Maximising Accuracy in Surveying with Ground Control Points
Drones have become a preferred method for aerial surveys due to how easy they make collecting geographical data and aerial photos. To ensure survey accuracy, ground control points (GCPs) are used. They are points defined on the ground with known coordinates and that are highly visible so they can be seen in the photos taken by aerial survey drones. After the drones take these images, software processing turns them and the geodata collected alongside them into digital elevation models, 3D models, orthomosaics, and other types of output.
What are Ground Control Points?
These consist of anything that can easily be seen in an aerial photograph. They typically use a target with a checkerboard pattern or targets with clearly defined center points. The checker box pattern is preferred because of the high contrast between the different parts of the pattern which makes them much easier to see and differentiate from the surrounding features. Additionally, a 2×2 checker box pattern has a definite center, and this center can be placed at the required coordinates when doing a survey.
When you hire companies like CompassData to do a survey, they will create their own ground control points for the purpose of that project. This data is stored which means you can get CompassData GCP data from their archives of surveys done in the past or those done recently.
Using Ground Control Points to Improve Survey Accuracy
There are two kinds of accuracy relevant when doing drone surveys. Relative accuracy is how accurate different points are in relation to each other. Absolute accuracy is how accurate the items on the photos are relative to their position on earth. If points on a survey are not well aligned with the base map used, then the absolute accuracy would be very low. GCPs can help improve this accuracy.
To ensure a high degree of accuracy, you have to place the ground control points accurately and where they would be most useful. Start by ensuring the ground control point targets you use are large and have high contrast so that they are easy to see.
Next, spread them out in the survey area, ensuring you cover the perimeter as well as notable features. For areas with varying elevations, place them in low, high, and midpoints for the best results. Also, leave some space between the GCPs and the edge of the survey areas so they do not get cut off by accident when taking your photos.
Then, you need to mark the geo-coordinates of the center of each GCP with a high-precision GNSS receiver. Because each GCP will need to match up with the images taken, it is best to number them. Lastly, the images plus the location data of all ground control points are uploaded into post-processing software.
When to Use Ground Control Points
You should always use ground control points in projects that require a high or specific degree of accuracy. Construction and engineering projects fall within these bounds, requiring a 1/10-foot accuracy.
Ground control points remain the best way to ensure surveys have a very high degree of accuracy. Working with them is fairly simple and knowing how to place them for the best results is critical.