A Digital Elevation Model (DEM) is a georeferenced image (or collection of images) where each pixel corresponds to a local elevation. DEM are useful for tasks involving sensor to ground and ground to sensor coordinate transforms, like during ortho-rectification (see section 3.2.4). These transforms need to find the intersection between the line of sight of the sensor and the earth geoid. If a simple spheroid is used as the earth model, potentially high localisation errors can be made in areas where elevation is high or perturbed. Of course, DEM accuracy and resolution have a great impact on the precision of these transforms.
Two main available DEM, free of charges, and with worldwide cover, are both delivered as 1 degree by 1 degree tiles:
The Orfeo ToolBox relies on OSSIM capabilities for sensor modelling and DEM handling. Tiles of a given DEM are supposed to be located within a single directory. General elevation support is also supported from GeoTIFF files.
Whenever an application or Monteverdi module requires a DEM, the option elev.dem allows set the DEM directory. This directory must contains the DEM tiles, either in DTED or SRTM format, either as GeoTIFF files. Subdirectories are not supported.
Depending on the reference of the elevation, you also need to use a geoid to manage elevation accurately. For this, you need to specify a path to a file which contains the geoid. Geoid corresponds to the equipotential surface that would coincide with the mean ocean surface of the Earth (see ). We provide one geoid in the OTB-Data repository available here.
In all applications, the option elev.geoid allows to manage the path to the geoid. Finally, it is also possible to use an average elevation in case no DEM is available by using the elev.default option.