Elevation4 DTM Technical Information

Elevation4 products
  4-m posting DTM + Stereo or Tristereo pair
Method DTM generated by automatic filtering of DSM  produced from Pleiades stereo imagery, complex areas are completed by manual stereo editing, 
Processing
  • Manual stereo editing of complex areas including areas with a high density of above ground features
  • Detection of water bodies (sea, lake, large river) and DTM flattening.
  • Hydro Enforcement – To insure hydrologically correct drainage network, this will be limited to the major drainage network, where continuous stretches of water are visible on the imagery.
Source data Pleiades Stereo pair(s),
Level Primary Regular, Format DIMAP JPEG2000
Cloud coverage < 5%
(Where cloud appears in the source images ,it will not be possible to calculate terrain elevation values)
Available Option Orthomosaic 50cm - Contour lines 5m
Grid spacing 4m
Accuracy Absolute XY*
  • With accurate GCPs: 1.5m CE90.
  • With Ref3D GCPs: 6 to 10m CE90**.
  • Without GCPs: 8.5m to 10.5m CE90.
Absolute Z*
  • With accurate GCPs: 2m LE90.
  • With Ref3D GCPs: 6 to 10m LE90**.
  • Without GCPs: up to 10m LE90.
Relative
  • With GCPs : 1.5m LE90
  • With : Ref3D GCPs 4m LE90 (A global slopes of up to 0.02% may be present across the dataset)
  • Without GCPs: up to 4m LE90 (A global slopes of up to 0.02% may be present across the dataset)
Format
  • DTM – ASCII Grid / GeoTIFF(32bit float)
  • Ortho Image -  GeoTIFF (3 bands 8bit)
Projection Geo WGS84 or UTM / WGS84 (custom projection on request).
Vertical Unit Metres
Vertical reference Elevations above mean sea level (ref. = EGM96).
Accuracy level The accuracy specification of Elevation4 (with GCPs) is similar to the HRE40 NGA classification.
GCPs

It is recommend that 5 GCP’s distributed across each stereopair are recommended to reach  a metric accuracy level.
The customer needs to provide accurate GCPs (~20cm XYZ) that are visible in the stereopair.

AOI
  • Large AOIs can be covered by adjacent stereopairs; the DTM mosaic will be seamless with no edge effect.
  • A minimum width of 10km is required.
  • Minimum area = 100 sq.km.
NoData value
  • The value –32767 is set for areas where the elevation is not determined (around AOI).
Metadata No additional metadata is provided with the DTM.
B/H ratio
  • The optimal B/H ratio is in the range of [0.3 – 0.6].
  • A high ratio (i.e. 0.6) is suitable for flat areas with small buildings.
  • A low ratio (i.e. 0.3) is suitable for steep terrain or high buildings.
Remark

GeoTerrain 4 DTM will provide very good results where a high percentage of the ground is visible on the imagery. While areas  that have a high percentage of ground that is obscured by above ground features, such as buildings & trees, where the DTM will be interpolated from surrounding areas of visible ground, will not be as detailed. In cases where the ground is continuously obscured for large areas, it will not be possible to generate GeoTerrain4.  Areas of clouds (and their shadows) will be masked to NoData in the DTM as ground elevations cannot be computed. 

 

Elevation4 DTM Technical Information