Ground Control Points for Drone Flights

Freelance Contributor

In a recent webinar (watch it here), Aaron Ames from POWER Engineers told us about a recent project, a transmission line project in Arizona. Aaron explained that drones were the best option for aerial imaging for this project as they offer a reduced footprint in an area with sensitive plants and wildlife, namely saguaro cacti, which enjoy heavy protections. Under Arizona state law, any development in this area must account for each cactus when planning for future structures and roads. Prior to UAV technology, POWER Engineers would likely have used manned aircraft (at a greater cost) or a ground survey crew (more expensive, more disruptive, more overall risk). For this particular job, drones were the most efficient and effective tool. 

This use case was full of fascinating lessons and takeaways; the one thing that really stuck out to me was that over half the crew’s on-site time was spent setting up ground control points (GCPs). This has raised some interesting questions about job planning and efficiency, which we’ll explore below in greater detail.

What are ground control points (GCPs) and how are they used with drones?

A GCP is a physical object that represents a point with known coordinates. They range in appearance, though generally they are designed to be highly visible and distinct from the surrounding terrain—as a rule, non-reflective material is better, as reflections can cause error in recognizing the GCP. They can be made of various materials, ranging from painted wood to molded plastic, and all that really matters is that they are readable by your software, easy to set up, and unlikely to move due to wind or weather. The resolution of your camera is an important factor as well, with an inverse relationship between resolution and maximum flight altitude/size of GCP.

In other words, the lower your resolution, the larger your GCP needs to be.

Ground control points can be made from a variety of materials, including ceramic tiles, five-gallon buckets, or any other visible, portable, non-reflective material.

When GCPs Are Used—and How Many

Like so much about drone operations, whether or not you should use GCPs depends on the deliverables you’re producing. Do you want to produce a 2D map and straightforward imaging? In that case, GCPs are not strictly necessary, but your results will have no scale, orientation, or absolute position information.

Does your project require higher accuracy or 3D imaging? If so, there is correlation between the number of GCPs and the level of accuracy you can expect to achieve, especially in areas with unusual or complex topography. In their project, POWER Engineers didn’t need absolute, survey-grade accuracy yet—that comes in the design phase—but they did need enough GCPs to seamlessly stitch together images from 17 flights. 

Tips for Using GCPs

  1. Three GCPs is the required minimum, though at least five is recommended.
  2. One should be placed at the center of the area, and avoid placing them at the extreme edges, as they will only appear in a small set of images.
  3. Pix4D suggests thinking of GCPs as the legs of a table: they should be distributed well apart from each other at regular intervals in order to “support” the overall area of the site.

Structuring Drone Workflow around GCPs

The ideal time for aerial imaging is mid-day, when shadows are at a minimum, and preferably in clear weather. In the U.S., night flights are prohibited without a waiver under Part 107, and you’ll need anti-collision lights in order to operate a drone during civil twilight. GCPs can be placed any time you have access to the site even in conditions that are unsuitable for flying. If you decide GCPs are necessary for your project, establishing them ahead of time can help save time that would be better spent flying, and possibly avoid costly and inefficient multiple visits to the job site.

By reserving viable flight time for flight itself, you can ensure you’re gathering data at the best possible moments, and perhaps even leaving you time to double-check your data on-site and patch any errors on the same day.

Using Skyward to make your drone operations more efficient

Aaron Ames of POWER Engineers also cited Skyward as an important tool in planning and executing this infrastructure project.

“The biggest thing that we used Skyward for was flight planning and to make sure that we weren’t in airspace that required further authorization,” he said. “We didn’t really have time in our schedule to obtain waivers from the FAA or work with the military. So we were able to tailor this flight into a small area.”

By using Skyward, POWER Engineers increased the efficiency of their workflow and avoided costly delays from wrangling with the legal process. Thanks to careful planning, they had no safety incidents in four days of flying, demonstrating their professionalism, high standards for operation, and the overall quality of their processes.