11 Ground Control Point Placement Strategies
You can create a great-looking 3D module using just drone imagery. Because the GPS location and other metatdata are embedded in the image file, the model will be reasonably scaled and within 3ft+/- of its absolute location. However, the best way to increase relative and absolute accuracy is by added aerial ground control points or GCPs. GCPs are targets with known coordinates that can be seen in drone imagery. However, there is a bit of an art to where you place your GCPs and how many you need. Here are even tips for GCP placement.
Tip#1: You need a minimum of 3 GCP to technically “triangulate,” but always have at least 4 no matter how large your site is. That extra GCP gives the software more data to work with and significantly increases the accuracy. The more, the better, but you’ll get diminishing returns.
Tip#2: Spacing should be no more than 20,000 pixels. This would be approximately 500 ft for a DJI Phantom 4 Pro UAS at 100 ft AGL.
Tip#3: Place GCPs throughout the center and slightly beyond the area of interest. (The “area of interest” is the scene you want to capture in the model) See the image below.
Tip#4: Locate enough GCPs to provide for at least 3 to 5 checkpoints. Checkpoints are like normal GCPs, but they aren’t used in the model. They are used to “check” the accuracy after the model is rendered.
Tip#5: Never put your GCPs in a straight line. They should be evenly distributed throughout the area of interest.
Tip#6: If you have a site with significant vertical elevation change, make sure you have GCPs at different elevations.
Tip#7: The majority of GCPs should not be at the outermost perimeter of the mission grid area. A few are okay but focus on the area of interest.
Tip#8: GCPs should be visible from the air. A 12”x12” target may work, but a 24”x24” is better. As a general rule, the size of a GCP should be at least 10x the ground sampling distance.
Tip#9: GCPs should be bright with contrasting colors but not shiny or reflective. Reflective surfaces will change appearance in direct sunlight.
Tip#10: Label the GCPs so that they are visible from the air. Spray painting a sequential number next to the GCPs is a common practice. Draw a line under 6’s and 9’s to differentiate them.
Tip#11: Don't short yourself on data. If you're using a flight control app (highly recommended) like Pix4D capture or Site Scan LE, make sure you're grid pattern is larger than your area of interest. You want lots of overlapping images. See image below.