3. Set a Few GCP's Inside the Site
Once the site is boxed in with control it's time to place a couple more GCP's to dial in accuracy. These will help fine tune the photogrammetry process, too. Not something you need to worry about, unless you process your own data, but important nonetheless.
To start, place a GCP on roughly the highest and lowest spots on your project. Don't worry about placing them on EXACTLY the highest and lowest points, just do what you can to get fairly close. Your highest and/or lowest points might be one of the boundary points from principle two, and that's ok.
While your boundary points do much of their work by scaling in your drone data horizontally, these high/low points make sure you are scaled in to the full range of vertical elevations on your project. If your highest and lowest points are on the outer boundary, place a GCP close to the middle of the project. You should have a minimum of five GCP's for any flight. Why five? This blog post in our GPS Site Control series explains why, geometrically, five is the minimum necessary. Ideally, we like to see six to eight, or more for a larger site.
For most construction sites (20 acres or less), six to eight control points will cover it. On a larger site place more control points evenly between the others. It's good to see a GCP every 500' to 800', more or less. You can stretch it out further on really big sites but keep in mind that your accuracy could start to drift between GCP's. In areas were elevations are critical place a GCP or two in or near those areas. That will guarantee the best accuracy possible for your flight where it counts the most.
One more thing to keep in mind. It's good to have a little extra ground control in case you have a bad topo shot or one gets disturbed before you fly. Take a few extra minutes to make sure you have at least the minimum GCP's required in place and consider adding a couple more for "extra credit" On an active construction site you rarely get a second chance to collect the topo data you need. A couple "insurance" GCP's are well worth the time.
Take a look at the sewer plant below. There are three GCP's inside the site boundary. One at the high point near the buildings in the north center of the site. Another is in a drainage basin to the south of it, the low point. There is a third on the middle of berm between the two lagoons. Any areas between bodies of water should have a GCP. The photogrammetry process does not work on water covered areas and we need to give it a little help with a GCP in that situation.
The high point on the pre-construction flight for this golf course project was along the cart path, roughly in the middle of the project area. The low point was on the southwest corner, where one of the boundary GPC's was able to do double duty as the low point.
The road section on the left has three control points between the ends of the project. It's best to alternate sides of the road and place a painted target in the middle of the road, if safely possible. Every 500' or so is enough in most situations, just make sure to hit the highs and lows and add more as you go around a curve. Consider placing extra GCP's in critical areas like near intersections or access roads.