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Are GCPs Necessary for my projects?

My organization uses drones for measuring stock tanks. Stock tanks are impoundments that catch water runoff for livestock. Our goal is to determine the maximum water a stock tank can hold during monsoon events.

We have flown hundreds stock tanks, most using GCPs, and I am trying to confirm whether we should be using them or not. The way these areas are set up we often have to carry GCPs and surveying equipment long distances. Using GCPs typically adds an 30 minutes or two per stock tank (highly dependent on tank size and site conditions.

I have attempted to find discrepancies between no GCPs vs GCPs in projects and the changes often seem to be negligible for my projects. The stockponds are typically between 0.5 and 4 acres in area and I usually run two separate flights with DJI GS Pro at 150 feet with 75%-80% overlap. The projects are flown with Phantom 4 Pro V2s. One flight is normally flown with the gimble at 90% and another flight at 60%. The projects normally finish with about 600+ pictures. Also to note, we don’t care about the stock tank being accurately geolocated and only worried about the volume.

The results are so accurate that I can look up the dimensions for our vehicles and measure them in Pix4D and they are spot on.

Does anyone have any suggestions? Our projects have deadlines and we want to fly as many stockponds as we can, but using GCPs can slow down a day by hours and hours and don’t seem to give us any meaningful changes. Is there a certain flight area where the error from not using GCPs becomes significant? For example, maybe we could start using GCPS on flights over 6 acres or so? Any help on this would be greatly appreciated!

Hello Vincent,

In my understanding what you care about the most, is the relative accuracy and not the geolocation of the project (absolute accuracy).

The relative accuracy will depend on the quality of the reconstruction of the project, which itself depends on the overlap between images, the visual content of the images, and many other parameters.

Generally, one can expect an error of 1-3 times the Ground Sampling Distance (GSD) of the original images for the relative position of a point in a project that is correctly scaled and reconstructed.

To increase and assess the relative accuracy we recommend using Manual Tie Points (MTPs), scale constraints, and orientation constraints.

If the overlap is adequate for the image content, then you do not need GCPs.

But generally, using GCPs will increase the chances for you to have accurate results.

If some areas though need help, then you might have to add them.

If you would like to check how accurate your results are you can always use checkpoints or verify lengths on the model and compare them with measurements on the field.

I hope my answer helped.