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Getting ground control points right comes down to three things: choosing markers with high contrast and clear geometry, placing them strategically across your site, and flying a path that actually captures all of them at a GSD that makes them readable. This guide covers each step, including what to do when you can’t use conventional marker mats.
Ground control is the go-to option for turning drone data into highly accurate, survey-grade models. We have covered the benefits of ground control in a previous post. Here is what else you should know about marking and recording ground control points (GCPs) to get your data processed accurately the first time.
The most common method for recording ground control measurements is via an RTK-enabled GPS rover using correction data streamed from a nearby reference station. Known survey marks can also be used, with a marker placed on top to be found in your aerial imagery.
Alternatively, Propeller’s AeroPoints achieve survey-grade accuracy without the time-consuming task of manually measuring and marking chosen locations.
Most effective GCP markers meet two criteria:
High contrast
The marker must stand out clearly from the surrounding terrain so it can be identified in aerial imagery without ambiguity.
Clear geometry
The measured centre of the marker must be unambiguous. The design should make it obvious exactly where the precise measurement point is.
Crosshair or checkerboard patterns are the most widely used. Both meet both criteria when painted at the right size for your flight altitude.
Markers should be placed around the outer corners of a site, with the remainder distributed throughout the center.
Corners first
Place markers at the outer corners of the site first. Distribute any remaining markers throughout the center to achieve even coverage.
Check the drone’s view
Every GCP must be visible from above. Avoid locations that will be obscured by trees, buildings, or fences in the final imagery.
Stay at ground level
GCPs should always be measured at ground level. Elevated measurement points introduce elevation error into the terrain of the final 3D model.
Occasionally the nature of a site prevents the use of conventional markers. In that case, examine the site before flying and look for features that meet both criteria above. Examples of features that can work:
Two rules when using natural markers
The accuracy of outputs generated by photogrammetry is primarily limited by the ground sample distance (GSD) of the imagery, and the same applies to aerial surveys using ground control.
Match your marker size to your GSD
If your GSD is 5cm and your painted marker lines are only 3cm wide, those lines will likely be unrecognizable in the processed imagery. Adjust your flight altitude until your GSD is smaller than the width of your marker lines. If markers cannot be identified during processing, they cannot be used.
Fly over your markers
A common mistake is planning ground control carefully but then flying a path that falls short of capturing one or more markers. Plan a flight area slightly larger than the intended survey boundary to ensure all markers appear in multiple images.
When you are ready to submit your GCPs for processing, Propeller needs four things:
We’re happy to show you how Propeller can power your worksite, and boost productivity.
