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Tue Apr 22 06:41:17 2014

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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.



Colour properties for WMS and WMTS layers

An advantage of using map services like WMS and WMTS is that your styling has been done by someone else. But this can turn into a disadvantage as well in case the colours of your overlay are similar to the colours in your remote base layer. To show this I created an example using the […]


How to do some quasi 3d cartographic effects in QGIS

No words for this one, just pictures! Above: What we are trying to achieve… Above: Our symbol layers Above: The roof colour is data defined A shadow Layer And a highlight layer One other hint – don’t forget to enable symbol levels!


QGIS Flickr Group

The excitement about the upcoming 2.0 release is growing and to add some fuel to the fires, Mathieu founded the QGIS Flickr Group. Anyone can join and add their maps done with QGIS master.

I’m looking forward to seeing what you have come up with. Please note that this group is meant for maps only (therefore no screenshots of the application please).



België en Duitsland in RD met QGIS

De Basisregistratie Topografie (BRT/Top10NL) houdt ongeveer een kilometer na de landsgrens al op. Velen, bijvoorbeeld brandweermensen, hebben behoefte aan meer kaartbeeld over de landsgrens. Die kaartdata is er, en nog kosteloos ook. De Open Street Map (OSM) van de Europese landen is als Shapefiles te downloaden, o.a. via de Duitse GeoFabrik, http://download.geofabrik.de/europe.html. Ze wordt aangeboden in […]


Sextante Modeler Evolution 1.0.8 to 1.1

Sextante is quickly becoming the goto geoprocessing toolbox for me. I’ve been working with Sextante 1.0.8 on QGIS 1.8 and lately I’ve started looking into Sextante 1.1 for QGIS 2. This post highlights some of the main differences between the two versions. I’m sure there are many more hidden gems I have not discovered so far.

One thing you will notice if you have used previous versions of Sextante is that the new version comes with a simplified interface which groups tools into three categories: geoalgorithms, models, and scripts. If you prefer the old style grouping by algorithm source such as GDAL, GRASS, etc. you can switch to the Advanced interface.

Let’s start with the bad news: Models created in 1.0.8 are not compatible with 1.1 since many of the algorithms have been rearranged in new categories and Sextante cannot find them by their old names anymore, e.g.

1.0.8 … ALGORITHM:ftools:fixeddistancebuffer
1.1 … ALGORITHM:qgis:fixeddistancebuffer

The great news is that the modeler has been improved greatly. Model representations now show the flow of input and output data through the model steps much more clearly:

Sextante 1.0.8 Modeler

Sextante 1.0.8 modeler

Sextante 1.1 Modeler

Sextante 1.1 modeler

I also found the new modeler much more stable – no crashes so far. *fingerscrossed*

Another nice new feature is Sextante commander which can be started using the shortcut Ctrl+Alt+M. It’s a quick launch solution for all Sextante algorithms:

sextante_commander

At FOSS4G, I’ll be presenting some work I did evaluation OSM using Sextante 1.0.8. I’d love to hear how you are using Sextante.



Print Composer 2.0 – Take #7

Today’s post: More print composer overview magic!

Inverted Map Overviews

Thanks to the “Invert overview” option, we can now chose between highlighting the detail area (left example in the image) or blocking out the surrounding area (right example).

printcomposer_overviews

The “Lock layers for map item” option can come in very handy if you want to reduce the number of layers in the overview map while still keeping all layers of interest in the main map.



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



QGIS and WFS service caching

The dutch PDOK-services plugin is a simple plugin to load some national (PDOK) dataservices. One of the available services is a Web Features Service (WFS), while a very handy service, calling the WFS service to load all roads in the Netherlands shows you the following result: As you can see the blue bar are actually […]


Building QGIS on Ubuntu 13.04

It seems upgrading to Ubuntu 13.04 breaks a few things in the QGIS build process. Here’s some quick workarounds I’ve found to get it building again.

Firstly, Ubuntu defaults to Qt 5, causing the error:

CMake Error at /usr/share/cmake-2.8/Modules/FindQt4.cmake:1216 (message):

 Found unsuitable Qt version "5.0.1" from /usr/bin/qmake, this code requires
 Qt 4.x

This can be fixed by switching the system default back to Qt 4, so that  qmake refers to the Qt 4 version. The “qt4-default” package handles this switch for you, so just run:

sudo apt-get install qt4-default

Update: A better solution was found by Pierre and Pvanb and is described here. Basically it involves changing the QMAKE_EXECUTABLE option from /usr/bin/qmake to /usr/bin/qmake-qt4.

Next, I was getting the error:

make[2]: *** No rule to make target `/usr/lib/libpython2.7.so', needed by `output/lib/libqgispython.so.1.9.0'. Stop.

