Create Ground Control Points using NAIP & DEM

Here I will explain how to create Ground Control Points (GCP) by using NAIP and DEM.

As Ortho mosaicking software (e.g. TerraPhoto & PhotoScan) need GCP to accurately align the mosaic to true geographical co-ordinates. Often aerial imagery is captured over difficult to reach terrain. This makes collection of GPS’s less practical/feasible.

Solution: In lieu of actual GCP’s, I create ground control points using NAIP & DEM. Following are the steps for creating GCP’s:-

  • Download latest NAIP  & DEM from Geo Spatial Data Gateway. or any other source.
  • Mark down easily identifiable features on NAIP imagery in ArcMap, by creating new point shape file.
  • Export shape file as XY csv file in ArcCatalog.
  • Use csv file’s XY co-ordinates in Gdal’s command line executable to get the elevation for each pixel.
  • Now we can create new XYZ text file by joining columns of xy (shape file csv) and Z (output from gdallocationinfo)
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    gdallocationinfo elevation using DEM & xy shape file
    gdallocationinfo elevation using DEM & xy shape file
  • Import XYZ file into TerraPhoto/PhotoScan  and identify same features in all overlapping images. This will force XYZ coordinates from DEM & NAIP into the software solution.