An interesting note about placing quality Ground Control Points.
Any other useful tips?
To make a quality ground control points to use for georeferencing in drone mapping software, there are 4 key things to remember.
1) Use a Large, Clear Targets
Ideal GCP targets could be a large X marked on the ground, a checkerboard mat, or a circular target with a center point. You want to make sure your target has high contrast colors like black & white and that it is large enough to be easily visible from your flight altitude.
2) Measure Your GCP Center with High Precision GPS
The GPS on your phone, tablet, or drone won't cut it!
You need an extremely precise GPS measurement to create a quality ground control point. To do this you will need a Real Time Kinematic (RTK) or Post Processing Kinematic (PPK) GPS receiver. Traditional examples include the Trimble R family of products (including the Trimble R8 pictured on the right) but recently low-cost alternatives like the Emlid Reach have entered the market.
3) Evenly Spread Your GCPs Through Your Map
Typically you will need a minimum of 4 GCPs but no more than 10 are usually required even for larger maps. A good strategy is use 5 GCPs located in the 4 corners of your map and the center. As you place your GCPs make sure you leave a minimum 50 ft buffer from your GCPs to the boundaries of your map.
In this example, the subjects of interest are the 2 star shaped buildings so we place our GCP as best we can around the corners and in center of our subjects.
4) Make Sure Your GCPs are Unobstructed and Clearly Visible
To create quality GCPs for drone mapping, your markers must be clearly visible from your flight altitude to be successfully processed. Visual obstructions like overhangs, snow, shade, or glare can make your GCPs difficult to identify in your map photos.