Surveying on the Ground
Even the best in drone surveying technology still requires some surveying on the ground. Whether it is for technical, legal, or verification reasons, a good drone survey will always be complemented by at least some ground-work. Here are the major categories that need to be surveyed on the ground.
Ground Control Points
Ground Control Points are critical to the accuracy of photogrammetry missions, especially if the aircraft does not have a dual-band RTK/PPK capable GNSS antenna. Without ground control points, an aerial survey can’t be very accurate.
Ground control points need to be surveyed on the ground using traditional land surveying methods. This is typically either done by high precision GPS equipment, or laser total stations. The final accuracy of a survey will never be higher than the accuracy of the ground control used in the project.
For more information about ground control, check out our article on how to set ground control points.
Check points are identical to Ground Control Points in the way that they are set, but quite different in how they are used in process. Learn more about the difference between ground control points and check points. Check points are there only to validate the 3D model and verify the accuracy of a final survey.
Check points are not necessary to every project, but they are strongly encouraged. And when check points are set, they must be surveyed on the ground.
Extremely high accuracy points
Drone surveying continues to get more accurate every year, but it still has its limitations. Usually, after accounting for the real-world noise caused by lens aberrations, atmospheric errors, focus errors, vibration, and a thousand other things, drone surveying can’t get all that much better than 0.1’ RMSE. That is fine for the vast majority of surveying, but there are some points that simply need higher accuracy.
Some shots, such as building corners or ADA-compliant ramps, must be accurate down to a few hundredths of a foot. For this, a drone simply isn’t good enough yet, and these should still be measured on the ground. Often, even a ground based GNSS receiver isn’t good enough for these types of measurements, and a laser total station will be required.
Areas that can’t be seen from the air
Drone based photogrammetry can only map what it can see. This means that anything that can’t be seen from the air will have to be measured on the ground. Typically, this means anything that is covered by dense tree canopy or the eaves of buildings.
Objects required by law
There are some things that are required by law to be surveyed on the ground, and cannot legally be surveyed from the air. Typically, this includes things like property boundaries and set survey monuments. However, these rules vary by state and survey type, so consult your state survey laws to know what legally must be performed on the ground.