() function is used to locate the text "population"=>" within the string attribute value. The population value is then extracted using the left() function to get the characters between "population"=>" and the next occurrence of ". This value can ten be cast to integer using toint() and then compared to the population limit:

5000 < toint( 
   left (
      substr(
         "other_tags",
         strpos("other_tags" ,'"population"=>"')+16,
         8
      ),
      strpos(
         substr(
            "other_tags",
            strpos("other_tags" ,'"population"=>"')+16,
            8
         ),
        '"'
      )
   )
) 

There is also one additional detail concerning label placement in this style: When zoomed in closer than 1:400,000 the labels are placed on top of the points but when zoomed out further, the labels are put right of the point symbol. This is controlled using a scale-based expression in the label placement:

Screenshot 2014-07-12 13.32.47

As usual, you can find the style on Github: https://github.com/anitagraser/QGIS-resources/blob/master/qgis2/osm_spatialite/osm_spatialite_tonerlite_point.qml


QGIS – Mapping Election Results, pt 2: Adding and overlaying the data in QGIS

Continuing on from the previous tutorial:-

Return to QGIS. Add the westminster_const_region.shp file if necessary

  1. Press the Add Delimitated Text file button, and select the .csv export of the cleansed electoral data
  2. The two options I changed from the default settings are:-
  • First record contains field names
  • No geometry (attribute only table)
QGIS - Create layer from text file

QGIS – Create layer from text file

Step 3 – Joining the data

Joining the polygons in westminster_const_region.shp to the data imported from the Results_Cleansed spreadsheet will allow the data to be presented in a spatial and visual format which will be much easier to interpret, allow for spatial analysis and also give the viewer an idea of the geographic spread. Using QGIS’ Join function will hopefully save a lot of copying and pasting!

Right click on westminster_const_region.shp and select Properties to open the Properties dialog

  • Select the Joins button from the left panel
  • Join Layer – the layer that you want to join to
  • Join Field – the field that you want to join to
  • Target Field – the field in this layer that contains the matching data
QGIS - Add vector layer

QGIS – Add vector layer

The join will now appear in the layer’s Joins list:-

QGIS layer properties

QGIS layer properties

The attribute table will now show the combined  data for both layers:-

QGIS attribute table

QGIS attribute table

This data can now be used to create a thematic map that colours each constituency according to party that won the seat in 2010.

I won’t go through all the steps of creating a thematic map as an earlier tutorial does this.

I’ve used the same colours that the different parties in the UK use:-

QGIS Layer properties

QGIS Layer properties

The thematic map shows the results across the entire UK. It is easy to identify patterns in the result, for example

  • The Liberal Democrats mostly won seats in Scotland, the North East, Wales and South West.
  • There is strong Labour support in South West Scotland, North West England, West Midlands, South Wales, London, Liverpool and Manchester.
  • The Conservative support covers much of the rest of England, especially South East England, excluding London.
2010 election results map

2010 election results map


Shapeburst fill styles in QGIS 2.4

With QGIS 2.4 getting closer (only a few weeks away now) I’d like to take some time to explore an exciting new feature which will be available in the upcoming release… shapeburst fills!

As a bit of background, QGIS 2.2 introduced a gradient fill style for polygons, which included linear, radial and conical gradients. While this was a nice feature, it was missing the much-requested ability to create so-called “buffered” gradient fills. If you’re not familiar with buffered gradients, a great example is the subtle shading of water bodies in the latest incarnation of Google maps. ArcGIS users will also be familiar with the type of effects possible using buffered gradients.

Gradient fills on water bodies in Google maps

Gradient fills on water bodies in Google maps

Implementing buffered gradients in QGIS originally started as a bit of a challenge to myself. I wanted to see if it was possible to create these fill effects without a major impact on the rendering speed of a layer. Turns out you can… well, you can get pretty close anyway. (QGIS 2.4′s new multi-threaded responsive rendering helps a lot here too).

So, without further delay, let’s dive into how shapeburst fills work in QGIS 2.4! (I’ve named this fill effect ‘shapeburst fills’, since that’s what GIMP calls it and it sounds much cooler than ‘buffered gradients’!)

Basic shapeburst fills

For those of you who aren’t familiar with this fill effect, a shapeburst fill is created by shading each pixel in the interior of a polygon by its distance to the closest edge. Here’s how a lake feature polygon looks in QGIS 2.4 with a shapeburst from a dark blue to a lighter blue colour:

A simple shapeburst fill from a dark blue to a lighter blue

A simple shapeburst fill from a dark blue to a lighter blue

You can see in the image above that both polygons are shaded with the dark blue colour at their outer boundaries through to the lighter blue at their centres. The screenshot below shows the symbol settings used to create this particular fill:

