Monday, October 8, 2012

Calculate the difference between ground control points and a DEM using Global Mapper

Somebody asked me how to find out the elevation difference between the digital elevation model (DEM) and independently surveyed ground control points (GCP). Any terrain software such as Global Mapper should have the tools to calculate the differences. In this post, I shall illustrate the steps to use Global Mapper to find out the elevation differences.

It is not as straightforward as other software as there is no single tool in Global Mapper. Basically, the DEM and the ground control points layer must be loaded together first; then the DEM elevation under the control point must be added to the GCP layer as an ELEVATION database attribute field. With the database attributes having the GCP elevation and DEM elevation fields, a database field subtraction can be done to calculate the difference between the DEM elevation and the GCP elevation.

Load the DEM and GCPs

  1. Start Global Mapper. Load in a DEM layer e.g. dtm.asc.

  2. Load and display the vector GCP layer e.g. gcp.shp.

  3. Optional. Select Search | Search by Attributes, Names, and Description.

    The Search by Attributes, Names, and Description dialog box appears.

    Note the GCP elevation field e.g. GPS_ELEVAT

Append the DEM elevation to the GCP
  1. Select File | Export Vector Format.

    The Select Export Format dialog box appears.
  2. Choose a format e.g. Shapefile. Click OK.

    The Shapefile Export Options dialog box appears.
  3. Toggle on Generate 3D Features Using Loaded Elevation data.
  4. Toggle on Export Points.

    The Save As dialog box appears.
  5. Type in the new GCP layer name e.g. gcp_dsm_accuracy.shp. Click Save.

    The GCP layer is exported out to a new vector file. The DEM elevation for each GCP point is appended to the output file
  6. Press ALT+C.

    The Overlay Control Center dialog box appears.
  7. Select the loaded GCP layer e.g. gcp.shp. Click Close Overlay.

    The GCP layer is unloaded

Calculate the difference between GCP and DEM elevations
  1. Load in the GCP layer appended with the DEM elevations exported previously e.g. gcp_dsm_accuracy.shp.

  2. Press ALT_C.

    The Overlay Control Center appears.

  3. Right click on the GCP layer.

    A pop up menu appears.

  4. Choose CALC_ATTR.

    The Setup Attribute Calculation dialog box appears.
  5. In the Select Existing or Create New Attribute to Assign Calculated Values to field, type in a name e.g. hgtdiff.
  6. In the Source Attribute field, choose the GCP elevation field, e.g GPS_ELEVAT.
  7. In the Operation field, choose Subtract.
  8. Toggle on Use Attribute Value. Choose the DEM elevation field e.g. ELEVATION.

  9. Click OK.

    The differences are calculated and inserted into the hgtdiff field.

    The Feature Information dialog box below shows the new field with the calculated difference for one of the GCP feature.

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