Thanks to the SRTM project we can access to a comprehensive world coverage of land elevation data. It has become very simple to create one’s own topographic maps with QGis .
We will discuss here :
– how to download elevation data for the area of interest
– how to format the display style to fit the standards of the topographic maps
– how to enhance the viewing using a “shady” layer type
– and finally how to generate contours
To download the elevation data of your area of interest , go on page http://viewfinderpanoramas.org/dem3.html
This video shows you the progress of the download .
The data available corresponds to a definition of 3 ” arc , about 75m. Every pixel of your DTM will display to this definition. In order for the DTM and the corresponding map to be correct, you must work with an area of about a fifteen kilometers wide. For smaller areas, pixelation becomes visible and the quality of the topographic map fades.
The downloaded tiles depict a size of 1 degree by 1 degree and are in a hgt format.
This format is immediately readable by QGis . You can, therefore, load the tiles using adding raster in the Cartographic window.
How toFormat the display style
By default, the raster is displayed in shades of grey. Therefore it is necessary to apply some colouring adapted to accentuate the topography. Although it is possible to create your own color ramp or to use a color choice provided by QGIS, you can download by using the plugin Color Ramp Manager .
To install it, go in the menu Extensions -> Install / Manage Extensions, select the plugin Color Ramp Manager and click Install the extension . You will a new tool added to your tool bar :
Click the tool to open the configuration window . Indeed, all the colour ramps are not installed automatically . The one that interests us specifically, cpt-city must be installed manually.
- Check the Full box -City cpt package
- Choose the directory to install the package. Warning! by default this is the first option checked , but due to the fact that the Program Files directory is writing- protected , it will not work . You’d rather choose QGis customization directory.
- Click on Check to start the update of the installation package.
Once the package is downloaded, exit the dialog.
Follow the video instructions to apply a colours ramp to your raster layer
Add a Shady type layer “( Hillshade)”
To create this layer just go to the Raster Menu-> Field Analysis -> Shading
Give a name to this file to archive this new layer .
However, it is essential to modify the Z factor. Indeed, the order assumes that you are working with a planar projection, i.e. the XY coordinate of your raster layer are in meters. Since the layer is in decimals degrees (latitude / longitude) you have to enter an appropriate conversion factor? The value to enter is 11120.
Click OK to retrieve your new shading layer.
Now you only have to change the style of this layer to have a topographical image.
Open the properties of the shading layer
- Change the Min value to 125 and the Max value to 255
- Change the Merge Mode to Multiply
- Change the Brightness to 45
- Change the Contrast to 20
The final result is as follows:
Create the contours
If you want to add contours, just go to the Raster menu -> Extraction -> Creating contours:
- Give a location and a name to the new contours file
- Re-enter the interval, in meters, between the different contours
- Click Attribute Name if you want to have the elevation as an attribute for the field contours (if not, you will not be able to label the contours)
Once the order executed is , you will have the contours loaded :