Ground Control - Principle 4
4. Collect Topo Check Shots
Another important consideration when laying out your ground control is topo check shots. Check shots are just that - topo points we use to quality check the final 3D surface built from your drone flight.
What's the difference between the two? Ground Control points locate your drone data in the correct place, check points are independent and what we use to verify accuracy. If we don't have check shots we will only know if your data is correct right on your GCP's, which it always is - because we told it to be.
We suggest taking check shots more-or-less in between GCP's and in any critical areas. Try to mark them with about a 6" spot of paint, you don't need to number them. If you don't have any paint on hand, shoot them on a spot that will be recognizable from your drone flight, like the end of paint line on pavement, sawcut junction or a sidewalk corner. If places like that are not an option, just collect them where you can.
Any check shot is better than no check shot. The general guidelines regarding where it's ok to place ground control applies to check points, too. The drone has to be able to see them from overhead and they need to be on spot with bare earth, a hard surface or mowed grass.
If you are using your drone data for design and need to tie-in to an existing hard surface or drainage feature be sure to take check shots at each tie-in and flowline. We like to see about as many check shots as you have GCP's plus any critical areas and tie-ins. Of course, more is better to a point, but don’t get hung up on the exact number or feel like you need to capture 100 of them.
In your field controller, when taking topo shots, just use a simple point descriptions. If they are an important spot like a tie-in or flowline, label them clearly. Otherwise, labeling them "check" is sufficient. Good point descriptions will make your field work much easier to understand in the office. They might even save a trip back to the field to collect more data.
When it's time to export the points you can use the proprietary file type your GPS brand generates or a universal file type like CSV or TXT. However you export the data, be sure to select the Point #, Northing, Easting, Elevation and Description fields.
One common mistake we see when contractors take check shots is they let the point of their rover sink into the dirt. This will give you a check shot a 0.1' or two lower than the actual surface. This will make the final 3D surface derived from your drone data appear to be high, when it is actually right on. If you have one, put a blunt topo shoe on the bottom of your rover pole.
Here is the sewer plant we looked at in principle 2. Suggested topo check shot locations are shown with a yellow target labeled "CK". They are roughly between control with one on the berm between the lagoons.
This road project has a few check shots on either side of the right-of-way and at two key intersections. Remember, check shots don't don't take a lot of planning. Just capture then as you travel between control points. Collecting them should add very little time to your field work, but give you a big piece of mind when it comes to proving your drone data is accurate.