A simple shapeburst fill from a dark blue to a lighter blue

Creating a simple shapeburst fill from a dark blue to a lighter blue

Here we’ve used the ‘Two color‘ option, and chosen our shades of blue manually. You can also use the ‘Color ramp’ option, which allows shading using a complex gradient containing multi stops and alpha channels. In the image below I’ve created a red to yellow to transparent colour ramp for the shapeburst:

Colour ramp shapeburst with alpha channels

Colour ramp shapeburst with alpha channels

Controlling shading distance

In the above examples the shapeburst fill has been drawn using the whole interior of the polygon. If desired, you can change this behaviour and instead only shade to a set distance from the polygon edge. Let’s take the blue shapeburst from the first example above and set it to shade to a distance of 5 mm from the edge:

Shapeburst fills can shade to a set distance only

Shapeburst fills can also shade to a set distance from the polygon’s exterior

This distance can either be set in millimetres, so that it stays constant regardless of the map’s scale, or in map units, so that it scales along with the map. Here’s what our lake looks like shaded to a 5 millimetre distance:

Shading to 5mm from the lake's edge

Shading to 5mm from the lake’s edge

Let’s zoom in on a portion of this shape and see the result. Note how the shaded distance remains the same even though we’ve increased the scale:

Zooming in maintains a constant shaded distance

Zooming in maintains a constant shaded distance

Smoothing shapeburst fills

A pure buffered gradient fill can sometimes show an odd optical effect which gives it an undesirable ‘spiny’ look for certain polygons. This is most strongly visible when using two highly contrasting colours for the fill. Note the white lines which appear to branch toward the polygon’s exterior in the image below:

Spiny artefacts on a pure buffered gradient fill

Spiny artefacts on a pure buffered gradient fill

To overcome this effect, QGIS 2.4 offers the option to blur the results of a shapeburst fill:

Blur option for shapeburst fills

Blur option for shapeburst fills

Cranking up the blur helps smooth out these spines and results in a nicer fill:

Adding a blur to the shapeburst fill

Adding a blur to the shapeburst fill

Ignoring interior rings

Another option you can control for shapeburst fills is whether interior polygon rings should be ignored. This option is useful for shading water bodies to give the illusion of depth. In this case you may not want islands in the polygon to affect their surrounding water ‘depth’. So, checking the ‘Ignore rings in polygons while shading‘ option results in this fill:

Ignoring interior rings while shading

Ignoring interior rings while shading

Compare this image with the first image posted above, and note how the shading differs around the small island on the polygon’s left.

Some extra bonuses…

There’s two final killer features with shapeburst fills I’d like to highlight. First, every parameter for the fill can be controlled via data defined expressions. This means every feature in your layer could have a different start and end colour, distance to shade, or blur strength, and these could be controlled directly from the attributes of the features themselves! Here’s a quick and dirty example using a random colour expression to create a basic ‘tint band‘ effect:

Using a data defined expression for random colours

Using a data defined expression for random colours

Last but not least, shapeburst fills also work nicely with QGIS 2.4′s new “inverted polygon” renderer. The inverted polygon renderer flips a normal fill’s behaviour so that it shades the area outside a polygon. If we combine this with a shapeburst fill from transparent to opaque white, we can achieve this kind of masking effect:

Creating a smooth exterior mask using the "inverted polygons" renderer

Creating a smooth exterior mask using the “inverted polygons” renderer

This technique plays nicely with atlas prints, so you can now smoothly fade out the areas outside of your coverage layer’s features for every page in your atlas print!

All this and more, coming your way in a few short weeks when QGIS 2.4 is officially released…

Toner-lite styles for QGIS

In my opinion, Stamen’s Toner-lite map is one of the best background maps to use together with colorful overlays. The only downsides of using it in QGIS are that the OpenLayers plugin can not provide the tiles at print resolution and that the projection is limited to Web Mercator. That’s why I’ve started to recreate the style for OSM Spatialite databases:

toner-lite

So far, there are styles for lines and polygons and they work quite well for the scale range between 1:1 and 1:250000. As always, you can download the styles from QGIS-resources on Github.


A guide to GoogleMaps-like maps with OSM in QGIS

Using OSM data in QGIS is a hot topic but so far, no best practices for downloading, preprocessing and styling the data have been established. There are many potential solutions with all their advantages and disadvantages. To give you a place to start, I thought I’d share a workflow which works for me to create maps like the following one from nothing but OSM:

osm_google_100k

Getting the data

Raw OSM files can be quite huge. That’s why it’s definitely preferable to download the compressed binary .pbf format instead of the XML .osm format.

As a download source, I’d recommend Geofabrik. The area in the example used in this post is part of the region Pays de la Loire, France.

Preparing the data for QGIS

In the preprocessing step, we will extract our area of interest and convert the .pbf into a spatialite database which can be used directly in QGIS.

