Ground Control Points
Orthomosaics in Mavis are directly georeferenced using the GPS coordinates of the UAV log file or the GPS geotagging information in the exif tags of the images. The positional accuracy thus depends on the accuracy of the GPS. If Ground Control Points (GCP) are available the orthomosaics can be rectified in this post processing step. This requires a GCP.xyz tab delimited ASCII text file with the columns X, Y, and Z. Coordinates must be given in the same UTM coordinate system and zone as the orthomosaic.
While loading the orthomosaic Mavis asks for the corresponding GCP.xyz file. The GCPs are then displayed over the orthomosaic since both dataset are already georeferenced. In a next step the GCP locations can be digitized based on the orthomoasic by double clicking on the corresponding location. The closest GCP from the GCP.xyz file is automatically determined.
Rectification works either for one to three GCP which results in a an general xy(z) shift or for four and more GCPs resulting in a perspective transform. The DSM can also be adjusted if required. It can be chosen between an XY transform or an additional average Z-shift or a Z-fit.
In general the more GCPs the better. For a Z-shift at least four are required. Digitizing in Mavis: Using refresh updates the point size according to the zoom level. The nearest GCP is automatically selected and displayed in the same color.
The following screenshot shows automatically aligned GCPs pairs. Additional GCPs with no corresponding digitized locations are ignored.