Ground control points (GCPs) can be an invaluable tool for aerial mapping. When used correctly, they help ensure a high degree of global accuracy, which is important for most surveying and construction projects. But the fact is, the majority of drone maps don’t require GCPs. Because of this, it’s important to understand when to use ground control points and when to skip them. But how do you decide if your drone mapping project needs GCPs?
In an earlier post, we wrote about using ground control points to improve the accuracy of aerial maps. Today, we take a look at what types of aerial maps benefit from ground control points and why using them may — or may not — be important for your next project.
When to Use Ground Control Points
In general, there are two reasons to use GCPs: if you need a high degree of global accuracy, or if you are contracted to provide measurements within a certain range of accuracy.
If your project requires a high degree of global accuracy, then you should use ground control points. Land surveys and DOT road inspections are two examples of projects that require true global accuracy.
Have questions about measurement accuracy with GCPs? We tested the most common DJI aircraft to determine exactly how accurate they are. Read our free whitepaper for more information.
Ground control points were important to Bon Air Drone’s Shelly Engle when she used aerial mapping to generate a contour map of a quarry in Virginia. Strategically placed GCPs helped Shelly ensure the map had a high degree of global accuracy. This was important when she orthorectified the final contour map.
As Bon Air co-owner Kyle Falwell puts it, this level of accuracy sets their company apart from the competition. “Our top priority is the quality control side of it,” says Kyle. “There are a lot of people out there doing aerial surveys, but we want to hang our hat on accuracy.”
Not sure what global accuracy is? Read our post about the basics of accuracy in drone mapping.
Likewise, GCPs are important if you use an aerial map to take volumetric or linear measurements and you are contracted to provide those measurements within a certain range of accuracy. This is the case for many civil engineering and construction projects, where 1/10 foot accuracy is often required.
Construction company Brasfield & Gorrie considers the purpose of each map before deciding whether to use GCPs. “If we’re going to make a critical determination off of the data set, absolutely we’re going to use GCPs,” says VDC coordinator Hunter Cole. “If it’s more for a marketing effort or we’re making smaller relative volumetric measurements, we may not bother with GCPs on that site.”
If you’re unsure about using GCPs, check out our short diagram below. It will quickly help you decide if they are necessary for the next project you’re working on.
GCPs When a Surveyor’s Signature is Required
If your aerial mapping project requires a surveyor’s signature, you should use ground control points. Surveying companies generally use GCPs because a high level of global accuracy is important to their work.
Common surveying projects where ground control points are useful include:
Property boundary surveys: Accuracy is essential here, because property boundary surveys are usually the last word in property line disputes. An aerial map can be used to compare plat data and determine if a property line has moved.
Insurance assessments: A good example of this is a FEMA survey for flood insurance, which uses accurate elevation data to determine a property’s flood risk.
Subdivision and new construction surveys: Local and federal regulations often require surveys in order to document the presence of utilities like gas and power lines, or water mains.
General liability: If structural damage occurs to a building, it is likely a lawsuit will be filed against the contractor who built it. Having survey records that show the structure was built up to code helps protect the contractor from being held liable.
Using Ground Control Points to Prove the Accuracy of Measurements
In general, when you use a map to take volumetric or linear measurements within a map — say, to measure stockpile volumes or the length of a fence — ground control points aren’t necessary. In most cases, as long as the resolution of a map is good, the margin of error for measurements is going to be the same with or without GCPs. This is because there is a positive correlation between high-resolution images and stitching accuracy. Following a few best practices can help ensure you produce high-resolution maps.
That being said, engineering and construction projects still use ground control points, even for maps that don’t require global accuracy and don’t need a surveyor’s signature. In many cases, contracts require that measurements taken on a project fall within a certain range of accuracy. Ground control points may not do much to increase the accuracy of measurements, but they are a good way to prove that these measurements are accurate.
Best Flight Practices When Using GCPs
If you decide ground control points are the right choice for your aerial mapping project, the next step is choosing the best flight parameters and following best practices to ensure the accuracy of your map. Ultimately, your flight parameters will depend on your mapping subject, but here are some general guidelines to keep in mind when using GCPs.
Flight Altitude
- 250–350 feet for most maps. This range will give you a good amount of resolution, but won’t bog down the stitching process with too many images.
- 400 feet for large projects. For especially large maps, such as in the forestry industry, a higher altitude is necessary. Less resolution is needed for this type of map, and a higher altitude lets you cover a large area quickly while collecting enough data relative to the tallest feature in the map.
Overlap
75/75 overlap recommended; don’t exceed 85/85. Higher overlap increases accuracy, but too much overlap slows down processing speeds unnecessarily.
Ground-truthing with vertical checkpoints
Ground-truthing your data is very important to ensure the accuracy can be replicated using multiple methods.
Test shots
Taking a few test shots helps you verify that you are seeing the correct output. It also gives you another number that you can stand by when presenting the accuracy of your map.
EPSG Codes
Before processing your map in DroneDeploy, you must enter the EPSG code that relates to your GPS measurements. Choose your EPSG code by modifying the settings of your GPS measurement device.
In most cases, we recommend using WGS84 (EPSG: 4326). One exception is when you need to compare your aerial map to existing plat data from a previous land survey. In that case, be sure to collect data in the same reference system that was used when the original land survey was taken.
Using GCPs: Where to Learn More
No matter what subject you are mapping, or what your aerial mapping project requires, understanding when — and when not — to use GCPs is an important lesson for any drone operator. Likewise, knowing how to correctly position, measure and record ground control points can make all the difference when your project requires that you use them.
- Our guide to using ground control points is a good first step for understanding how to use them.
- Read our ground control point request checklist before submitting a GCP request when using DroneDeploy.
- To learn more about global accuracy, including tips on how to improve it, read Part 1 and Part 2 of our accuracy in drone mapping series.
- For more information about accuracy standards in surveying, consult the ASPRS Positional Accuracy Standards.
