@aragalie Thats a method I have seen being used to great effect. Infact even a basic DJI Phantom 4 can produce great images from height, there is a youtube video of someone doing that for grain crop monitoring. I don’t see why it could not be used as you said to gather data. The only problems I can see would be battery life (Maybe not in near future) and mapping the coordinates of the weed location to match the gps coordinates on the land drone.
By mounting the compute and cameras on the land drone you simplify the location problem, but you got me thinking about the drone as in theory if you had a preplanned spray map made using drone images you could really ramp up the speed of the sprayer drone without worrying about bumps in the field.
Maybe you could tether an air drone to a land drone with a power cable to combine the best of both worlds. 
I have made a gaggle database. IF anyone wants to try and make a detector but finds my pictures are no good please let me know what would work better and I can try and get some more.
Thanks