This can be done in one step using ogr2ogr:

C:\Users\anita_000\Geodata\OSM_Noirmoutier>ogr2ogr -f "SQLite" -dsco SPATIALITE=YES -spat 2.59 46.58 -1.44 47.07 noirmoutier.db noirmoutier.pbf

where the -spat option controls the area of interest to be extracted.

When I first published this post, I suggested a two step approach. You can find it here for future reference:

For the first step: extracting the area of interest, we need Osmosis. (For Windows, you can get osmosis from openstreetmap.org. Unpack to use. Requires Java.)

When you have Osmosis ready, we can extract the area of interest to the .osm format:

C:\Users\anita_000\Geodata\OSM_Noirmoutier>..\bin\osmosis.bat --read-pbf pays-de-la-loire-latest.osm.pbf --bounding-box left=-2.59 bottom=46.58 right=-1.44 top=47.07 --write-xml noirmoutier.osm

While QGIS can also load .osm files, I found that performance and access to attributes is much improved if the .osm file is converted to spatialite. Luckily, that’s easy using ogr2ogr:

C:\Users\anita_000\Geodata\OSM_Noirmoutier>ogr2ogr -f "SQLite" -dsco SPATIALITE=YES noirmoutier.db noirmoutier.osm

Finishing preprocessing in QGIS

In QGIS, we’ll want to load the points, lines, and multipolygons using Add SpatiaLite Layer:

Screenshot 2014-05-31 11.39.40

When we load the spatialite tables, there are a lot of features and some issues:

  • There is no land polygon. Instead, there are “coastline” line features.
  • Most river polygons are missing. Instead there are “riverbank” line features.

Screenshot 2014-05-31 11.59.58

Luckily, creating the missing river polygons is not a big deal:

  1. First, we need to select all the lines where waterway=riverbank.
    Screenshot 2014-05-31 13.14.00
  2. Then, we can use the Polygonize tool from the processing toolbox to automatically create polygons from the areas enclosed by the selected riverbank lines. (Note that Processing by default operates only on the selected features but this setting can be changed in the Processing settings.)
    Screenshot 2014-05-31 13.40.16

Creating the land polygon (or sea polygon if you prefer that for some reason) is a little more involved since most of the time the coastline will not be closed for the simple reason that we are often cutting a piece of land out of the main continent. Therefore, before we can use the Polygonize tools, we have to close the area. To do that, I suggest to first select the coastline using "other_tags" LIKE '%"natural"=>"coastline"%' and create a new layer from this selection (save selection as …) and edit it (don’t forget to enable snapping!) to add lines to close the area. Then polygonize.

Screenshot 2014-05-31 14.38.48

Styling the data

Now that all preprocessing is done, we can focus on the styling.

You can get the styles used in the map from my Github QGIS-resources repository:

  • osm_spatialite_googlemaps_multipolygon.qml … rule-based renderer incl. rules for: water, natural, residential areas and airports
  • osm_spatialite_googlemaps_lines.qml … rule-based renderer incl. rules for roads, rails, and rivers, as well as rules for labels
  • osm_spatialite_googlemaps_roadshields.qml … special label style for road shields
  • osm_spatialite_googlemaps_places.qml … label style for populated places such as cities and towns

qgis_osm_google_100k


Topology in QGIS

Introduction

Topology rules define the permissible relationships of features within a given GIS layer or between features in two different GIS layers. An example is that features in a road dataset must be connected to other roads at both ends, unless the road is specified as a dead end street.

Advantage of topology over queries

A lot of the checks that topology rules carry out could be achieved using spatial queries. You may have to use queries if the GIS software you’re using doesn’t have a topology feature.

Topology rules have the advantage that they only need be created once and then they can check your work as you go.

Queries would need to be re-created each time they are run. They can be saved, depending on the GIS being used, but this is still more time consuming and it is a task that must be carried out separately at the end of a work session.

Rules

QGIS 2.2 topology tool has the following rules pre-defined:-

  • End points must be covered by (e.g. a railway line usually begins and ends at a station)
  • Must contain (e.g. a building polygon must contain at least one address point seed)
  • Must not have dangles (a line must begin and end at another line)
  • Must not have duplicates (each feature should be unique, e.g. postcode areas)
  • Must not have gaps (e.g. administrative area polygons cannot have gaps)
  • Must not have invalid geometries
  • Must not have multi-part geometries (each feature should be a separate entry)
  • Must not overlap (e.g. administrative area polygons cannot overlap each other)
  • Must not overlap with (a feature from layer must not overlap with another layer)

Example 1 – Roads must not have dangles

The following example uses the “Must not have dangles” rule to identify polylines from a roads dataset that are not snapped to other lines. Roads usually begin and end at a junction with another road, so this is a useful rule to identify where lines were not correctly snapped together.

To create and validate a Topology Rule

  • Open the Topology Panel, by selecting Vector menu, Topology Checker, Topology Checker
  • The Topology Panel appears in the lower right corner of the QGIS desktop window

Image

  • Press the Configure button to open the Topology Rule Settings dialog
  • The top of the box will have 2 or 3 pull down boxes depending on the layer and rule that is chosen. Use these to build the rule and then press the Add Rule button.

