I'm looking to do a topographic survey of a 700 acre site where the vertical measurement accuracy needs to be within 12 inches. So dividing that by 3 means I can have a HSD of 4in/pixel or better (it will be better due to altitude limits.) So I have a few questions.
There seems to be no information anywhere regarding what determines the number of ground control points you need. I HAVE found conclusions that it's the law of FAST diminishing returns. Most example use 3-10 ground control points but those have been for site sizes in the 20-300 acre range. The site I am looking at doing is about 40% flat and 60% has hills with lots of elevation change up to about 500ft. It's a desert-like landscape so there are sparse buildings and vegetation, roads, but not forest or water.
The other question is, given the objectives, what's a good minimum frontlap and sidelap to shoot for? I was thinking 75/65.
And finally, I have a processing question. The elevation measurement of my GCPs is going to be more accurate than the drone's. I'm planning on processing with Photoscan. Wouldn't you need to basically launch from one of the GCPs and make a note of the ASL elevation of that GCP then modify the EXIF elevation data for all the photos so that the drone elevation has a more accurate ASL elevation that matches up with the GCP instead of using AGL, or an uncorrected ASL from the EXIF? Or would you use the option in Photoscan to just ignore the coordinates in the EXIF data and only use the GCPs?
There seems to be no information anywhere regarding what determines the number of ground control points you need. I HAVE found conclusions that it's the law of FAST diminishing returns. Most example use 3-10 ground control points but those have been for site sizes in the 20-300 acre range. The site I am looking at doing is about 40% flat and 60% has hills with lots of elevation change up to about 500ft. It's a desert-like landscape so there are sparse buildings and vegetation, roads, but not forest or water.
The other question is, given the objectives, what's a good minimum frontlap and sidelap to shoot for? I was thinking 75/65.
And finally, I have a processing question. The elevation measurement of my GCPs is going to be more accurate than the drone's. I'm planning on processing with Photoscan. Wouldn't you need to basically launch from one of the GCPs and make a note of the ASL elevation of that GCP then modify the EXIF elevation data for all the photos so that the drone elevation has a more accurate ASL elevation that matches up with the GCP instead of using AGL, or an uncorrected ASL from the EXIF? Or would you use the option in Photoscan to just ignore the coordinates in the EXIF data and only use the GCPs?