Dang cows!
The one job we did get flown yesterday is for a licensed land surveyor. His clients pays for a survey and he is on the ground doing his part. If the customer wants a nice new photo to go behind our under the survey that is our part. The client wanted photos and we provide them. Of course everyone wants our photo to match up real close to the points he surveyed in.
The surveyor puts down aerial targets. Those are just a large white X that he has to put on a piece of level ground in the open. Obviously they have to be seen by the camera so under trees or bushes will not work. We then find that target in the image and tell our software the X, Y and Z values that the surveyor provided to us. That helps put our image right on top of the survey. Remember it is a target in 3 dimensions. The final image is an orthophoto and is corrected to fit the Earth real well.
https://online.wr.usgs.gov/ngpo/doq/doq_basics.html
This survey was in a field and there are cows. Cows are dumb machines people invented to turn grass into tasty steaks. If there is something different in a field they will go investigate it and either eat it, pee on it, crap on it or just tear it up. We have photographic evidence of a cow with her head right on the remains of the target. It is scattered all over the place and much of it is gone. That target is unusable.
Dang cows.