Optimal tolerance value for GCP auto detection

[_danny_]

Hi I've made up a stencil for 12 bit circular GCPs with a 54cm outer diameter. I have a Phantom 4 pro and a Phantom 4 multispec, and  use both drones to survey agricultural field trials.

I'm wondering if anyone can tell me how to optimise the tolerance value in the auto detection menu of Metashape pro? I can't seem to find any documentation on this, but assume it's a function of the GSD?

The altitude with which we fly is generally between 20m and 40m, meaning the GSD of the P4pro falls between 0.55 and 1.1cm, whilst the P4m GSD is between 1.05 and 2.10cm.. I've attached a picture of the test GCP i made up, obviously purple isn't ideal, but Agisoft didn't seem to have troubles detecting it with the P4Pro, haven't finished aligning the multispec to test whether it works on that yet though..

[Alexey Pasumansky]

Hello danny,

The image of the target on the provided DJI photo is not quite clear, there could be problems with the proper target recognition, as binarization of the target may not provide proper information on black and white segments. However, if it is properly detected only on one of the bands, but this detection is stable - it will be sufficient.

For aerial surveys you could consider "cross" non-coded targets, as it should be easier to detect them from long distance. But also the selected colors for the background and target itself should be selected in a way of providing the max. contrast on one of the multi-spectral bands from the multi-sensor system.