Image

  • Press OK when done, the dialog box closes and the window returns to the QGIS Desktop.
  • Press either the Validate All or Validate extent, depending on whether you wish to validate the entire dataset or just the current view extent.
  • The errors will be listed. Double click on a row will make the map window zoom and pan to the error.

Image

 


Scottish QGIS User Group Overview

Scottish QGIS User Group
Stirling, 19th March 2014

It was a long time coming but the wait was worth it. Forty two excited QGIS users and open-source GIS enthusiasts arrived at the Stirling Management Centre on a brilliantly sunny March day.  People had traveled from all over the UK to make the day happen: Charley Glynn from OS in Southampton, Pete Wells, Martin Dobias and Saber Razmjooei from Brighton as well as others from Aberdeen, Inverness, Dundee, Edinburgh, Glasgow, Cumbria and most places inbetween. The event was supported by thinkWhere, based in Stirling, and Neil Benny and Heikki Vesanto provided suitably geeky geo entertainment.

Neil Benny, QGIS EvangelistFirst up was Neil Benny (thinkWhere) who provided us with an overview of QGIS through the years to the current top features available in version 2.2 “Valmiera”. The questions on everyone’s minds were answered when he presented a series of slides outlining the benefits of using open source software, highlighting the savings and investments and the importance of investing in training. His top 10 feature comparison of proprietary v open source desktop GIS provoked much discussion.

After a coffee break I presented a short talk on how Angus Council is moving to a mixed hybrid GIS environment to take advantage of the flexibility of the open source licence and the variety of tools available to deliver results. Available here http://vimeo.com/89959143

Martin Dobias of Lutra Consulting and core QGIS developer revealed some of the performance enhancements available in the development version of QGIS. The multi-threaded multi-core rendering impressed everyone and will prove a huge draw card to seasoned GIS’ers used to single threaded applications.Martin Dobias, Core Developer

Saber Razmjooei (Lutra) filled in an open slot talking about the autoTrace plugin they developed for a group of Local Authorities across the UK. Modeled on the MapInfo trace tool it forms a key part of a lot of Council workflows and is a good example of how future plugin development work can deliver savings.

Pete Wells, plugin developerPete Wells (Lutra) delivered a very comprehensive overview of Python and QGIS and how they interact at different levels through the python bindings. There was a lot of interest in this and this was reflected in the feedback forms we collected where Python, plugins, hands-on workshops and tutorials feature high on the list of wants.

Charley Glynn (OS) unveiled some fantastic cartography using the OS vector products of MasterMap, VectorMap Local and District. He also revealed the work OS has been doing to make corporate styles available to the public and the Ordnance Survey’s bias towards open source software. Again the feedback forms revealed a desire to get hands on with QGIS to create good looking custom cartography. The next Scottish user group meeting will definitely be having some hands-on workshops.Charley Glynn, OS Cartographer IMG_20140319_152620

Heikki Vesanto (thinkWhere) bravely ventured into live demos of how to connect to just about any spatial data format available. Local files, local databases, WMS feeds, WFS feeds, text files, CSV and URLs with images and custom map templates using the Atlas generator. An excellent overview of just how flexible QGIS is when it comes to consuming data and converting data to almost every format supported by OGR and GDAL.

Thanks must go to the generosity of thinkWhere in supporting a feature filled programme of presentations and keeping us topped up with coffee. As a result the first Scottish QGIS user group meeting was a success and there is definitely a desire for more events like this.

Slides and videos of the presentations will be available here shortly.


QGIS – Two neat features in 2.2

Here’s a quick run-down on two nice new styling options which I’ve recently added to QGIS 2.2.

Map styling for compositions

This little feature was suggested by Mathieu Pellerin, who is always pushing the boundaries of QGIS’ cartographic tools and coming up with great ideas for new styling features (you can check out some of his work via Flickr). Mathieu’s idea was for a new ‘$map‘ variable for the expression builder. This variable holds the id of the map item which is drawing the map, and allows for some nice tweaking of maps in the composer.

The $map variable is most useful when you have more than one map in your composition. The example below shows $map being used to change the styling of a single layer from the main map to the smaller inset map:

Using $map to style two maps with different colours

Using $map to style a single layer in two maps with different colours

In this example the composition has two maps, the larger has an id of “main_map” and the smaller has “inset_map“. The boundary layer has been styled using the rule based renderer, with one rule for $map=’main_map’ and one for $map=’inset_map’, as shown below:

Rule based rendering using the $map variable

Rule based rendering using the $map variable

The end result is that the layer will be rendered using the two different styles depending on which composer map item it is being drawn into. This trick can also be used to tweak labelling rules between the maps. In the example above I’ve restricted the labelling to only show in the main map. This is achieved by setting an expression for the data defined “Show label” property. I’ve used the expression “$map=’main_map’” so that labels are only shown in the main map and not the smaller inset map.

