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How Do I Use Ground Control Points?
May 9, 2017
If you work with drone mapping software, you’ve no doubt heard talk of ground control points (GCPs). Used often in the surveying industry, as well as in virtual design and construction, GCPs greatly increase the global accuracy of drone maps. Although they are not necessary in every situation, GCPs are a vital tool for precision mapping. But what exactly are ground control points? And how do you go about using them correctly?
To help crack the code on ground control points, we’ve put together this short guide to using GCPs with drone mapping software.
What Are Ground Control Points?
So what exactly are ground control points? Ground control points are large marked targets on the ground, spaced strategically throughout your area of interest. If you use ground control points with your aerial map, you first need to determine the RTK GPS coordinates at the center of each. (We’ll explain how to do this a little later.) The ground control points and their coordinates are then used to help drone mapping software accurately position your map in relation to the real world around it.
It might be helpful to think of your GCPs as a series of thumbtacks placed on your drone map. Because the drone mapping software knows the exact location of each of these “thumbtacks”, it can reference their locations when it matches up all of the other points on the map.
When and Why Are GCPs Important?
When used correctly, ground control points greatly improve the global accuracy of your drone map. That is to say, they help ensure that the latitude and longitude of any point on your map corresponds accurately with actual GPS coordinates. This is important in situations where precision mapping and true global accuracy are needed. As we mentioned above, surveying companies generally use GCPs, because a high level of global accuracy is important in most of the work that they do. Virtual design and construction is another sector that often requires this level of precision drone mapping.
Landpoint, a surveying company based in Louisiana, uses ground control points when creating drone maps used for topographical surveying. Using GCPs on an 85-acre map, their team conducted an accurate aerial survey, saving over 80 man hours compared to traditional land survey methods.
Each drone mapping project is unique, and not all projects require a high level of global accuracy. Because of this, it is important to assess each project individually before you decide to take the extra step of using GCPs. But generally speaking, projects like geo-referenced overlays, design documents and land title surveys benefit from the use of ground control points. In an upcoming post, we’ll take a deeper look at which types of projects are best suited for using GCPs.
How to Construct a Ground Control Point
There is no one right way to make a ground control point. One important thing to remember is that the GCP must be easily visible in your aerial imagery. This is achieved by using high-contrast colors and by making sure the ground control point is large enough to be seen from your particular flight altitude. We generally recommend flying at 300 feet with a frontlap and sidelap of 70/75 when using ground control points. Keep in mind that this may change dependent upon the area you are mapping. Learn more by reading our whitepaper about mapping accuracy and reviewing our GCP support documents.
A number of companies do sell pre-made, portable ground control points. However, many drone users simply fashion their own.
This well constructed GCP was spray painted onto the concrete using a stencil. Notice that the marker is large enough to be visible from far away. A center mark helps eliminate any confusion as to where the center point is located.
If you’re unable to mark GCPs with paint, there are a variety of low-cost ways to make markers with items available from any local hardware store. The weather-resistant rubber and vinyl markers seen above end up costing about $5 each and are very durable.
Measuring the Location of Your GCPs
As we mentioned above, it is important to measure the GPS coordinates at the center of each ground control point. To do this, you need either a Real Time Kinematic (RTK) or Post Processing Kinematic (PPK) GPS receiver. Trimble and Leica products are commonly used for high-accuracy GPS measurements. New, lower-cost alternatives have recently come onto the market as well. Hiring a surveyor to measure the location of your ground control points is also an option.
Do not use a phone or tablet to measure the location of your ground control points. The accuracy of these devices is very similar to that of a drone’s onboard GPS system and will not deliver precise results. Instead, use one of the previous methods listed above, such as an RTK or PPK GPS receiver.
Best Practices for Using Ground Control Points with Drone Mapping Software
Of course, if you use ground control points with drone mapping software, it is important to use them correctly. Follow thesebest practices to help ensure your GCPs serve their intended purpose and improve the accuracy of your map.
Use a minimum of 4 large GCPs: DroneDeploy requires a minimum of 4 ground control points. Each should measure at least four feet. No more than 10 are usually needed for larger maps.
Evenly distribute your GCPs on the ground: For most maps of moderate size, we recommend 5 GCPs, one located near each corner and one located in the center, as pictured above. Also, make sure GCPs are spaced far enough apart, to avoid confusion. As a general rule, if you can see more than one GCP in an image, they are too close together.
Create a buffer zone around your map’s perimeter: We recommend a buffer zone between the edges of your map and any ground control points. This ensures there is enough image coverage to carry out reprocessing. The size of your buffer zone should be somewhere between 50–100 feet, depending on the overlap of your flight. A higher overlap produces more images and generally requires less buffer zone.
Be aware of elevation changes: If the area being mapped has noticeable elevation changes like hills, mines and valleys, make sure to place at least one ground control point on each of the different major elevations.
Make sure your GCPs are unobstructed: Visual obstructions like overhangs, snow, shade or glare make ground control points difficult to identify on your drone map.
Know Your EPSG code: 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).
For more information regarding the complexities of ground control points, watch our webinar on streamlining your workflow with self-serve GCPs, or contact us.