I'm planning to develop Digital Surface Models (DSMs) of a development site using a hexa equipped with Nex 7 camera. Hopefully to achieve around 1cm~2cm ground resolution.
1. What is the recommended height I should fly?
2. What is the lens I should use?
3. Recommended overlap horizontal/vertical (w.r.t image plane).
I calculated and found 100m height and a 16mm lens would give me somwhere around 2cm ground resolution, however, I like to hear what most of you use in a similar application (e.g. what are the practical limitations, why I should use 16mm instead of 20mm).
Thank you
Ruwan.
Replies
I recently did an evaluation of this: http://diydrones.com/profiles/blogs/accuracy-evaluation-is-better-t...
The altitude determines your GSD, but it also has an impact on perspective, namely that high objects become more ambiguous for orthomosaics. You'll get more artifacts around edges on buildings. Overlap depends on how stable your platform is, if it rolls or pitches around then you need more. If you have geometrically complex objects, then your overlap also needs to increase. For areas with trees and other complexity, I use 85% forward and 70% side overlap. For other areas you can get away with less, which increases your mapping area. So it's a trade-off, not a rule-of-thumb.
The choice of the lens should be guided by its sharpness, radial distortion and chromatic aberration qualities. If the 16mm has a lot of distortion, it reduces the accuracy and quality of your end result significantly.
Thanks Gerard. That blog post + your comments in that are awesome.