Tweaking label settings using the $map variable

Tweaking label settings using the $map variable

This small addition to QGIS 2.2 allows for some rather powerful improvements to multi-map compositions!

Drawing polygon borders only inside the polygon

The second new feature I wanted to highlight is a new option for polygon outlines which causes the outline to be drawn only on the inside of a polygon feature. The usual behaviour is for outlines to be drawn directly over the centre of the feature boundary, so that half of the outline is drawn inside the feature and half on the outside.

Simple Line Fill before

This means that the outline in a simple line symbol layer overlaps into the neighbouring polygons, and the result is that outlines from these features blend together:

Shaded borders pre QGIS 2.2

Shaded borders pre QGIS 2.2 – see how the colours bleed into the neighbouring features and overlap

This looks like a big muddy mess. A feature I’ve wanted for a long time is the ability to restrict these outlines so that they are only drawn inside the feature. This effect is commonly seen in world atlases and National Geographic maps, where each neighbouring country is shaded with it’s own unique outline colour. Now it’s possible to do this in QGIS just by ticking a single box!

The new "Draw line only inside polygon" option

The new “Draw line only inside polygon” option

As you can see in the above image, the simple line outline style has a new checkbox, “Draw line only inside polygon“. Ticking this box will clip the outline so that only the portion of it which falls inside the feature is rendered. Here’s the result:

Shaded borders with "Draw line only inside polygon" checked

Shaded borders with “Draw line only inside polygon” checked

So much nicer then the earlier output – now none of the borders overlap into their neighbouring regions! Ok, so it is possible to achieve a similar result by creating a specially crafted layer consisting of negatively buffered polygons subtracted from the original polygons, but this takes a lot of fiddling around. It also has the major disadvantage in that the result is scale dependant, and zooming in or out of the map will alter the size of the polygon outlines. But using this wonderful new checkbox in QGIS, we get proper scale-independent borders, and zooming in or out of the map keeps a consistent border width!

Zooming in keeps a consistent border width...

Zooming in keeps a consistent border width…

So there we go – two small new features added in QGIS 2.2 which have huge potential for your cartographic outputs! As per usual, if you come up with some fancy way of utilising these, don’t forget to add your maps to the QGIS Showcase on Flickr.

Vienna elevation model

Since I finally managed to download the elevation model of the city of Vienna, I thought I’d share some eye candy with you: The map uses layer blending to combine hillshade and elevation raster, and the elevation raster’s color ramp is a modified “garish14″ from QGIS’ cpt-city color ramp collection.

wien_elevation by underdarkGIS
wien_elevation, a photo by underdarkGIS on Flickr.

Update

Here is how you get access to the “garish14″ color ramp:

Start by selecting the "new color ramp" option in the raster's style window.

Start by selecting the “new color ramp” option in the raster’s style window.

Chose the "cpt-city" color ramp type.

Chose the “cpt-city” color ramp type.

In the "cpt-city color ramp" window, you will find lots of different premade color ramps. "garish14" is part of the "Topography" collection.

In the “cpt-city color ramp” window, you will find lots of different premade color ramps. “garish14″ is part of the “Topography” collection.


Using the 25m EU-DEM for shading OpenStreetMap layers

Inspired by Václav Petráš posting about “Did you know that you can see streets of downtown Raleigh in elevation data from NC sample dataset?” I wanted to try the new GRASS GIS 7 Addon r.shaded.pca which creates shades from various directions and combines then into RGB composites just to see what happens when using the new EU-DEM at 25m.

To warm up, I registered the “normally” shaded DEM (previously generated with gdaldem) with r.external in a GRASS GIS 7 location (EPSG 3035, LAEA) and overlayed the OpenStreetMap layer using WMS with GRASS 7′s r.in.wms. An easy task thanks to University of Heidelberg’s www.osm-wms.de. Indeed, they offer a similar shading via WMS, however, in the screenshot below you see the new EU data being used for controlling the light on our own:

OpenStreetMap shaded with EU DEM 25m

OpenStreetMap shaded with EU DEM 25m (click to enlarge)

Next item: trying r.shaded.pca… It supports multi-core calculation and the possibility to strengthen the effects through z-rescaling. In my example, I used:

r.shaded.pca input=eu_dem_25 output=eu_dem_25_shaded_pca nproc=3 zmult=50

The leads to a colorized hillshading map, again with the OSM data on top (50% transparency):

eu_dem_25m_PCA_shaded_OSM_trento_rovereto_garda_lake

OpenStreetMap shaded with r.shaded.pca using EU DEM 25m (click to enlarge)

Yes, fun – I like it :-)

Data sources:

Vintage map design using QGIS

This post describes the three simple steps necessary to create a vintage-looking map using the blending feature in QGIS 2.0′s print composer. This is what we are aiming for:

alaska_oldpaper

1. Prepare the map

Like any other map, this one starts in the QGIS main window. Try to stick with earthy colors which will go well with the old paper look. For labels, try fonts which look like handwriting.

alaska_oldschool_overview

Once you are happy with your map

2. Prepare the composition background

To get that vintage feel, we need a background image with a great texture. You can find such textures on sites like lostandtaken.com. Download one you like and add it to an empty print composer. Make sure it covers the whole paper:

alaska_oldschool_bg

Lock the image by right-clicking it once – a small lock icon should appear in the upper left corner.

