Ground Control- Best Practices
Ground control is a survey essential in ensuring accuracy of your photogrammetry and ultimately, your linework deliverables. Knowing how to shoot ground control points is important, however, ground control distribution, quantity and visual representation are all equally important and each is dependent on the other to produce high quality data.
The number of Ground control points needed is dependent on the size of the site in question, and the altitude at which you want to fly the site. The lower you fly the site, the higher the photo resolution will be, but the less amount of data will be included in each photo, and more ground control will be required to fill in the large number of photos that will not contain a GCP.
Below is an example of a flight at 400’ compared to a flight at 100’ with identical GCP quantity. The flight at 400’ has a GCP in every photo, while the flight at 100’ has gaps where no GCP appears in photos. The more photos there are between GCPs, the more the accuracy can stray.
Aerotas recommends a metric of 5 GCPs per flight battery. This metric works across all flight altitudes, because as you fly lower, you will use more batteries and as you fly higher the opposite happens, requiring less GCPs. This rule of thumb is pretty universal unless you are working with an RTK drone such as the Phantom 4 RTK. With the P4RTK, ground control points are no longer needed as anchors on the map, but only as checkpoints, therefore their quantity and distribution are not as critical.
The visual quality of a Ground Control Point is going to be crucial in maintaining accuracy throughout the project. The targets should be large enough and visually contrasting and precise enough to be able to be seen from altitude with great precision. We find that black and white kitchen tiles, such as these (picture of tile) tend to work the best and show the most precise center point so there is no ambiguity in the target representation.
Aerotas recommends a control point that has an unmistakable center point, like the point below.
The center point of a tile like this is very precise, and there is no guess as to where the center might be. This allows our photogrammetrists to pick out exactly the right point in the photo, without question. Black and white vinyl kitchen tiles are also an effective alternative.
Along with visual representation comes stability. If your target is an object that can be moved, be sure it is anchored properly to the ground. If a ground control point gets kicked or blows away in the wind after you shoot it, it will be misrepresented by a moving, or missing target.
Good distribution helps true up accuracy across the entire project. Distribution really makes sense if you think of the photos as a sheet of data that is going to be thrown over the topography, and the ground control points are spikes that hold the sheet and pull it tight and true over the land. If there is a project requiring 5 GCPs by flight battery, but 4 of those GCPs are in one corner, and one is up in the opposite corner, it leaves a lot of area that is not tied down by anything and thus has a great chance of skewing and becoming very inaccurate. If you take these same 5 GCPs and same site, and distribute them on the corners and put one in the middle, you now have effectively pinned down the entire site and the targets should show up in enough photos to process accurately.
As projects get larger, and more complex than a simple square or rectangle, the distribution can change, however it is always best to start each project (aside from linear missions which will be explained next) by pinning down the corners. Once this has been done, be sure that there is at most 1000’ between one GCP and the next, and fill out the perimeter as such. With your remaining points, try to create (as best you can) a grid, with 1 GCP in the middle of each box. The end result should look like this in a perfect world. More often than not though, we do not live in a perfect world. In these cases we have to adjust our layout a little, which is perfeclty ok.
Another couple other things to keep in mind are topographic high and low points of the project. It is ideal to shoot targets in both areas, as they help suck the data down into troughs and keep the data true up through the highest point.
Linear missions are an animal all their own. As in all missions, we want to make sure our GCP’s are spaced no more than 1000’ apart. However, for linear missions the distribution is key. It is not so important to pin the “corners” of a linear mission as it is to make sure you DO NOT lay your points in a straight line. You want to stagger the points as you move along your linear mission.
Following these rules for ground control will ensure that you collect great reliable data every time.