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)
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.