Options for Visualization of Heights and Surface Morphology
An often needed mapping application is the visualization of heights and the relief of a site. Among the many topographic elements, hydrological processes are primarily guided by
– thalwegs and channel networks,
– breaklines,
– slope areas,
– dams, dykes, embankments and
– depressions.
High resolution digital elevation models, which are provided by geoinformation authorities or generated by other organizations, are highly suitable for their visualization. Based on the original DEM, contour lines, hillshades and further datasets can be derived and used as supportive visualization techniques. For data processing I use SAGA GIS, for map creation I use QGIS. The figure at the end of this post depicts few possible variants of height and relief visualization. Parameter values in brackets show settings used for these maps.
Contour lines belong to the family of isolines. Identical height values are connected by one line. They can be used as overlay together with aerial images. Thereby a map viewer can spot land use and the relief simultaniously. It is common to add the height value at each contour line. In precision agriculture contour lines can be used as ploughing trajectories. This measure counteracts erosion induced by surface water runoff in sloped areas and promotes infiltration. In SAGA GIS one can create countour lines with the module Contour Lines from Grid:
– GRID SYSTEM
– GRID: Input DEM
– CONTOUR LINES: Output contour lines [create]
– MIN and MAX Contour Value: Defines the interval in which contour lines will be generated. In areas with negative height values the minimum values should be adjusted accordingly. [70-105]
– EQUIDISTANCE: The vertical distance value in which contour lines will be drawn (based on the definded MIN height value) [5]
For enhancing clarity in maps it might be useful to draw index contour lines (e. g. contour lines with 10 m equidistance) thicker than secondary contour lines.
Obviously height values inherit by the DEM itself contain infomation about the earth surface structure. More suitable for highlithing relief however is a hillshade model. The underlaying function is part of the standard equipment in every GIS. In the parameter settings one can choose the position of a fictional light source, which is directed towards the DEM and produces shadows due to its relief. Within SAGA GIS a hillshade can be created with the module Analytical Hillshading. For the standard shading method given settings can be used.
-GRID SYSTEM
-ELEVATION: Input DEM
-ANALYTICAL HILLSHADING: Output hillshade [create]
-SHADING METHOD: [Standard]
-SUN’S POSITION: [azimuth and height]
-AZIMUTH: Direction of which light source radiates [315 = north west]
-HEIGHT: angle above horizon [45]
-EXAGGERATION [4]
-UNIT: [degree]
An interesting shading method to look at is the Ray Tracing Method, which is capable of simulating realistic shadows at low light source heights or angles, respectively. Light conditions during dusk and dawn can be depicted with this parameter setting. Even higher information content can be achieved by combining the hillshade with a transparently coloured overlaying DEM.
The use of hillshade rasters is pretty common and a good choice in most cases. Especially for hydrological driven questions, the Visible Sky Value can be a better visualization option. The approach of hillshading delivers – depending on the relative position of a channel to the light source – a shadowed and an illuminated embankment side. The Visible Sky Value expresses the percentage of the sky, which is visible from a pixels position. This approach leads to a more distinct representation of channels, since morphologically similar embankments are visualized in a similar manner. In SAGA GIS one can use the Sky View Factor module for generating Visible Sky Values:
-GRID SYSTEM
-ELEVATION: Input DEM
-VISIBLE SKY: Output Visible Sky Value, Visible Sky percentage for each pixel [create]
-MAXIMUM SEARCH RADIUS [100]
-NUMBER OF SECTORS [8]
-METHOD [multi-scale]
-MULTI SCALE FACTOR [3]
Data Sources:
Aerial image: WMS DOP 10 @Geobasis NRW
Elevation Model: DEM1 tile 32314_5640, Data-Server Bezirksregierung Köln – published under the licence „dl-de/by-2-0″ (licencing text on https://www.govdata.de/dl-de/by-2-0)
Used Software:
SAGA GIS 6.4 (Conrad, O., Bechtel, B., Bock, M., Dietrich, H., Fischer, E., Gerlitz, L., Wehberg, J., Wichmann, V., and Boehner, J. (2015): System for Automated Geoscientific Analyses (SAGA) v. 2.1.4. Geosci. Model Dev., 8, 1991-2007, doi:10.5194/gmd-8-1991-2015. )
QGIS 2.18