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DSM Elevation Value

Hi Pix4D Community

Should my DSM and DTM have the same elevation as my GCP at the exact point?
I know GCPs are added to rectify any co-ordinate errors but with regards to elevation should my model/map have the same height as the GCP at the exact point or is that not necessarily the case?

May you kindly explain the purpose of adding GCP points with regards to elevation?

Kind Regards

Hi Mogapi,

The purpose of GCPs is to improve the absolute accuracy of a 3D model (i.e. to locate it on Earth). In general, the accuracy of the outputs depends on the accuracy of the inputs given to Pix4Dmapper (i.e. the ground sampling distance/GSD), the accuracy of the image geotags (coordinates of the initial images), the accuracy of the GCPs measured in the field and the visual quality of the images.

The absolute accuracy of the outputs should be expected to be 1-2 pixels (GSD) horizontally (X, Y coordinates) and 2-3 pixels (GSD) vertically (Z coordinate). For instance, for a project with a GSD of 2 cm, the horizontal accuracy expected is about 2-4 cm and the vertical one 4-6 cm.

Please note that the DSM and DTM are models generated using other algorithms (point cloud densification, spatial interpolators). This results in a final model that is additionally refined and smoothed, thus modifying the underlying elevation values (including the Z values of GCPs). So, my answer to your question is: no, the elevation values of a DSM/DTM will often not coincide with the Z values from GCPs.

Hopefully this was helpful.

Best,
Teodora