Jul 30, 2018 BLOG

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.

Ground control points were important to Bon Air Drone’s Shelly Engle when she used aerial mapping to generate a contour map of a query 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.

Bon Air Drone used GCPs to improve global accuracy for their map of a quarry. This level of accuracy was important when they generated a contour map of the quarry.

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 and 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.

Ground control points are often used to prove the accuracy of measurements on construction sites.

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.


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.