How to calculate

a Digital Elevation Model

In ILWIS, you can obtain a Digital Elevation Model (DEM) by interpolation of segments, by an interpolation of points, or by an interpolation of a raster map which contains rasterized value segments and points.

From Segments

Digitize the contour lines of a segment map with a value domain. Make sure that contour lines that consist of more than one segment have the same value. Code consistency of segments can be checked in the Segment editor.

Then perform a Contour interpolation. Contour interpolation is an operation which first rasterizes the contour lines in a segment map with a value domain, and then calculates values by means of a linear interpolation for pixels that are not covered by segments. For the Contour interpolation operation, you can also use a segment map with a class or ID domain as input, then the segment map must have an attribute table with a value column representing height values.

To create a Digital Elevation Model from Segments:

from the Operations menu in the Main window, choose Interpolation, Contour Interpolation, or
in the Operation-tree, open the Interpolation item, and double-click the InterpolSeg operation, or
in the Operation-list, double-click the InterpolSeg operation, or
in the Catalog, click a segment map with the right mouse button, and select Contour Interpolation from the context-sensitive menu.

The Contour Interpolation dialog box appears.

Tip:

When later on you want to create a georeference orthophoto or a georeference direct linear, which will use the created DEM, it is strongly advised to mark the Interpolation check box in the Raster Map Properties dialog box of the DEM.

From Points

Perform a point interpolation on your point map. A point interpolation performs an interpolation on randomly distributed point values and returns regularly distributed point values. This is also known as gridding. In ILWIS, the output values are raster values.

The input map for a point interpolation is a point map where:

points are values (point map with a value domain), or
points are identifiers (point map with an ID domain) and for which elevation values are stored in a column in a linked attribute table.

To create a Digital Elevation Model from Points:

from the Operations menu in the Main window, choose Interpolation, Point Interpolation, Moving Average or Moving Surface, or
in the Operation-tree, open the Interpolation, Point Interpolation item, and double-click the MovAverage or MovSurface operation, or
in the Operation-list, double-click the MovAverage or MovSurface operation, or
in the Catalog, click a point map with the right mouse button, and select Interpolation, Moving Average or Moving Surface.

The Moving Average dialog box or the Moving Surface dialog box appears.

Tips:

Point interpolation is a time consuming operation, to speed up the process the user is advised to use a relatively large pixel size during the interpolation and perform the Densify operation after the interpolation. Densify is a raster operation which lets you reduce the pixel size of your map. The number of rows and columns is increased, and new values are assigned to the pixels by means of a bilinear or bicubic interpolation.
When later on you want to create a georeference orthophoto or a georeference direct linear, which will use the created DEM, it is strongly advised to mark the Interpolation check box in the Raster Map Properties dialog box of the DEM.

From Segments with additional Point data

Digitize the contour lines of a segment map with a value domain. Rasterize the segment map with the Segments to Raster operation:

from the Operations menu in the Main window, choose Rasterize, Segment to Raster,
in the Operation-tree, open the Rasterize item, and double-click the SegRas operation, or
in the Operation-list, double-click the SegRas operation, or
in the Catalog, click a segment map with the right mouse button, and select Rasterize, Segment to Raster from the context-sensitive menu.

Rasterizing the point data:
1. If you have a point map with a value domain, rasterize the point map with the Points to Raster operation:
2. If you have a point map that contains points with identifiers (point map with an ID domain) for which elevation values are stored in a column in a linked attribute table,
Then, combine the two raster maps by typing the following MapCalc expression on the command line of the Main window:

pntseg

iff(isundef(pntras), segras, pntras)

where:

	pntseg	is the output raster map name of the combined point and segment map
	pntras	is the name of the rasterized point map
	segras	is the name of the rasterized segment map

Finally, interpolate the pntseg raster map which contains the rasterized segments and the rasterized points. Type on the command line of the Main window:

DEM

mapinterpolcontour(pntseg)

Visualize a DEM

To visualize a DEM, you can simply the DEM in a map window or apply the Shadow filter on the DEM and display the filtered map in a map window; you can overlay the contour lines in one color. Further, you can use Display 3D to create a three-dimensional view of your study area. For more information and an example, see How to display Digital Elevation Models.