3. Finish the composition

The final step is to add the map on top of the background image. To make our nice background texture shine through, we enable the “multiply” blending mode in the map’s rendering options:

alaska_oldschool_print

Feel free to add north arrows or drawings of dragons as finishing touches.


Raster Data Extraction using QIS

Raster files consist of a grid of cells, each cell contains a numeric value, which is used to determine how to colour each cell.  This value may be based on the elevation of the cell, flood water depth, or soil quality. It is possible to extract this information by point sampling or using a terrain profile. Point sampling copies the cell’s value to the overlying point. A terrain profile tool plots a graph with the cell’s value (elevation) on the Y axis and the distance along the section on the X axis.

Point Sampling Tool

DEM’s are often used to then update the elevation values of overlying points, for example I have used data from DEM’s to update the elevation values of address points and utilities. This isn’t as accurate as surveying each point, but it is a lot quicker! This process is also referred to image extraction, raster/vector conversion.

For this tutorial, you will need:-

  • The Point Sampling tool in QGIS is an optional plugin. You can download it by using the menus to select Plugins, Fetch Python Plugins.
  • Nasa’s srtm data, which you can download from here: http://srtm.csi.cgiar.org/
  • Some point data. If you can’t think of any, then they’re easy to create, for example use the Open Layers plugin to load Open Streetmap or Google Maps of your area, and then create points over a few cities.

I’m going to add the elevation value from the srtm rasters to a selection of UK towns and cities:-

Raster Data Extraction - UK srtm

  1. Use the menus to select Plugins, Analyses, Point Sampling Tool
  2. The Point Sampling Tool dialogue box opens. Select:-
  • The layer that contains the points to be sampled
  • The layer(s) with the field(s)/band(s) to get values from
  • The output (results) file
  • Press OK

Raster Data Extraction - Point sampling tool

The results file just contains the elevations:-

Raster Data Extraction - Elevations

It is possible to add these to the original layer:-

  • Create a buffer around the new points
  • Use the menus to select Vector, Data Management Tools, Join Attributes By location
  • Select the original points as the target and the buffer as the join layer

Another option is to update the x and y co-ordinates for both points using the Field calculator and then to match the rows in Excel on the co-ordinate column.


Data-defined properties in QGIS 2.0

In QGIS 2.0, the old “size scale” field has been replaced by data-defined properties which enable us to control many more properties than jut size and rotation. One of the often requested features – for example – is the possibility for data-defined colors:

datadefinedproperties

Today’s example map visualizes a dataset of known meteorite landings published on http://visualizing.org/datasets/meteorite-landings. I didn’t clean the data, so there is quite a bunch of meteorites at 0/0.

To create the map, I used QGIS 2.0 feature blending mode “multiply” as well as data-defined size based on meteorite mass:

meteorites1

Background oceans and graticule by NaturalEarthData.


Raster Based Terrain Analysis Techniques pt2

Continuing from last week’s post, I will show you how to use terrain analysis to calculate:-

  • slope,
  • aspect
  • hillshade
  • ruggedness index

Slope

Slope is calculated by comparing the pixel value at a particular location relative to the surrounding 8 pixel values. This gives the steepness of the slope.

The Slope dialogue box is very simple:-

Calculate slope dialogue box

Calculate slope dialogue box

  • Select the elevation layer (this will be the DTM raster)
  • Select the output layer
  • I have left the Ouput format and Z factor as default. If the ground is very flat, then exaggerating the z factor might make the slopes easier to visualise.

Aspect

The aspect shows the compass bearing of the slope

The raster has been given values from 0-360 depending on the slope aspect. The darker areas with the lower values are the north to north east facing slopes; the lightest areas with the highest values are the west to north west facing slopes.

Aspect shading

Aspect shading

Hillshade

This calculates the amount of sun or shade for a 3D surface. The dialogue box is similar to the previous ones, however there are new options for the sun angle:-

DEM hillshade dialogue box

DEM hillshade dialogue box

This analysis uses a fixed location of the sun to accurately simulate the effects of bare hillside and shaded valleys. I positioned the sun to the south west (200 degrees), the east facing slopes around the River Medina estuary in the north of the island are very shaded, in contrast to the brightly lit west facing slopes on the other side of the river.

