Drone Mapping 101: What are Ground Control Points?
Mapping seems like one of the commercial applications that maximizes the advantages of drone technology. By leveraging the portability and speed of drones, mapping surveys can be done faster, cheaper, and at much less risk to the surveyors. Still, the use of modern technology has not rendered all traditional mapping practices obsolete.
One practice that was carried over from traditional mapping to drone mapping is the use of Ground Control Points or GCPs. In this article, we take a detailed look at what GCPs are, how to use them in a drone mapping survey, and assess if this practice still has a place in today’s landscape of modern mapping technology.
What are Ground Control Points?
Ground Control Points are visible, marked targets that are strategically placed within the area of interest. The locations of these GCPs are then determined using a highly accurate GPS receiver. The GPS measurements of the GCPs are used to “calibrate” the reproduced model according to a global geodetic coordinate system.
Why should I use GCPs?
Although GCPs are a great tool for enhancing the absolute accuracy of your map or model. It is also a time-consuming and labor-intensive practice that is not always necessary. It is recommended to use GCPs if the application of the map you are generating requires a high level of global or absolute accuracy. Common applications that require the use of GCPs include:
- Land title and property boundary surveys
- Maps that will be integrated with other geospatial data
- Maps for monitoring of changes over time (such as land subsidence or loss of vegetation)
- As-built surveys
However, if the main objective of your survey is to generate a map with excellent visual detail, then you probably will not need to bother with using GCPs. It might also be unnecessary in applications where having a high degree of relative accuracy is sufficient to meet the objectives, as in measuring the distance between buildings. The following are some of the applications that may not need the use of GCPs:
- Advertisement materials
- Site surveillance and management
- Measurement of length, area, and volume
- Routine inspection and scouting
How do I integrate GCPs in my survey design?
The reason why GCPs are not recommended for surveys that do not require them is the fact that the implementation of GCPs in a survey inevitably takes more time and requires more effort for the surveyors. A lot of thought needs to into planning where the GCPs will be located, and the surveyors have to physically access these areas to place the GCPs. If you are planning to integrate GCPs into a future survey project, then here are some of the best pointers:
1. Pick an EPSG code and stick with it
An EPSG code is a quick reference to a spatial coordinate system as compiled by the European Petroleum Survey Group. The EPSG Geodetic Parameter Set is a comprehensive database of coordinate reference systems, datums, and geodetic parameters that continues to be used and updated until today. For most jobs that require high-accuracy maps, the EPSG code 4326 is most commonly used, referring to the World Geodetic System 1984 (WGS84) coordinate reference system.
Many experienced surveyors will attest that one of the first things you need to do before starting on any mapping project is to choose an EPSG code and use it consistently throughout the whole project. The choice of which EPSG code to use may depend on the client who is requesting the job, or the surveyor who is recording the GCP locations. You may also be compelled to use the EPSG code of other existing map data if these are to be combined or compared with the new map data you are collecting.
2. Use at least 5 GCPs that are strategically located within the survey area
GCPs have to be positioned within the mapping survey area so that they influence the accuracy of the whole map. The easiest method is to put GCPs in each of the 4 corners of the survey area, although not all geometries are amenable to just having 4 “corners”.
The best way we have heard it described is to imagine the survey area as a desk, with the GCPs acting as legs to support the weight of the table. Whatever geometry your survey area is, it is generally recommended to add a 5th GCP right at the middle. It is not a good idea to have GCPs too close together, as this may lead to Scale and Orientation errors due to uneven correction.
Obviously, larger maps will require more GCPs but there should be no reason to have more than 10. Beyond 10 GCPs, the addition of more GCPs does not significantly contribute to improving the accuracy of a map.
3. Be wary of elevation changes
Survey areas with huge changes in elevation or complicated topography will typically require more GCPs. The general rule of thumb is to place a GCP on every major elevation in the map, whether it’s a hill or a valley. This practice results in better and more accurate reproduction.
4. Make sure that GCPs are visible in your aerial image
To ensure that the GCP you established will be usable in post-processing, you have to make sure that it will be visible in the final aerial images. GCPs need to have a well-defined center which is usually achieved by using high-contrast colors. A GCP may be as simple as an X-mark spray-painted on the bare surface, or a pre-made checkerboard pattern that you can simply lay on the ground. In any case, it must be large enough to be highly visible from the drone’s hovering altitude – around 4 feet across should be good enough for most applications. You can do a couple of test flights to verify the visibility of your GCPs.
Aside from the design of the GCPs, you must also ensure that they are clear of visual obstructions. Buildings, bridges, and trees can cast shadows on your GCPs depending on the time of day, so you need to do a final visibility verification before you proceed with the drone survey.
5. Ensure good overlap
An overlap rate of 75% both along and across tracks is generally recommended for mapping surveys that use GCPs. This can be increased up to 85%, which should result in more accurate reproduction, but will also produce more data to be post-processed. Another simple rule to follow is to ensure that the GCP will show up in at least 5 overlapping images.
6. Set a buffer zone between your GCPs and the map perimeter
Although we recommend placing GCPs near the boundary of your survey area, it is also important to leave a buffer zone between the borders and the GCPs. Placing GCPs exactly on the edges of a survey area tends to limit the number of images where they will be visible. A buffer zone of 50 to 100 feet should be sufficient for most applications.
7. Measure your GCP location using high precision GPS
The accuracy of the map that your survey will produce is heavily influenced by the accuracy of the location measurements of your GCPs. For this purpose, the GPS on your phone or drone just won’t do. Most professional surveyors use either a Real Time Kinematic (RTK) or Post Processing Kinematic (PPK) GPS receivers. There are several GPS receivers from Trimble and Leica that are capable of highly accurate GPS measurements, but there are also newer and more affordable alternatives available in the market now.
Is it possible to map accurately without using GCPs?
Recently, drones with advanced GPS capabilities have boasted the ability to conduct mapping surveys with fewer GCPs. This evolution of technology has led many to ask: will it eventually be possible to conduct a drone mapping survey with no GCPs? To answer this, we must look into all the factors that influence the accuracy of a drone survey.
While it is true that an RTK-enabled drone can achieve centimeter-level locational accuracy, the process of geotagging each of the image it captures involves technology other than its GPS receiver. The inertial measurement unit (IMU) measures the rotational position of the drone, the timing calibration of the camera trigger factors in a slight delay, and the camera gimbal records the position and angle of the drone’s camera. This means that, while an RTK module allows the drone to capture highly accurate locational data, it is no assurance that each pixel of each image it captures is assigned to its accurate location based on a global coordinate system.
Even with more advanced technology, it will still probably be very poor practice to conduct a mapping survey without a “ground truth” complement. The use of GCPs allows surveyors to compare the results of the mapping software’s data processing algorithm with real-world measurements. It provides a level of confidence with the accuracy of survey data that cannot be achieved by relying on post-processing algorithms alone. There may be value in RTK-enabled drones lessening the required number of GCPs, but we cannot imagine the practice of using GCPs to disappear anytime soon.
Final thoughts
Ground control points allow surveyors to reconcile their drone survey data to data collected at the ground level. The practice of using GCPs is invaluable in ensuring that the reproduced maps and models of a drone survey is consistent with a global geodetic coordinate system, allowing them to be used reliably for legal matters. This also allows clients to compare and combine their newly generated maps with existing maps.
There are a couple of rules that surveyors need to follow when using GCPs to maximize their benefits. Although the use of GCPs is not necessary for all types of surveys, it is not a practice that we can see disappearing, despite advancements in drone GPS technology.