I’m not sure if this is the correct solution, but setting up some links and re-running ldconfig gets around this:

sudo ln -sf /usr/lib/x86_64-linux-gnu/libpython2.7.so.1 /usr/lib/libpython2.7.so.1
sudo ln -sf /usr/lib/x86_64-linux-gnu/libpython2.7.so /usr/lib/libpython2.7.so
sudo ldconfig

Update: a better solution is to change the PYTHON_LIBRARY option from /usr/lib/libpython2.7.so to /usr/lib/x86_64-linux-gnu/libpython2.7.so

These two changes were enough to get QGIS building again. If you’ve got a better solution for these errors, let me know…


A neat trick in QGIS 2.0 – images in atlas prints

Here’s a cool trick which you can do in QGIS 2.0. It builds on two new features introduced in version 2.0 – atlas prints and html labels.

Atlas prints (previously available as a plugin, now integrated into QGIS core) were developed by Oslandia, and allow you to create data driven map layouts from a specified coverage layer. You can read more about them here.

Another new feature in QGIS 2.0 is the ability to render composer labels as html (courtesy of Olivier Dalang). This allows all kinds of fantastic effects, such as formatting text in the middle of a label (using <b>, <i>, and <font> tags) or creating labels which contain HTML tables. You can even use CSS stylesheets and rules to format the HTML! I’ve been told JavaScript also works inside the labels, but I’ve yet to try this out.

You can combine these two new features for some great tricks. Let’s say we’d like to create a set of maps of local councils, and we want each map to have a watermarked logo of the council on it. For this example I’ve created a basic basemap of Victorian councils, and I’ve downloaded all the council logos (in a variety of formats) to a local folder. Next, I’ll add an extra column to the council layer containing the name of the logo image:

adding_logo_column

Adding a logo column to the table

Then, we’ll throw together a simple composition containing the map and set it up as an atlas print:

setting_up_atlas

Generating an atlas

Now for the fun bit. I’ll add a label item to the composer, set it to “Render as HTML”, and insert some specially-crafted html:

The magic HTML label...

The magic HTML label…

For copy/paste purposes here’s the label contents again:

<style>
* {margin: 0px; padding: 0px}
</style>
<img src="file:///home/nyall/GIS/council_logos/[% "logo" %]"
style="width: auto; height: 100%; display: block; margin: 0px 0px 0px auto;" />

There’s a couple of things to note here. First, the magic happens in the image’s src attribute (“file:///home/nyall/GIS/council_logos/[% "logo" %]“). When the composer is exported, QGIS will replace the [% "logo" %] part with the contents of the logo field for each row in the councils table. This means the image source will then point to the local copy of the council’s logo, eg “file:///home/nyall/GIS/council_logos/glen_eira.jpg” for the first row.

Secondly, I’ve styled the image with the css:

width: auto; height: 100%; display: block;

This allows the image to resize to 100% of the height of the label while maintaining its correct aspect ratio. I’ve also added the rule

margin: 0px 0px 0px auto;

to force the image to right align within the html label. This ensures that all the watermarked logos will appear in a consistent size and position for each map.

Lastly, I’ll remove the label’s frame and background by unchecking these options, then set its transparency to 80% under the new “Rendering” section:

Yet another new feature in QGIS 2.0...

Yet another new feature in QGIS 2.0…

Ok, we’re all done. Now, when I select Composer -> “Export as Image”, we’ll get a lovely set of council maps complete with watermarked council logo!

A watermarked atlas!

A watermarked atlas!

There we go — all ready for print, with no manipulation in external programs required at all!

Bonus post-credits section

Here’s a kicker — the linked images don’t need to be local. That means you can even piggy-back off an existing web service to generate an image on the fly! Let’s say you were asked to add QR codes to your maps to link directly to the council website. All it takes is adding a new column to the table, then modifying the image src to read:

src="http://qrfree.kaywa.com/?l=1&s=8&d=[% "url" %]"

Now when we export the maps we’ll also get a QR image generated on the fly and inserted into the layout!

Complete with dynamically generated QR code!

Complete with dynamically generated QR code!

Combining HTML labels and atlas prints in this way is extremely powerful. This example is just touching the tip of the iceberg – I’m keen to see what the community can do with this when QGIS 2.0 is released!


Advanced Python Field Calculator

Advanced Python field calculator is one of the numerous tools in Sextante for QGIS. It’s extremely powerful but it doesn’t use the syntax of QGIS’ default field calculator (the one you can access via attribute table). Therefore, here comes a short introduction:

If you want to reproduce this example, I used a dataset of town areas from the new open government data site of Lower Austria.

