table of contents

Generate the Hillshade, Slope Shade, and Color Relief

About

Digital Elevation Models (DEM) are raster data about topography. DEM's do not include features such as trees and buildings and are therefore different from Digital Surface Models (DSM) which include those features. DEM's are rich sources of data can be used generate hillshades, slope, and relief maps.

Data

Process

Create a Hillshade

We will create a hillshade from the DEM called sf-dem.tif. To do this you can use the gdaldem hillshade command. The gdaldem hillshade command minimally takes the following arguments:

gdaldem hillshade -of PNG inputDEM.tif outputHillshade.png

where

Run the following in the terminal to generate a hillshade:

gdaldem hillshade -of PNG sf-dem.tif sf-dem-hillshade-default.png

Change the Hillshade Light Direction

You can adjust the direction of the light by adding an argument to the -az (Azimuth) flag. Changing the direction of light will change the shadows cast on your DEM.

Give it a try by running the following:

gdaldem hillshade -of PNG -az 135 sf-dem.tif sf-dem-hillshade-az135.png

Make a Colored Relief Map

Color relief or hypsometric tints depict elevation as bands of color, to enhance elevation zones so map readers can better see differences in relief. The colors selected for the tints are assumed to relate to the ground cover typically found at various elevations in the area being mapped. A typical color scheme progresses from dark greens for lower elevations up through yellows/browns, and on to grays and white at the highest elevations - Bjørn Sandvik.

In order to create color our DEM, we need to create colored breaks for a selected number of elevation bins. First, let's check what the min and max values are using the gdalinfo command with the -stats flag:

gdalinfo sf-dem.tif -stats

We should get something like this:

Josephs-MacBook-Pro-2:XX_digital_elevation_models Jozo$ gdalinfo sf-dem.tif -stats
Driver: GTiff/GeoTIFF
Files: sf-dem.tif
Size is 676, 675
Coordinate System is:
GEOGCS["NAD83",
    DATUM["North_American_Datum_1983",
        SPHEROID["GRS 1980",6378137,298.2572221010002,
            AUTHORITY["EPSG","7019"]],
        TOWGS84[0,0,0,0,0,0,0],
        AUTHORITY["EPSG","6269"]],
    PRIMEM["Greenwich",0],
    UNIT["degree",0.0174532925199433],
    AUTHORITY["EPSG","4269"]]
Origin = (-122.437530223697820,37.812525827311823)
Pixel Size = (0.000092539239086,-0.000092652993030)
Metadata:
  AREA_OR_POINT=Area
Image Structure Metadata:
  INTERLEAVE=BAND
Corner Coordinates:
Upper Left  (-122.4375302,  37.8125258) (122d26'15.11"W, 37d48'45.09"N)
Lower Left  (-122.4375302,  37.7499851) (122d26'15.11"W, 37d44'59.95"N)
Upper Right (-122.3749737,  37.8125258) (122d22'29.91"W, 37d48'45.09"N)
Lower Right (-122.3749737,  37.7499851) (122d22'29.91"W, 37d44'59.95"N)
Center      (-122.4062520,  37.7812554) (122d24'22.51"W, 37d46'52.52"N)
Band 1 Block=676x3 Type=Float32, ColorInterp=Gray
  Minimum=-2.314, Maximum=126.607, Mean=19.995, StdDev=24.762
  NoData Value=-9999
  Metadata:
    STATISTICS_MAXIMUM=126.60684204102
    STATISTICS_MEAN=19.995340656419
    STATISTICS_MINIMUM=-2.3140776157379
    STATISTICS_STDDEV=24.762441881228

We can see that: Min=-2.4 and Max=126.7. Using this information we can make some elevation breaks and some corresponding colors for each break. The example below shows how gdal will use apply a color to the pixels with the corresponding elevation.

elev1 r g b
elev2 r g b
elev3 r g b
elev4 r g b
...

Make a color-relief.txt using the echo command, like so. In this case, echo takes everything between the quotations and stuffs it into a file called color-relief.txt usng the > operator:

echo '0 0 0 0
26 110 220 110
51 240 250 160
76 230 220 170
101 220 220 220
127 250 250 250' > color-relief.txt

Now we can generate our colored relief:

gdaldem color-relief -of PNG sf-dem.tif color-relief.txt sf-dem-coloredRelief.tif

Make a Slope Map

First create the slope terrain

We can also use our DEM to create a map of slopes on the terrain. We use the gdaldem slope command to achieve this. There are a variety of formats we can export to, but we can't export .png, so in this case we use GTiff (.tif):

gdaldem slope sf-dem.tif sf-dem-slope.tif

Second make Color Ramp and Create a Colored Relief Map

Using the same method as above, create a color palette for the slope degrees in a file called color-slope.txt:

echo '0 255 255 255
90 0 0 0' > color-slope.txt

Now we can use the gdal color-relief command to take our color-slope.txt colors and apply them to the slope map generated above:

gdaldem color-relief sf-dem-coloredRelief.tif color-slope.txt sf-dem-slopeShade.tif

And voila! We've now made a hillshade, a colored-relief map, and a slope map!

Last, let's make a contour map:

We can now use the gdal_contour command to generate the contour lines from the DEM. The gdal_contour command takes a number of arguments, but a minimum execution would include:

gdal_contour -a heightFieldName DEMfile.img outputFileName.shp -i contourInterval

where:

gdal_contour -a height sf-dem.tif output_contour_10m.shp -i 10.0

If you now list the files in the folder, you will see the "output_contour_25m.shp" file.

And now you have beautiful contour lines! 🗻

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