How to compute the 3D location of a 2D point on the ground?

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Do you simply want the range from the camera? Or do you need precise 3d location?

Either way you need

1. The focal length in mm or the size of the FPA in mm.
2. The height above the ground.
3. If you want the precise 3d location, you need where the camera is, over the ground.

This assumes the point you want is level with the ground. If it is notably off the ground, you can't do it with one stationary camera.