The upper half of the Advanced Python field calculator is rather self-explanatory but the lower half is where it gets interesting: Code in the global expression section will be executed only once before the calculator starts iterating through all the features of the input layer. Therefore, this is the correct place to import necessary modules or to calculate variables that will be used in subsequent calculations. Code in the formula section will be evaluated for each feature in the input layer. As shown in the following example, this is where we can calculate new values, e.g. the area of the polygons in km²:

sextante_pythonfieldcalc_area

As you can see, the feature geometry can be accessed using $geom.

If you want to access an existing attribute, that’s possible using <attribute_name>.

Anyway, this is the resulting layer’s attribute table including the new areaKM2 field:

sextante_pythonfieldcalc_area_results

Thanks to Victor for pointing me to the documentation of FieldPyculator which Advanced Python field calculator is based on.



New forms related features coming in QGIS 2.0

View here on youtube: https://www.youtube.com/watch?v=rnqRAOvuJEU


QGIS 2 Annotations

View here on youtube: https://www.youtube.com/watch?v=0pDBuSbQ02o


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


Dynamically updating layers in QGIS with PostGIS views

View here on youtube: https://www.youtube.com/watch?v=oOhbbEkl4Kg

Dynamically updating layers in QGIS with PostGIS views

View here on youtube: https://www.youtube.com/watch?v=oOhbbEkl4Kg


Coming soon in QGIS Part 2 – Color control for raster layers

Continuing on from part 1, another feature I’ve recently pushed to QGIS is the ability to control the hue, saturation and colour of a raster layer. This builds off the excellent work done by Alexander Bruy (who added brightness and contrast controls for raster layers), and it’s another step in my ongoing quest to cut down the amount of map design tweaking required outside of QGIS. Let’s step through these new features and see what will be available when version 2.0 is released in June…

First up is the ability to tweak the saturation of a layer. Saturation basically refers to the intensity of a colour, with low saturation resulting in a more washed out, greyer image, and high saturation resulting in more vibrant and colourful images. Here’s a WMS layer showing an aerial view of Victoria at its driest, least appealing and most bushfire ready state:

Original layer

Raster layer before saturation controls…

Let’s tweak the saturation a bit to see if we can make it more appealing. In the Style tab under raster layer properties, you’ll see a new “Saturation and hue” section. For this layer I’ll bump the saturation up from its default value of zero:

Saturation settings

Saturation settings

Which results in something like this:

Resultant layer!

… and after increasing the saturation!

Ah, much better. This actually looks like somewhere I’d like to live. A bit over-the-top perhaps, but it IS handy to make quick adjustments to raster colours in this way without the need for any external programs.

How about turning an image grayscale? I regularly have to do this with street directory basemaps, and until now couldn’t find a satisfactory way of doing this in QGIS. Previously I’ve tried using various command line utilities, but never found one which could turn an image grayscale without losing embedded georeferencing tags. (I did manage to achieve it once in QGIS using a convoluted approach involving the raster calculator and some other steps I’ve thankfully forgotten.)

But now, you can forget about all that frustration and quickly turn a raster grayscale by using a control right inside the layer properties! You even get a choice of desaturation methods, including lightness, luminosity or average. Best part about this is you can then right click on the layer to save the altered version out to a full-resolution georeferenced image.

grayscale

Street map in grayscale… woohoo!

Lastly, there’s the colourise option. As expected, this behaves in a similar fashion to the colourise tools in GIMP and Photoshop. It allows you to tint a layer to a specified colour. Let’s take a WMS layer of Melbourne, tweak the brightness and contrast, and colourise it blue…

colorize_settings

Tweaking the colourize settings

… and the end result wouldn’t be out of place in Ingress or some mid 90′s conspiracy flick!

colorized

Colorized WMS layer

These changes are just a tiny, tiny part of what QGIS 2.0 has to offer. It’s looking to be a sensational release and I can’t wait for final version in June!


Print Composer 2.0 – Take #6

The latest version of Print composer features new

Rulers and guide lines or “alignments”

Rulers are a well-known feature in graphics programs such as Gimp and Photoshop. Now you can also find them in QGIS Print Composer. Click onto the ruler, hold the mouse key down and move the cursor to position guide lines for map feature alignment.

printcomposer_grid

Of course, there’s also the handy “Snap to grid” functionality.



Print Composer 2.0 – Take #5

This fifth part in my series on QGIS 2.0 Print Composer presents

Map grids

There are numerous different options for map grids in the new composer but a picture is worth a thousand words:

printcomposer_grids

The upper-left map features a zebra frame style and coordinate labels aligned horizontally and vertically.

The upper-right map shows a normal frame with labels written inside the frame instead of outside. This grid shows an additional offset.

The lower-left map has no frame but customized, colored and dashed grid lines.

Finally, the lower-right map shows a cross grid with default horizontal coordinate labels.


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