DEM optimization

Functionality

The DEM optimization operation can be used to enhance a Digital Elevation Model (DEM), on which you wish to use the Flow direction operation later on. The DEM optimization operation will 'burn' existing drainage features into your Digital Elevation Model (DEM); a subsequent Flow direction operation will thus better follow the existing drainage pattern.

The DEM optimization operation offers the following possibilities:

Gradual drop of (drainage) segments in the output DEM, over a certain distance to the (drainage) segments.
Gradual raise of (watershed-divide) segments on the output DEM, over a certain distance to the (watershed-divide) segments.
Additional sharp drop or raise of segments on top of the gradual drop or raise.
Simple drop or raise of polygons in the output DEM.

The result of using the DEM optimization operation is a 'corrected' DEM in which existing drainage features are more pronounced.

Tip: Before using the DEM optimization operation, you can use the Fill sinks operation to remove local depressions from your DEM.

Explanation of terms:

Figure 1 and Figure 2 below show cross sections through the terrain.

The dotted line shows the original height value(s) in the input DEM, the blue line shows the position of a drainage.
The Buffer distance is shown in pink, the influence of Smooth drop in greenish and the influence of Sharp drop in red.

		DEM pixel size: 10 m Buffer distance: 35 m Smooth drop: 30 m Sharp drop: 10 m
Fig. 1: Effect of Buffer distance, Smooth drop and Sharp drop on a DEM, using a single drainage segment as example.
		DEM pixel size: 10 m Smooth drop: 0 m Buffer distance: n.a. Sharp drop: 40 m
Fig. 2: Effect of Smooth drop 0 and Sharp drop on a DEM, using a single drainage segment as example. If using Smooth drop 0, then Buffer distance will not be taken into account.

Buffer distance:	Buffer distance determines the width at either side of a segment where height values should be adapted.
Smooth drop:	Smooth drop determines the height with which segments and their surroundings (as specified by the Buffer distance) should be gradually dropped (positive value) or raised (negative value) in the terrain.
Sharp drop:	Sharp drop determines the height with which segments themselves should be dropped (positive value) or raised (negative value) in the terrain.

For Smooth drop and Sharp drop goes: values > 0 represent a drop; values < 0 represent a raise.
When Smooth drop <> 0 and Sharp drop <> 0, Sharp drop is an additional drop or raise on top of the Smooth drop (Figure 1).
Smooth drop 0 means no gradual drop; Buffer distance will then automatically be disregarded; Sharp drop will ensure a drop or raise at the position of the segments themselves (Figure 2).
When polygons are used, only Sharp drop applies.
Undefined height values in the input DEM will remain undefined in the output DEM.

For more information on the calculation of output values, see DEM optimization : algorithm.

Specifying buffer distance, smooth drop and sharp drop values via an attribute table:

If you wish to a apply the same buffer distance, smooth drop value and sharp drop value to all segments/polygons in the drainage map, there is no need to use an attribute table. You can directly specify the values in the dialog box or on the command line.

To apply specific buffer distance(s), smooth drop value(s) and sharp drop value(s) for specific classes or IDs of segments, you can use an attribute table (see Figure 3 below).


Fig. 3: Example of a segment drainage map and its attribute table in which different values are used for Buffer distance, Smooth drop and Sharp drop for individual segment classes.

An attribute table that is linked to a segment map or to the domain of a segment map is automatically recognized by the dialog box and the command line.
Column names Buffer_dist, Smooth_drop, Sharp_drop are automatically recognized by the dialog box and the command line. Of course, you can also use your own column names.

Input map requirements:

An input Digital Elevation Model (DEM) is required (value raster map).

Furthermore, an input 'drainage' map (vector map) is required:

You can use a segment map (domain type Class or ID) and optionally an attribute table for the segments.

Segments may represent genuine drainages, for instance when the position of segments should be dropped in the output DEM.
Segments may also represent watershed-divides, for instance when the position of segments should be raised in the output DEM.
An attribute table should be used when you wish to apply specific buffer distances, smooth drop and/or sharp drop values for individual segment classes or IDs.

For the 'drainage' map, you can also use a polygon map (any domain) to drop or raise areas in the output DEM.

Domain and georeference of output map:

The output map will use system domain Value; the value range for the output map is automatically calculated.

The output map will use the same georeference as the input map.

Tip: The output map of the DEM optimization operation can be used as input map for the Flow direction operation.

Reference:

The DEM optimization operation complies with the AGREE method, developed by Ferdi Hellweger at the University of Texas, Austin, 1997. For more information, refer to: http://www.ce.utexas.edu/prof/maidment/gishydro/ferdi/research/agree/agree.html