Photo identifiable Ground Control Points are obtained using pre-existing features that will be visible from aerial or satellite photos, for the purpose of orthorectification, as the basis for measuring a coordinate. CompassData considers many factors when selecting a Ground Control Point location.
- Visible color contrast
- Persistent structural placement
- Unobstructed sky view
- Scale of usability
When selecting a GCP in any area, we first look for a good color contrast at a right angle. The reason for high color contrast is to ensure the GCP will be visible in any potential customers’ imagery for years to come. The reason for the selection of right angles is to further determine the fixed position of a GCP. This becomes more and more important to us as we produce higher precision in our post-processed data.
Equally important is the selection of a permanent feature such as the corner of a concrete sidewalk (as opposed to the corner of a paint stripe). This is done not only to ensure the GCP will be in any number of different image sets, but also to provide the customer with a verifiable point that is in its exact location, unmovable, not faded by years of weather and wear.
Another technique used by both the CompassData office as well as the field data collector is the selection of a GCP that is – and will stay – unobstructed by overhanging objects such as trees, building awnings or vehicles parked across a painted stripe.
Extremely important in this GCP selection is the field data collector, who makes decisions in the field and is responsible for locating points that will remain in future imagery.
Another determinate for choosing GCPs is the ability to use them for all scales of imagery, from 6-inch pixels to 6-foot pixels. This relates to collecting the tightest, most minute angle of a large contrast feature so that every GCP can be used for any image set.
Although there are some situations in which location of an area of interest limits us to the quality of a Ground Control Point, most of our archive is located in urban areas in which finding a suitable permanent GCP is little to no problem.