Monday, October 12, 2015

Digital Elevation Data

Digital Elevation Data may refer to a digital elevation surface or elevation type as discussed below:

Digital elevation surface is one among the following five types:

  1. Digital Surface Model (DSM) is the highest reflective surface of ground features captured by the sensor. It includes, tree tops, roof tops, tops of towers, telephone poles and other natural or man-made features. Photogrammetry, IFSAR, LIDAR and sonar can provide this type of surface however, accuracy and resolution my vary with technology.
    1. With sonar, DSM may include sunken vessels and other artifacts whereas the bathymetric surface reflects the underwater terrain
    2. With photogrammetry, LIDAR and IFSAR reflective surface includes passing cars and trucks not normally considered a part of digital terrain model.
  2. Digital Terrain Model (DTM) or Bare Earth Surface represents the surface of "bare-earth" after removal of ALL vegetation and man-made features. Such surfaces are generally called Digital Terrain Models (DTMs). Photogrammetry has traditionally generated DTMs when elevations are generated by manual compilation techniques. Unless specified, bare-earth surface includes top surface of water bodies.
  3. Bathymetric surface represents the submerged surface of underwater terrain
  4. Mixed surface is a hybrid of the above surface types. For example, coastal studies may require a DTM of the bare-earth surface merged with the bathymetric surface. Multiple surface representations are useful for a number of applications like, forest inventory studies that require a vegetation surface and a bare-earth surface for the same site.
  5. Point cloud elevation file is a raw data file containing single points with multiple elevations (Three dimensional point samples). An example of a point cloud file is a LIDAR multi-return dataset where there may be multiple z-values for each 'x' or 'y' coordinate.
Elevation types are discussed below:
  1. Orthometric height is the height above the geoid as measured along the plumbline between the geoid and a point on the Earth's surface, taken positive upward from the geoid. It is obtained from conventional differential levelling where the survey instruments are leveled to the local direction of gravity.
  2. Ellipsoid height is the height obtained from GPS surveys,  prior to corrections for the undulation of the geoid. Ellipsoid heights are independent of the local direction of gravity.
  3. Other types of elevation
    1. Bathymetric depth is measured in terms of water depths expressed as positive numbers downward, below the tidal datum.
    2. Geoid height is the difference between the ellipsoid height and the orthometric height at a specified location. It equals the undulation of the geoid.