A DEM with hillshading

A DEM with hillshading

This is the most visually appealing and easily understood result and so it is often used as a backdrop for maps with other layers added.

Ruggedness Index

The ruggedness index value is calculated for every location, by summarizing the change in elevation within the 3×3 pixel grid.  Ruggedness index values are grouped into categories to describe the different types of terrain.  The classifications are as follows:

Ruggedness Classification

Ruggedness Index Value

Level 0 – 80m
Nearly Level 81 – 116m
Slightly Rugged 117 – 161m
Intermediately Rugged 162 – 239m
Moderately Rugged 240 – 497m
Highly Rugged 498 – 958m
Extremely Rugged 959 – 4397m

The dialogue box for the ruggedness Index is the same as it is for the other types of analysis mentioned above. The IOW is all categorized as level or nearly level in the ruggedness index. This is despite it being quite hilly! I used the Stretch to MinMx contrast enhancement on the layer properties box:-

A DEM with ruggedness index displayed

A DEM with ruggedness index displayed

The result is quite different to the relief and hill shade raster’s. This is because, it doesn’t attempt to show actual slopes, rather it shows the change in elevation categorised as shown in the ruggedness index table. It is still easy to see the line of hills that cross east to west across the island.


Raster Based Terrain Analysis Techniques pt1

In the previous tutorial, I showed you how to create a raster terrain model. This is useful by itself for visualising the relief of an area. However, it can be even more useful when used as the basis of further analysis.

Over the next few tutorials, I will show you how to carry out the following types of analysis:-

  • Slope
  • Aspect
  • Hillshade
  • Ruggedness Index

I am going to use srtm data for the UK, you can download the file for your area from here: http://srtm.csi.cgiar.org/

Displaying the raster, resolving display problems

  • Add the image to the project using the Add Raster Layer button. At first the image opens completely grey, to stretch the black to white gradient to fit between the minimum and maximum values found in the image:-
  • Press the Stretch Histogram to Full Data Set on the Raster toolbar
  • Alternatively, right click on the layer in the Layer Panel, and
    • Select Properties.
    • Select the Style tab.

    At the bottom, change the Contrast Enhancement pull down to Stretch to Min Max.

  • If a grid displays as a continuous grey box, check the Transparency for null cells setting
    • Open the Layer Properties
    • Select the Transparency tab

      QGIS Layer Properties

      QGIS Layer Properties

  • Check that the correct band is selected in the Transparency Band pull down
  • Check the No data value and Percent Transparent entries in the Transparent Pixel list

The DEM Models plugin should appear on the Raster Menu. If it isn’t installed, it can be downloaded by using the menu to select Plugins, fetch Python plugins.

Its operation is similar for all the types of analysis that can be undertaken

  1. Select the input raster layer
  2. Select the output raster layer that will contain the results
  3. Use the pull down to select the analysis:
  • Hillshade
  • Slope
  • Aspect
  • Color Relief
  • Terrain Ruggedness Index
  • Topographic Position Index
  • Roughness
DEM Terrain Module

DEM Terrain Module


Image Analysis Using QGIS

Introduction

Rasters are created from gridded data. Each pixel is coloured according to an interpolated value, e.g. triangulation (TIN), nearest neighbour analysis.

A raster file is comprised of a pixels arranged in a grid formation. Each pixel contains a colour value that instructs the computer as to what colour to use when displaying it. Raster images tend to be used for grids as they are a more efficient method of showing large areas of coloured pixels than vector maps.

The following illustrates how a raster grid represents terrain, and how the information might be extracted:-

For simplicity’s sake, imagine that we’re back in the days of 256 colours with 1 being white and 255 being black. I tend to display relief with the highest ground as white or red, then to show lower ground as green or blue.

Let’s take a cross section through a hill:

A grid raster image of the terrain would appear similar to below (please note that I have drawn this in Inkscape using a gradient fill to keep the demonstration simple!):-

The numeric values of the raster grid that the computer would see would be similar to this:

5 5 5 5 5
5 100 150 100 5
5 100 250 100 5
5 100 150 100 5
5 5 5 5 5

Note the values are not the actual elevation, just the colour values of the pixels. The elevation that each pixel value corresponds to (the legend) is contained in the accompanying shape file along with image registration (the x, y coordinates).

By analysing the grid and determining the relationship between pixel values and the elevation that they represent the GIS software can accurately model the terrain. Once the terrain has been modelled, it is possible to undertake further analysis such as slope calculation, predicting hill shade or water runoff.

The Image toolbar

Firstly, let’s have a look at QGIS’ image tool bar:-

QGIS image bar

QGIS image bar

This can be added by right clicking on any toolbar and selecting Raster from the short cut menu. The buttons are from left to right:-

  • Stretch Histogram to Full Data Set
  • Local Histogram Stretch
  • Geo-referencer
  • Interpolation
  • Zonal Statistics

ColorBrewer NoFlash-Version

Did you know that there is a version of ColorBrewer that does not require Flash?

