DEM Raster Display

With Bentley Descartes it is possible to display DEM raster so that the surface it represents can be easily interpreted.

DEM Raster

This section explains what a DEM raster is and the kind of file formats that can be used to stored a DEM raster.

What is it?

The acronym DEM stands for Digital Elevation Model. A DEM raster is a particular kind of raster for which a pixel's value doesn't represent a color but an elevation. DEM raster can be generated from a variety of technics such as aerial lidar and stereoscopic photographies.

Since an elevation measurement can be negative a the pixel values of a DEM raster are usually signed. The pixel values of some high precision DEM raster are encoded as 32 bits floatting point values, allowing the DEM raster to have sub-meter precision. 

Some other information that are commonly required for describing a DEM raster are :

1. No Data Value : The special value that is used to describe pixel for which no elevation measurement is available. Those "missing" elevation measurement are often caused by adverse atmospheric condition during the remote sensing procedure. 
2. Vertical Unit : The unit used for the elevation measurements (i.e. : the DEM raster's pixel values).

File Formats For DEM Raster

There are two kinds of file format that can be used to store DEM raster, those who are be invented especially for storing DEM raster and those who can store raster as well as DEM raster.

Common file formats that can contain DEM raster data for which Bentley Descartes is able to display as a surface are listed below.

1. File formats for raster or DEM raster data
   • Erdas IMG (*.img),
   • SPOT Digital Image Map (*.dim)
   • GeoTiff (*.tif;*.tiff)
   • Tag Image File Format (*.tif;*.tiff)
   • GeoRaster
2. File formats for DEM raster data only
   • Digital Terrain Elevation Data (*.dt0;*.dt1;*.dt2)
   • USGS DEM ASCII (*.dem)
   • USGS SDTS DEM (*.catd.ddf)

Working With DEM Raster As Surface

Because DEM raster's pixel values are elevation values and color values there are hard to visualize when attached normally in Raster Manager. The principal reason is the elevation values contained in a DEM raster usually occupy only a small range over the range of valid values.

For example, on earth, the maximum depth (i.e. : minimum elevation) doesn't exceed -11000 meters while the maximum elevation doesn't exceed 9000 meters. A DEM raster using 16 bits signed integer value for storing its pixel values will have possible value range of -32768 to 32767. Clearly, even if a particular DEM raster contains measurements in the order of the maximum depth and elevation found on earth its pixel values will be far from occupying the range of valid value.

To solution this visualization problem of DEM raster, Descartes perform a special kind of filtering which convert the elevation measurements to RGB 24 bits color value.

Attaching DEM Raster As Surface

When Descartes is loaded a new sub-menu item under the menu item File called Attach Surface is added in Raster Manager as shown in the picture below.

When a DEM raster needs to be visualized as a surface it must be attached as a surface by using the sub-item "Attach Surface", NOT as a normal raster. The picture right below shows the same DEM raster attached twice in Raster Manager, the difference being that the left attachement is a normal raster attachment while the right attachment is a surface attachment.