Ground Control Points 101 (GCPs)

What are they? Why do they matter? How are they used?

With the growing constellation of commercial earth-observation satellites, there has been an associated growth in the inclusion of spatial data in our everyday lives. Once the realm of surveyors and rocket scientists, Global Positioning Satellite (GPS) technology is now in our automobiles, wrist watches, and cell phones, and is becoming increasingly tied to image data, be it a picture of the Earth or the front of the restaurant we just made cell phone reservations with while stuck in traffic.

To enable such generalized use of aerial, satellite and ground-based imagery, geospatial professionals process the collected images with ground coordinates in order to correctly position the imagery in relation to the Earth. To ensure that the image is in the right position, and that the correct image of the restaurant comes up on the screen of the onboard navigation system in our car, it is necessary to provide accurate ground coordinates to tighten the accuracy of the overhead imagery.

This is the role of Accuearth. We are continually collecting new Ground Control Points (GCPs) across the world to provide accurate coordinate data to support an increasing variety of applications.

Reliable GCPs are an essential input for precise orthorectification of:

  • Remotely sensed imagery
  • LiDAR and IFSAR Surface Models
  • Image and mapping data quality assessment
  • Photogrammetric mapping
  • A fundamental building block of Geographic Information System (GIS) development

Bringing the Real World to Remotely Sensed Imagery

In order to be of any use spatially, Airborne and Spaceborne imagery must be referenced to the real world. By recording precise coordinates of photoidentifiable locations within an image, those coordinates form the basis for remapping that image, through the use of robust software such as Esri ArcGIS, into a usable, accurate map image, ideal for a variety of applications and industries.

Without ground control, accuracies for spaceborne images are typically 10-50 meters off, based on onboard position and attitude sensors and known orbital parameters.

Once ground control has been applied, those accuracies improve to .5-2 meters or better.

Through the use of ground control and sophisticated terrain modeling techniques, aerial images can be generated into orthophoto maps with sub-foot relative accuracies and absolute accuracies of under 2 feet.

So, contrary to popular belief, higher quality aerial or satellite imagery cannot replace the need for ground control. In fact, GCP collection becomes increasingly more important as image accuracy is required and user expectations increase. Fortunately, collecting ground control is now a much faster, more accurate, and cost-effective process thanks to the use of GPS technology.