Ground Control - Principle 1
1. Location Matters
In our mostly 2D life on the ground we usually don’t get too worried about what is overhead. When placing ground control, it is critical to look for overhead obstructions. If the drone can't see your GCP as it flies overhead, that GCP can't be used to make your data as accurate as it can be.
As best as possible, make sure the drone will be able to capture photos of the GCP from all sides. If you need to place one near a tree, try to place the GCP at least the height of the tree away from it. Avoid placement under powerlines. If you can’t avoid powerlines entirely get out from directly under them. Move a few feet away from a fence line and don’t set a post or lathe right next to a target. Flatten or cut vegetation over about 1' tall right next to your GPC.
Once you have done your best to clear overhead obstructions take a look at the ground. Place the target on a fairly flat area a few feet away from sudden grade changes. Instead of placing a target on an embankment, set it at the toe or top of the slope. Avoid placing a target in a parking space or intersection (we have seen more than one GCP with a F-150 or Camry on top). Don't set your target on the back of a curb or on a block wall.
Now that you have a good location identified, all you have to do is place the target. Make sure the target is pinned or weighted down well enough to keep it in place for the duration of the flight. Then just place your rover in the center, level up and take the topo shot - just as shown in the photo below. If you are placing it on an existing survey nail or mark, the entire target needs to be flush with the surface and centered on the existing survey point. Give the point a simple description in your controller, like GCP1SW, for "ground control point 1 south west". A simple description like that will make your field work much easier to sort out for your data company or someone back at the office. It's nice to paint the GCP # next to the target, but that's not 100% necessary.
It is mostly common sense here, but you would be surprised how many drone photos we receive where a target or two is obscured in most of the photos.
One other thing, if someone is measuring in your GCP's while you are flying, remind them to avoid occupying a point as the drone passes over or near it!