Setting coordinate system different than GPS (WGS84)

[kajak]

Hi
I have images with GPS geotags (WGS84) and markers in a national coordinate system. What is the workflow to align the images using reference method (using positions of the images in WGS84), but refine the alignment and set the output coordinate system based on markers?

Thank you and sorry if the question is repeated.

[Jeremiah_ROWE]

Try this...
Orthomosaic and DEM Generation with Agisoft PhotoScan
http://www.agisoft.com/pdf/PS_1.3%20-Tutorial%20(BL)%20-%20Orthophoto,%20DEM%20(GCPs).pdf

[SAV]

Hi kajak,

Here is my workflow which should solve your problem (start with an empty project):

1. Import images with GPS tags (WGS84)... I assume they are from a drone/UAV 
2. In the reference pane, click the 4. icon from left (CONVERT)
3. Choose your (national) coordinate system to match the coordinate system of your markers/ground control points. The X/Y/Z coordinates of your images will be converted from WGS84 to your chosen coordinate system.
4. Import your markers. Both your images and markers should now have the same coordinate system.

Then process as usual. Don't forget to place the markers in your pictures. For photo alignment choose Reference. Remember to run 'optimize cameras' after alignment (magic wand icon in reference pane). You might also want to uncheck a few markers so they can be used as check points.

Let me know if that worked for you.

Regards
SAV

[kajak]

Hello
Yes it worked fine, thank you.
Now when I'm exporting a 3D model, my choice is either to export it in local coordinates or the system that I used. Local coordinates work fine, however the other option seems to be transforming the model (why??). Does it mean that 'local coordinates' will use the previously set coord system?

[Alexey Pasumansky]

Hello kajak,

What do you mean under model transformation when exporting in the default coordinate system used for the chunk referencing?

[kajak]

Hi
Yes, sorry. When exporting the obj model, I have to choose a coordinate system (local coordinates is the default). My question is in what coordinate system is the model going to be if I leave it as default? Is it going to be the coord system that I used for georeferencing in the project?
Thanks

[Alexey Pasumansky]

Hello kajak,

If the chunk is georeferenced, then you can export the model in the coordinate system is referenced (WGS84 in your case) also you should be able to choose the coordinate systems from the list that can be reached by the conversion of the coordinates.

Note that some 3D modelling packages may not correctly work with the geographic WGS84 coordinates, as X and Y units are in degrees and Z is in meters. If they treat everything as the same scale units you'll be observing incorrect model display.

[kajak]

Hi
Maybe I didn't explain it properly. I georeferenced the project using a national coordinate system (not WGS84). Now what's confusing for me, is that the 'Export Model' window gives me an option to choose a coordinate system. When I choose local, the model looks correctly - is it using my national coordinate system though??

I hope it's clear now.
Thanks

[Alexey Pasumansky]

Hello kajak,

When you use "local coordinates"  option for the georeferenced model export, this local coordinate system would be tangential to the ellipsoid in the area of the model position. So the model scale and axis orientation would be the same as original (georeferenced) model, but the absolute values of the coordinates would not fit the projected coordinates.

[BobvdMeij]

We are in a similar situation, where our images are provided with WGS84 positional data and GCPs that have been measured using our native Dutch reference system. Our workflow is the following.

1. Load Photos (leave the automatically detected WGS84 reference as it is)
2. Align Images
3. Import markers (define the national reference in the import dialog)
4. Mark the markers in all images
5. In the Reference pane DESELECT al checks for the images, leave the checkmarks for GCPs ACTIVE
6. Optimize camera's
7. Continue the workflow as usual
8. Now, during DEM/Orthofoto/Pointcloud export, it should automatically define your national reference as the method for export.

The above workflow seems to align with the workflow as defined in the official manual, which recommends to deactivate your images during Camera Optimization and thereby forcing the optimization to be based on your GCPs only. Especially when both are in a different coordinate system! This also makes sense considering the relative poor accuracy of your image GCPs data compared to the much higher accuracy of your GCP coordinates.

[Jeremiah_ROWE]

I have done tests that agree with this workflow. You want to uncheck the camera positions before optimizing for your GCP and final coordinate system. The only time it seems suitable to keep camera and marker positions both active during optimization is if your actual UAV/EXIF imagery position data is RTK quality.

[kajak]

Great, thank you for sharing your workflow. It works perfectly for me