GPS data - Scaling / Orientation

[sam3d]

Does Zephyr reference available GPS (EXIF) data to help with scaling and orientation during reconstruction?

I've started experimenting with aerial drone images and so far the reprojections have been oriented nicely and scaled pretty close to real world values.

I'm just curious as i assumed the output would always be completely arbitrary.

[Andrea Alessi]

Hi Sam,

yes, if exif data has gps coordinates, zephyr will automatically georeference the project (you can disable that function in the options).

Please note that GPS data is not very accurate (usually 1 ore more meters of error) so if you aim to get a georeferenced proejct, you should also use more accurate devices for that.

Please also note that the GPS information is not used for camera orientation, but rather for a fast, rough guess, that helps speed up a little the process. GPS data or not, it won't matter on accuracy of the reconstruction.

[sam3d]

Thanks Andrea

I didn't realise that GPS data was of use - I appreciate that it is not very reliably accurate / consistent so it's good to hear it can help with certain steps.

How about the orientation of the scene as a whole; is this process aligned in some way to the GPS data?

I ask because my reconstructions so far have orientated exactly as i'd hope (unmeasured) with no manual adjustment necessary - how does this work?!

[Andrea Alessi]

GPS is just used to speed up the processing to estimate the neighbor cameras. If you strip the GPS data you'll get the same result (unless the images are really bad).

3DF Zephyr is completetly autocalibrated. We do not rely on any external data

[sam3d]

Andrea, do georeferenced scenes affect the selection tools in Zephyr?

I'm finding it difficult to measure/define plane etc without using local rendering. It's like the pointer is snapping to a huge invisible grid.

[Andrea Alessi]

There shouldn't be any difference. Maybe in this specific case you have a very big mesh and the rendering window is slower than usual? Can you take screenshots (or even better a video) to explain further your issue please? thanks!

[sam3d]

Andrea, i've sent you a video via wetransfer just now, which hopefully is clear. I've also included a Project file for the same site - it uses less cameras but demonstrates the same issue on my PC.

[Andrea Alessi]

thank you! I'll have a look at it as soon as possible!

[Andrea Alessi]

Hi Sam,

I had a look at your dataset. I understand now what you mean, it shouldn't happen regardless of it being georeferenced or not. I did a quick test with another georeferenced project i had ready, and i couldn't reproduce it, so i think it's not just the georeferencing alone that is causing that behaviour. We'll investigate - I'm afraid I'll have to wait Monday for Filippo, however we'll get back to you asap!

[sam3d]

Great ok.

BTW the images are from a mavic 2 pro - i thought i'd read a while ago there were some issues with the way the gps EXIF / XMP data was being logged, certainly with regards to elevation? Not sure if how that'd be a problem here but just in case.

[Andrea Alessi]

Well if you use the XMP workflow then zephyr will trust the XMP data for camera orientation. However if i recall correctly you are using 3DF Zephyr Lite, which does not support XMP workflow. So for that, you're okay.

As for GPS coordinates in exif data, Zephyr uses it only as first rough guess. So unless they are *extremely* wrong it shouldn't be an issue at all. You can however disable GPS data in the SfM phase from the application options. If you don't need a georeferenced dataset, you can safely do so as GPS exif data will not improve the accuracy of the reconstruction.

[sam3d]

Hey Andrea

did you have chance too look in to this?

[Andrea Alessi]

Hi Sam,

yes we're looking into it it's probably something related to sampling on that tool. We'll hopefully have a fix soon!

[sam3d]

Okay