Digital Elevation Models (DEM)
DEMs are regular grids of elevation points formatted for use in Geographic Information Systems (GIS) and Computer Aided Design systems (CAD). DEMs are either the elevation of the full visible earth surface, the tops of trees and buildings or the elevation of the bare ground beneath the tree cover and buildings.
Digital Surface Models (DSM) are elevation grids of the full visible earth surface. Digital Terrain Models (DTM) are elevation grids of the bare ground.
PhotoSat DEM naming
Many of PhotoSat’s civil, mining and petroleum engineering customers are unfamiliar with GIS terms and acronyms. These engineers are the primary users of PhotoSat’s satellite elevation data. To communicate with these engineers we have learned to call our highly accurate stereo satellite DTMs “PhotoSat satellite survey grids”.
We shorten this to “PhotoSat survey grids”, “satellite survey grids”, “PhotoSat surveying” and “satellite surveying”. The PhotoSat DTMs are produced from stereo satellite photos that vary from 30cm to 70cm in ground resolution. The PhotoSat DTM accuracy varies from 15cm to 50cm in height.
PhotoSat’s DSM’s, produced from 1.5m and 2.5m stereo satellite photos, we call “PhotoSat satellite mapping grids”. We shorten this to “PhotoSat mapping grids”, “satellite mapping grids”, “PhotoSat mapping” and “satellite mapping”.
PhotoSat Digital Surface Models, DSM
PhotoSat produces DSMs from stereo satellite photos. Our Geophysical Stereo Satellite Processing System automatically matches ground features in both photos of stereo satellite photo pairs. From the relative location of each ground feature in the photos the system computes the elevation of the feature.
The accuracy of the computed elevation depends on the type and geometry of the stereo satellite photos. This is discussed more fully in PhotoSat surveying accuracy overview and PhotoSat Mapping Overview.
PhotoSat produces 3m posted DSMs from stereo 1.5m resolution SPOT 6 and SPOT 7 satellite photos. For more information on PhotoSat DSMs, see Satellite Mapping.
PhotoSat Digital Terrain Models, DTM
All of PhotoSat’s satellite surveying products are DTMs. These DTMs are produced from 50cm ground resolution stereo photos from a number of satellites. The accuracy of PhotoSat’s DTMs vary from 15cm to 50cm. For more information, please see PhotoSat surveying.
PhotoSat DTM production
PhotoSat produces a DSM and then identifies all of the above ground features such as buildings, vehicles and trees. We remove the buildings from the DSM by interpolating the ground level surrounding the buildings through the building footprint. We use the same process to remove the vehicles and individual trees.
To produce a DTM in areas of tree cover we first produce a grid of the tree canopy height. When we are confident that we have a reasonable grid of the tree canopy height we subtract the height of the tree canopy from the DSM. For more information on producing DTMs in forested areas please see PhotoSat bare ground surveys in forest covered areas.