How to create a gridmap with an jpg/png-picture (greyscale)

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Hey guys I would like to create a gridmap with an heighmap I created on blender (greyscale-picture where black is 4,5m and white 0,5m greyscale between is linear). I would like to have a gridmap with the different height parameters of my environment. Therefore I created an heightmap in Blender! I just want to inplement the png/jpg-file and create an gridmap with the (heightparameter (depending on the greyscale of the picture) for my A*-Algorithm. Can someone help me or give me any tipps how I can do that the best way?