The Ground Sampling Distance (GSD) depends on:
- The camera properties including the resolution, focal length, etc.
- The flight altitude and more generally the distance to the ground/target.
- The uniformity of the terrain surface as well as the presence of objects, trees or buildings.
The expected GSD can be adjusted in Pix4Dcapture using the flight altitude setting. For perspective cameras of DJI drones like Inspire 1, Phantom 4, Phantom 3 Advanced and Phantom 3 Professional, typical values are:
- At 50-m elevation, the GSD is close to 2.2 cm/pixel.
- At 70-m elevation, the GSD is close to 3.0 cm/pixel.
- At 100-m elevation, the GSD is close to 4.3 cm/pixel.
The GSD and the flight altitude are actually proportional. Flying at 75 meters will result in a GSD approximately close to 3.3 cm/pixel.
Note that DJI Phantom 2 Vision or Phantom 2 Vision+ have a fisheye camera. The GSD estimation is therefore more difficult and experimental. At an altitude of 50 meters, the GSD obtained with Pix4Dcapture is between 1.7 and 2.7 cm/pixel.