Enjoy! Pure Javascript and zero loading times: http://colorbrewer2.org/js/

colorbrewerjs


Queries in QGIS pt2 – Spatial Query

In the previous post (Queries in QGIS pt 1 – Attribute Queries), I showed how to select features based on their attribute data, e.g. select shops which have the address entered as London. Now let’s imagine there isn’t an address attribute, or alternatively, we want to select features within flood plains for example, or identify areas of woodland that have high voltage power lines running through them.

Layers for spatial query

Spatial selection selects features in one layer based on their spatial relationship to features in another layer

To begin a spatial query either:

  • Use the menu to select Vector, Spatial Query
  • Press the Spatial Query button

The spatial query dialogue box:

QGIS Spatial Query Dialogue Box

Select Source Features From – this is the table that the selection will be from.

Where the feature – the options will change depending on the exact combination of point, line or region features that are being used for the selection. They include contains (e.g. a region layer may contain points), crosses (e.g. a line layer may cross a region layer), Is Within (e.g. a point layer may be within a region layer), Touches (where one object may touch another but not actually be within it).

Reference Features Of – this is the second layer. It will be used to select features from the first layer, but its features won’t be part of the selection

The selected geometries only will only use features that have already been selected. For example if I wanted to select all the woodland within a particular county I could set up the query to read

  • Select Source Features from Farmland
  • Where the Feature is Within
  • Reference features of County Boundaries (selected features only)

Click OK, the query will run. Once it has completed, the results will appear:-

QGIS Spatial Query Dialogue Box

It is possible to run a further query based on the selection

Press the Create Layer from Selected to add the selection to the map as a new layer:-

Spatial Query Results

Combined Spatial and Attribute Query

Quite often GIS is used to select features that contain a certain attribute within a certain area (e.g. all the A roads and motorways within Greater London).

These queries are carried out by combining the above Spatial and attribute selections. They can be carried out either order depending on which is most logical.

In this example I would overlay the roads and OS Boundary Line features. I would then select all the London Boroughs by clicking on them with the Select Single Feature tool

  • Select Source Features from Roads
  • Where the Feature is Within
  • Reference features of OS Boundaryline (selected features only)
  • Press OK
  • In the Results box, press the Create Layer from Selected to create a new layer
  • Right click on the newly added Selection from Roads layer in the Layer Panel
  • Select Query
  • Use the Query Builder to enter the following SQL “Classification” = ‘A Road’ OR “Classification” = ‘Motorway’
  • Press OK

Geo-Processing in QGIS

I’m going to look at the geo-processing tools. The geo-processing tools are found on the Vector menu under Geo-processing tools. These tools do not edit the input tables; instead you are prompted to create a new layer for the results. Therefore the input layers don’t need to be editable. You can choose to carry out the operation on every feature in the chosen layers, or just the selected features. These functions can be combined with attribute updates and calculations to carry out more complex analysis (e.g. calculate proportional overlap) or to count the address points within set distances of proposed new roads.

I will carry out most of the operations on the green square and red circle shown below:-

QGIS map window

QGIS map window


Intersect creates a new feature based on the area of overlap (the intersection) between the two layers. The attributes from both source layers are copied to the new feature:-

Intersect in QGIS

Intersect in QGIS

To calculate the area of overlap, update the newly created feature’s attribute table with its area.

Union creates a new layer that covers the combined features

Union in QGIS

Symmetrical Difference creates new shapes based on the non overlapping areas of the original features:-

Symmetrical Difference in QGIS

Clip creates a new shape based on the area of the input layer that is overlapped by the clipping layer. It is similar to the intersection but differs in that the attributes of the chosen layer only are copied to the new feature. It is similar to MapInfo’s Erase Outside function.

Clip in QGIS

Difference creates a new feature based on the area of the input layer that isn’t overlapped by the clipping layer. It is similar to MapInfo’s Erase function.

Difference in QGIS

Dissolve breaks apart overlapping regions in the same layer.

Buffer creates a region around each feature in the source layer. I have used buffers to count address points within set distances of new roads, assign address points to local amenity catchment zones etc. In this case I’m going to apply a 100m buffer around overhead electricity lines. These can be downloaded from OS Open Data.:-

Buffer in QGIS

  • Input vector Layer – the layer that contains the source objects
  • Buffer Distance – the distance the buffer will extend from the source objects
  • Buffer Distance Field – alternatively QGIS can use a value from a numeric field, this makes drawing variable width buffers for features in the same layer easy e.g. Sites rated High Sensitivity could be updated with a buffer distance of 1,000m, sites rated Medium Sensitivity could be updated with a buffer distance of 500m.
  • Dissolve Buffer Results. The default is to combine the buffers into one region. Enabling this creates a separate region for each source object.

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