Fundamental Raster GIS procedures

Contour lines

Contour lines show the lines where the elevation of the terrain is exactly the same and has uses in many applications for mining, construction, city planning. Contour lines provide quick interpretations on how level a terrain is and an indication of the slope at different levels of elevation, so it helps to estimate volume and steepness. It is one of the simplest 2D mechanisms to do this and is a reasonably effective visualization method.

Quick Guide

In QGIS, load a DEM raster file, then "Raster | Extraction | Contour". Specify your settings and click OK. This generates an ESRI shapefile, which is a vector layer. The vector layer can be rasterized or saved (and reprojected?).

Extensive tutorials

http://www.qgistutorials.com/en/docs/working_with_terrain.html

http://www.gistutor.com/quantum-gis/20-intermediate-quantum-gis-tutorials/48-quantum-gis-qgis-raster-based-terrain-analysis-techniques.html

Cool visualization ideas

1. Export the contours layers as a KML file for visualization in Google Earth and superimpose them on 30m SRTM data (default elevation data in Google Earth).
2. Use the contour lines and layer this over a map.
3. Add the contour lines and then use "hillshading" effects to deepen the effects of the slopes and ruggedness of the terrain.

Hillshade

The hillshade is a raster that shows the lit and shadowed parts of a piece of terrain by virtually projecting sunlight from a specified aspect. This creates a chart that theoretically makes it easier to interpret the terrain.

Quick Guide In QGIS, open your DEM image. Then Select "Raster | Analysis | DEM". Provide your settings for the Azimuth and altitude of the light, set the destination image, then Run the analysis.

Important Note When you have a DEM with lat/long coordinates in degrees and decimals, but elevation in meters, you need to apply a scale to factor of horizontal vs. vertical units (111120 around the equator). If you exported and projected to UTM, then your horizontal and vertical units are the same, so you don't need to apply the scaling. (This is why exporting to UTM saves you pain, remember?)

Extensive tutorials

http://docs.qgis.org/2.2/en/docs/training_manual/rasters/terrain_analysis.html

https://vimeo.com/23458312

Cool visualization ideas

1. Use the hillshader to visually enrich maps and ortho's. If you ever played with layers in Photoshop, you know that there are ways to have two layers interact by setting the "normal", "screen", "lighten" and "darken" settings. You can do these operations in QGIS too, which you may want to do if any of the layers in use has a different projection and therefore wouldn't come out right in photoshop directly. This is an example of a hillshader applied on a regular orthophoto with a multiply operation selected and 70% transparency applied:

Slope map

The slope map provides important data with regards to the accessibility of land and suitability for construction or farming. The contour map already uncovers some of this information by looking at places where contour lines get close together or far away, but the slope maps provides a fuller picture how the terrain varies between such lines.

Quick Guide In QGIS, open the DEM. Then go to "Raster | Analysis | DEM" (same as for hillshading). Now instead, select the "slope" function. Use 111120 for the scale if you work with lat/lon data and not meters.

Extensive tutorials

http://docs.qgis.org/2.2/en/docs/training_manual/rasters/terrain_analysis.html

http://www.gsd.harvard.edu/gis/manual/raster/

Aspect map

The aspect map provides data with regards to the azimuth of the land; 360=north, 90=east, 180=south, 270=west. The grey band doesn't really visualize this well because of the way how pixels get represented on screen and the fact that pixel values are angles, not intensities. You should probably classify the directions to improve the visualization if that's required, but the map is probably more useful as data.

Aspect (and slope maps) are useful when installing solar panels (which should usually be perpendicular to the rays of the sun to optimize yield), in architecture to determine the angles of the sun at times of day in the room, etc.

Quick Guide In QGIS, open the DEM. Then go to "Raster | Analysis | DEM" (same as for hillshading). Now instead, select the "aspect" function.

Extensive tutorials

http://docs.qgis.org/2.2/en/docs/training_manual/rasters/terrain_analysis.html

https://www.youtube.com/watch?feature=player_detailpage&v=McnSdEpa9QU#t=383

Cool visualization ideas

1. Classify the azimuth angles to visualize the direction of hillsides.
2. Combine the slope map with the aspect map. Hue determines the aspect, the saturation determines the slope angle). See: http://blogs.esri.com/esri/arcgis/2008/05/23/aspect-slope-map/

Putting the fundamentals together

Here's an image after a bit of work in GIS. The DEM itself was 'classified' by dividing the entire dataset into five ranges of data. Now it's easy to see the bottom floor where work takes place, all the way to the tree tops in red.

The image however is very blotchy and it doesn't show anything about the terrain details. Adding a hillshade to this with 40% transparency and multiplying it over the classified dem, we see this:

A lot better. But we can exaggerate the boundaries a bit more by punching them out with the "slope map" in an effort to provide more information on the level of the terrain:

Remember: we started with a grey, georeferenced image containing only elevation data. The result is that we now have a colored image which is much clearer in depth perception and what the lay of the land is. What would you rather give your customer? The DEM we derived from your post-processing tools or this much nicer image which took 5 minutes to generate? The image has been scaled down for this wiki, but you can generate it in the same resolution as the source data.