I have some aerial images and I need to know how if I can have the ground sampling distance value from the image without calculate it from the plane fly's parameters.
2 Answers
In (my) question and answer Georeference single drone image from EXIF data, there is a calculation for the horizontal field of view (in metres) of a UAV image with given lens characteristics, pitch, and altitude.
For pitch -90 degrees, i.e. pointing straight down, this reduces to
ground_fov_horiz = altitude * sensor_width / focal_length
If you divide this by the number of pixels in the image horizontally, you should get the GSD.
Example for a Mavic mini at altitude 100m:
ground_fov_horiz = 100m * 6.17mm / 4.36mm = 142m
GSD = 142m / 4000 pixels = 3.5 cm/pixel
Alternatively, a lot of software for UAV route planning does it for you, for instance the mapping component of dronelink.com.
You have to be a bit careful since the camera focal length is usually specified prior to any sensor lens corrections. So you may be a few percent off.
I'm not sure I understand "ground sampling distance" for an area, but assume you mean the pixel resolution of the image.If the images are geo-rectified, then you can just zoom in until you can see the pixels, then use a measuring tool to get the pixel width in whatever software you have. Otherwise, I think you could estimate the distance across the image and divide by the number of pixels across the image.
