Before launch, the drone baro Elev would need to be set to zero.
Over a short flight, the air pressure change should be minimal but could be offset by using a recording barometric base station at the launch site (D) for the duration of the flight to adjust factor A, above, by matching the time tags on both data sets.
WRT the GCPs, I think they primarily correct for XY mis-alignment by controlling rubbersheeting in the “horizontal plane”. Not sure if the GCP could correct for errors in A or C above or not. Needs more thought on that one.
The GCPs provides the photogrammetry program references for X, Y and Z. As a consequence measurements on the model it produces can be cm-level accurate in XYZ across a large muli-acres site with well placed GCPs.
Most drones record an estimated elevation, based upon its barometer, along with GPS-based longitude and latitude on each photo it takes. The drone does not directly calculate surface elevations. These are calculated from the drone’s photos in the photogrammetry step. The drone’s barometric-based elevation is used, along with its GPS L&L data, as a starting point for camera placement. Then the alignment of camera positions are refined based upon tie-points (pixel-level features found in more than one photo which includes the GCPs) with more weight given to the GCPs. Once the cameras have been accurately aligned, then a dense point cloud of the surface is created and from this a 3D mesh model can be generated.
The more accurate the inputs, the more accurate the results. So your suggestion of adjusting A based upon a time-tagged base-station barometric recording could help but the biggest modulator of model accuracy is the GCPs. In the non-GCPs case, the improvement in elevation accuracy may be noticable but will not correct for the very real doming elevation errors on large sites discussed by James above. GCPs will correct for these errors. Some applications in precision agriculture are insensitive to absolute elevation errors if a good bare-earth model can be generated and thus require no corrections as described towards the bottom of:
Hi,
when we try to export the Contour DXF using the above option in dronedeploy. How can we get the line labels with the heights on the contour lines. Something like this.
