What size do April Tags need to be?

RE: What size do April Tags need to be?

John Harless — Mon, 21 Dec 2020 16:56:34 GMT

Thanks for the tips Oto. It seems like these tags are not going to be nearly as useful as I first thought. It's a shame too. I was hoping they could fit on at the max a 24"x24" corrugated plastic sheet (print size about 22"x22") so I could put some grommets in the edges to be able to nail/staple it down or weight it down with sand bags. Oh well.

Thank you for the replies again!

RE: What size do April Tags need to be?

Oto — Mon, 21 Dec 2020 16:24:38 GMT

Theoretically one tag pixel should be at least 1 pixel in photo so 35 pixels are to be on safe side. You could try to use smaller tags but honestly it is not worth the effort as they may not be readable or may be blurred or contrast is too low. If it will not be readable then you will have to mark them manually. What you could do is print/order print the targets on ISO A0/ANSI E sized paper which would be biodegradable and leave in place. Another way of course is use zoom-by-foot so after automatic mission zoom to each point by taking some photos in lower altitude above.

But for P4 RTK it would be enough to just mark two user tie points for constraining the model and use RTK georeferencing. Also reusing good calibration will help to get most of P4 RTK.

RE: What size do April Tags need to be?

John Harless — Mon, 21 Dec 2020 15:50:38 GMT

Good morning and thanks Oto. Not sure where I miscommunicated. I do not need to see the tags in the meshes. I just meant that I need the meshes to be accurate to +/-0.20' so I have to fly at a height with that in mind.

RE: What size do April Tags need to be?

Oto — Mon, 21 Dec 2020 15:35:27 GMT

Tags do not need to be visible in meshes so 3x the GSD is not needed. Tags are recognized in aerotriangulation step using full photo resolution(not sure what happens with downsampling).