Ground Control Point

[Annalisa]

Hello everyone!

I'm a PhD student in the field of Geomorphology.

I'm currently working on a photogrammetry project and I could use some advice on obtaining Ground Control Points (GCPs) for my drone survey. Any tips for me? I was considering using a red carpet and placing CDs on it to create GCPs. What are your thoughts?

All the best,

Thanks,
Annalisa

[Dave Martin]

Hi Annalisa, first few quick thoughts.

GCP targets can be coded or un-coded; I guess from your question you're thinking about un-coded. 

First thing I would suggest is that you need to work from your proposed flight(s) and know the GSD and the accuracy you aspire to, that will help guide the size of target you plan to lay.

I don't know what background or substrate you plan to lay your targets on, but from the air I would be concerned that the red carpet might not be that distinct? 

You mention using CDs - whilst the playing surface of a CD can 'blink' when, say, the sun strikes it, at other times they can be quite dark from other directions; the un-printed top white surface of a printable CD/DVD is though visible against a dark background.

A very significant proportion of targets used for UAS/drone aerial photogrammetry use black and white for maximum contrast, and the chequer board pattern of two quarters white and two quarters black, with the reference point at the central intersection - that type of target is distinctive and also draws the eye in when working with them on-screen. (If you were using coded targets, then that centre can be found automatically to pretty good accuracy).

If you are wanting to use carpet as your target material, could I suggest investing in cans of black and white paint and a roll of masking tape?

In terms of logistics, you also need to remember that as well as a suitably-distributed set of GCPs, you also need a similar number of check points in your area that won't be used to constrain the resultant model, but will be used to assess the accuracy of the survey/model.

As this is an issue wider than (just) cultural heritage, I might also suggest reading/searching/joining groups on UAS/drone aerial photogrammetry and photogrammetry more generally? Three that I could suggest on FaceBook are:
Drone Photogrammetry
Drone mapping, 3D modelling and GIS
Agisoft Metashape (not sure if you're using Metashape, but target info will transcend software used)
(disclosure: I'm admin/moderator for those groups)

Dave

[John Ethan]

Hi Annalisa,

Using a red carpet and CDs for GCPs sounds innovative! Just ensure precise measurement, visibility, and stability for accurate results. Best of luck with your project!

Best regards, John Ethan