Ground control points link a point in an aerial photo to its coordinate on the ground. Common ground control points include checkerboard patterns and targets with a defined center factor. However, there are plenty of strategies to use. Read on for more information!
A Strategy to Use Ground Control Points
A Strategy to Use Ground Control Points is essential when using drones for mapping projects. Not all mapping projects require high-level global accuracy, so you should evaluate each project individually. However, you can use CompassData Inc. GPCs if you need high-level global accuracy for your projects.
Record checkerboard GCPs
For effective mapping, it’s essential to use accurate and reliable checkerboard GCPs. Whether they are used on a road or an airport runway, the right GCP is critical in determining the position of an aircraft. The accuracy of GCPs depends on their geometry, including the center, which represents geographic coordinates. GCPs should be easy to locate from various heights and have accurate GPS coordinates. The most common types of GCPs include rigid boards with an “X”-shaped marker painted in the center or a checkerboard texture.
Avoid stacking too many GCPs
The best way to avoid clustering multiple Ground Control Points (GCPs) is to place them at similar distances. Ground Control Points should be at least 400 meters apart, but the closer they are to each other, the better. However, too many GCPs will result in gaps in coverage, and their location will skew your 3D model. But, if your project requires multiple GCPs, you must consider the practical implications.
A 20-acre landfill cell should contain at least 5 GCPs; however, if you are mapping a larger area, you can use more than seven. In the example below, we’ve chosen three GCPs to survey the perimeter of a 20-acre site, inset at least 15 m from the mapping boundary. We also set three GCPs in the center to avoid “doming” – where a GCP is located at the highest or lowest point.
Avoid placing them at the edge of the map.
Put your GCPs in the right spot to get the best accuracy from your aerial photos. Avoid placing them in the middle of a site. It will result in a giant “air pocket” in the center of the image. Place your target points on your map highest and lowest elevation areas. It will help avoid aerial photo pinning errors and ensure the highest level of accuracy.
If you have to use them for hand-sized samples, they will be hard to locate in the photo. Also, if you’re putting them on the map for other purposes, place them at equal distances away from the edges. Make sure to fasten them securely, as moving them will cause prominent positional errors.
Avoid placing them too close to trees.
Ideally, you will place your ground control points (GCPs) at the highest and lowest elevations possible. Otherwise, you risk creating a giant air pocket in the center of your aerial photo, making pinning the images more difficult. To help you determine the best placement of AeroPoints, we’ve included examples of two sites where optimal geometry has been achieved. The example uses all ten AeroPoints.
Avoid relying on “naturally occurring” points.
A ground control point (GCP) is a place where coordinates are determined. Aerial maps need to be accurate, so GCPs are vital for accurate mapping. Ground control points are placed at multiple locations to increase the degree of precision. The accuracy of these points is either absolute (a point corresponds to its true value), or relative (a point is determined by scaling against its absolute coordinates). Using multiple known locations makes it easier to establish the distance between two points, and the overall scale of the map.