Why Use Ground Control Points?
A Ground Control Point is a physically marked location with a fixed position. This fixed location has high accuracy GPS coordinates that are corresponding to the location of the GCP. If the accuracy of your GPS device is high enough the GCP will increase the spatial accuracy of your data.
With a standard DJI drone you will get accuracy values of +/- 1-5 meters. So without using Ground Control Points your final orthomosaic will be tied to the earth within +/- 1-5 meters of accuracy. With the use of well placed Ground Control Points you are able to increase the accuracy value to the levels of your high accuracy GPS unit. For more details on GCP placement see our Capturing Ground Control Points article.
Choosing a high accuracy GPS system for the collection of GCP's is very important. The Phantom 4 Pro has a GPS accuracy of +/- 1.5 meters. GPS accuracy from your average smartphone is only around 5-8 meters depending on your device. With this said if you are not using a survey grade GPS system like RTK or PPK GPS you are really wasting your time with the collection of Ground Control Points. You could potentially make your accuracy worse by using a smartphone or low accuracy GPS for the collection of these points.
Is the difference in the distance between points on a map corresponding to the actual distance between those points in the real world. To have high relative accuracy it does not matter where your map is located on earth just that size and shape are relatively accurate to the true depiction of your features.
You do not need Ground Control Points to have Relative Accuracy. All of our maps have good Relative Accuracy. This allows you to make measurements of length, area, and volume with accuracy regardless of where the survey is located.
Absolute accuracy is measured based on the distance of your features on the map from the actual locations of these features in the real world. Just like Relative accuracy it is still important to have correct size and shape of these features in your map. The difference is that the map as a whole is accurately located when compared to the location of the individual features in the real world. Having high Absolute Accuracy is a must if you are interested in precision location accuracy in your final outputs.
Ground Control Points are required to have high absolute accuracy. For an example if you are interested in detecting small 6 inch striations of a disease in your field you may need high Absolute Accuracy to find these small patches easily.
When you need GCP's
You will need to use GCP's when flying for absolute accuracy. Remember absolute accuracy is used for higher locational accuracy. This means that your images will be both precise and accurately located.
- Property Boundaries Survey
- Elevation land Survey for new development
- Survey of existing structure for As-Builts
- Vegetation Index data to be used in smart tractor (Variable rate fertilizer applications)
When you do not need GCP's
You don't need GCPs if you are flying for relative accuracy only. Remember relative accuracy means that your images will all be precise but not accurately located.
- To detect large scale trends in your data
- Pulling field level statistics
- Crop Counting