The quality of your ground control points (GCPs) directly affects the accuracy of your math model, and that, in turn, determines the outcome of your project. When you collect the GCPs:
- Choose features that you can identify accurately at the resolution of the raw image.
- Select features that are close to the ground. Features that rise above the ground, such as buildings, may appear to lean in the image. Therefore, a point collected from the top of the feature may be displaced from the actual ground coordinate.
- Avoid using shadows as GCPs. Although shadows may be easy to see in the image, they are not permanent and can move from one image to another.
- Beware of selecting common or repetitive features as GCPs such as parking lots or lines on a highway. When you try to identify the feature in the image, it may be difficult to select the right one.
- Identify the features in the raw image that you want as ground control before collecting GCP coordinates in the field using a GPS or ground survey.
- Collect GCPs from a variety of elevations in a wide distribution over the image and the project.
- Collect GCPs in an area of overlap between two or more images when possible. The same ground coordinate collected in multiple images helps to produce a more accurate model.