Hack for Cheap Ground Control Points
Making a 3D model or map with drone imagery is easy enough with the right software and training. Most commercial drones have GPS units and will embed the drones’ location in the metadata of the images. However, the accuracy of the GPS unit is often plus or minus a meter. Adding ground control points (GCPs) is a great way to improve the accuracy of your drone models.
Just Paint It
A GCP is just a target on the ground with known coordinates that can be seen in drone imagery. By locating the GCP in the images, your modeling software can accurately position the model on the earth’s surface and improve the scaling for better measurements. The GCP doesn’t need to be anything fancy. If you’re on a job site where you don’t care about marking the ground, spray paint is ideal. A “arrow” shape is often used as it’s easy to find the exact point that has been surveyed. Avoid a single-color shape with the survey point in the middle. It can be hard to know the exact center when locating the survey point in the images. Don’t forget to paint an identifying number large enough to see from the drone images. You’ll be glad you had when you’re trying to figure out which GCP goes with which coordinate. The higher you fly, the harder it will be to see the target. If you’re between 12 and 24 inches, you should be fine.
What about when you can’t spraypaint the ground? I’ve been on a lot of jobs where painting the new asphalt, decorative pavers, or landscaping isn’t an option. First, if you search online, there are plenty of pre-made GCPs. Some are fabric materials. They are nice because they can be rolled up and easily stored. I generally don’t like them because you need to use pins in each corner to keep them taut and from blowing away in the wind. Those pins don’t work on a hard surface. There are also “disposable” paper GCPs. These are fine too, but they have the same limitations as the fabric kind, plus they tend to roll up around the edges with humidity. Rigid GCPs are preferred, in my opinion, but they can get costly and are hard to store.
My solution…checkerboard kitchen floor tile! You can buy twenty, 12”x12”, black-and-white floor tiles for less than $30 on Amazon. They are heavier than the fabric or paper kind so they don’t blow away in the wind as easily. Also, they won’t curl up with moisture.
They are only 12” x 12”, so it’s hard to see them if you’re flying high, but you can always put four of them together to make a larger target. They have peel-and-stick backing, which you can leave on or apply to a ridged board. I put mine on a ½” sheet of plywood because I like the extra weight and to make sure it's 100% flat. If you go to your local hardware store, they may even cut down a sheet of plywood for you. The tiles are exactly 12” x 12”, so make sure when the plywood is cut you cut outside your 12” mark. I also recommend drilling a hole in the middle so I can put a 8” landscaping nail in it. Those work great for securing it to the ground especially if you’re on a slope. The one downside of these tiles is that they can be shinny and reflective. Reflective surfaces like windows and water are tough to model. You can rough up the surface or put a thin “misting” of paint to remove the shininess.