Camera Elevation AGL Ground Control Points Above Sea Level

[jazzyj]

So the photos from my Phantom 4, the camera elevation is Above Ground Level (AGL).  The 5 ground control points I had surveyed use Above Sea Level (ASL). 

Is this going to screw things up?

[amwade]

The elevation for your ground control points should over ride those from your P4. I've been running models using the same method without a problem.

[SAV]

Hi jazzyj,

If you took off near sea level, there won't be any problem 

If not, there are three things you could do to fix the problem:
1. Change the camera accuracy to a large value in the Reference settings
2. Uncheck all photos in the reference pane, which means only your GCPs will be used
3. Change the XMP/EXIF information of your pictures by adding the elevation of your point of take off to your flight height (point of take-off = 0m for DJI drones). This can be done with a Python script or with external software packages (e.g., ExifTool).

Regards
SAV

[jazzyj]

So I'm getting ready to do a topographic survey.  Vertical accuracy needs to be within 12 inches so GSD will be 4inches/pixel or better (will be better due to altitude limits)  It's a large area, 700 acres.  Hoping 7 GCPs will be enough.  The elevation data for the GCPs laid down by a land surveyor will obviously be more accurate than the drone. 

I'm wondering if let's say the GSD is going to be 3in/pixel.  Frontlap 75 probably and sidelap 65, is it going to be worth  noting the ASL elevation at the launch point right next to a GCP and then modifying all the EXIF elevation data or just uncheck the photos and only use the data from the GCPs when processing in photoscan.  I'm a coder (PHP) with some Python experience but if it's really not going to give me much benefit I'd just assume go the simpler route.