In the business of UAV mapping, accuracy is everything. As my surveyor partner indicates, “If you don’t have an accurate project, it’s nothing more than a pretty picture…”. Close enough is great if you are playing horseshoes or tossing hand grenades. However, if you are doing UAV mapping, being “close enough” is a waste of your time and your clients’ money. So, how do you obtain the highest amount of accuracy in your drone mapping project? The most common method is to use Ground Control Points. (Another method, Real Time Kinematics (RTK), requires a specially equipped drone and a ground reference base station, and will be the subject of a future blog article).
Purpose of Ground Control Points (GCP)
Why use Ground Control Points? In a word – accuracy. They are physically marked locations with a fixed position and their coordinates have been precisely determined. This will increase your accuracy in the X-Y and Z axes, (also commonly known as; X,Y,Z; Northing, Easting, Elevation; and Latitude, Longitude and Altitude). GCPs allow you to:
- Correctly geolocate the site being surveyed
- Accurately scale the project in all three axes
- Provide 3 dimensional measurements that you sell your client
Can you accurately topographically map a site without GCP? Without either GCP or an RTK aircraft, a project could easily be off 1%. This may not sound too bad, but a small 1,000 foot project being off one percent could create a 10 foot error! By using Ground Control Points with proper gathering and processing procedures, You will typically be able to create projects with 1/10th of a foot (2-3cm) accuracy.
A real world example of how Ground Control Points will significantly improve your results is below. What you see is the visual difference in a project with and without Ground Control Points.
In a nutshell, the way that GCPs work is that these fixed points in the project are used to physically adjust the project in 3 dimensions to align it with the GCPs, and therefore create both global and local accuracy. Check Points, which as a Best Practice are taken along with GCPs, are not used to adjust the project, but are used as an independent accuracy check. In short, Ground Control Points are essential to provide project accuracy and Check Points are used for verifying that your data and your solution is indeed correct.
Positioning Of Ground Control Points
To improve the resolution of our topographic mapping, we randomly distribute Ground Control Points throughout the dataset. Some of the things we’ve found make the most significant difference are:
- Random – Distribute well throughout the dataset. Don’t bunch them together.
- Die Face – Ground Control Points should be in the center as well as near the edges, but NOT on the edges. Think of how the dots are distributed on the 5 side of a die.
- Tight – keep the Ground Control Points no more than 1,000-1,500 feet apart
Pro Tip: If all else fails, imagine the site you are mapping is a tarp that you’re securing over a pile of mulch. The way you would put enough weights to secure the tarp from flying off the pile in a breeze is a good analogy to ideal GCP placement.
Using GCPs in your Post-Processing Software
To help understand what makes good and poor targets, understanding how the targets are used in the post-processing software is helpful. The targets need to be readily identifiable in the photographs captured by the drone. Within the post-processing software, you’ll be zooming in quite a bit on the target to precisely mark the center of the target per the following two images:
What Makes A Good Target?
So, we’ve established that Ground Control Points are useful and help accuracy. We understand the placement of the GCP’s. So, what makes for a good ground control point?
- Contrast – Easily visible with good relief. You want to make sure that the colors of the GCP contrast strongly with the background. Unless you’re marking in a very light colored area, white tends to be the best color, in which case, go with a color that provides good visual contrast with the surrounding area.
- Permanent – The GCP should be on a permanent ground feature, ideally with flat ground and no overhead obstructions to the view
- Precise Pick Point – The GCP needs an easily point whether it is the tip of an arrow, an “X” painted on the ground or a commercial available target. Amorphous shapes make determining the actual center of the GCP incredibly hard.
- Spray Paint – Just using a can of striper spray paint to mark a GCP can work fine. You might want to consider using a stencil to keep it neat and to keep from having ambiguous centers.
You don’t need to spend a lot of money to have quality Ground Control Points. Below are some examples of commercially available targets and some incredibly simple ones using just line striping spray paint.
What Makes a Bad Target?
While you might think a bad target is better than no target at all, if you are going to the effort of establishing Ground Control Points, do it properly. So, what makes for a bad target location?
- Relief – A common error is putting targets on walls, raised walkways, curbs or anything that has a strong vertical relief.
- Trees – Make sure that you don’t put your GCP under a tree. You need them to be fully visible without obstruction from above.
- Edges – You don’t want your Ground Control Points to be sitting too close to the actual boundaries of your mapping area since you need them to be fully visible in a number of the captured images. So, the rule is close but not on the edge.
- Movement – Using an unweighted or secured object that can be blown away is a bad idea since movement of the GCP’s destroy the purpose of the GCP
- Non-Flat Surfaces – Due to problems with Z axis resolution, you want to make sure to choose a flat target as well as a flat surface
- Size – Since they need to show up in photos taken anywhere from 200-400 feet in altitude, you’ll want to make them at least 36-48” across.
Methods for Measuring Ground Control Points
You have quite a few options for measuring the Ground Control Points. You can use GPS surveying equipment or have a surveyor locate your GCPs. We have some ideas on matching it with a lower cost GPS base station system, such as those made by Stonex https://www.stonexpositioning.com/index.php/en/products/gps. Some other less expensive, but not quite as flexible alternatives, such as the Propeller AeroPoints https://www.propelleraero.com/aeropoints/ and OBS (One Button Survey) is being developed by https://www.epocdronesurvey.com/ Everyone has different experience, time, and budgets so having these many varied options is convenient for mapping professionals.
Incorporating GCP into Projects
If you have used good targets and placed them correctly, your post processing won’t have to work as hard to integrate them into your projects. One of the great advantages is GCPs don’t need to be geolocated before you do your flight. They only have to be physically visible in your images at the time of data collection. The drone crew can mark and document the GCPs prior to the flight and a surveying crew can geolocate them afterwards.
Using GCP Data
Your Ground Control Points locations can easily be exported into your usual CSV or text file for uploading into your mapping software. If you are using Pix4d, even better. Pix4d has built in ground control point handling that is easily imported.
We’ve looked at what Ground Control Points are and why you should be using them. We’ve also looked at how easy they are to integrate into your data collection as well as dramatically improving your UAV 3d mapping accuracy. Given the convenience and accuracy, it’s hard not to use GCPs, but there is a definite workflow to follow to assure consistent, accurate results.
The case for GCPs is clear. What do you think? Do you already use GCPs or do you think they are a waste of time and resources? Please reach out to us for further help or guidance with Ground Control Points and Improving UAV Survey Accuracy.