Incorrect Coordinate Transformation

[Dragline]

I seem to be encountering a systematic coordinate transformation error. I am converting camera coordinates from EPSG 4326 to EPSG 26734. Coordinates as converted by Photoscan appear to be ~26 meters east and 10 meters south of where they should be. Any ideas why this is?

Example Coordinate Set:

EPSG 4326 From Geotagged Photo:
-148.7518243   63.97889333

Converted to 26734 by Agisoft:
700965.2522   3648730.228


Converted to 26734 by EPSG website (https://epsg.io/transform#s_srs=4326&t_srs=26734&x=-148.7518243&y=63.9788933):
700878.87403674 3648763.98909459

[Alexey Pasumansky]

Hello Dragline,

At the moment I can suggest the following:
- save EPSG::26734 system to the PRJ file,
- open it in text editor, name it differently and also modify the transformation parameters in TOWGS84 section (TOWGS84[-10,158,187,0,0,0,0]),
- then when using Convert option load this custom coordinate system from PRJ file (will appear in User-defined systems section) and select it as a target system.

Unfortunately, by the provided link I couldn't find the information about the transformation from NAD 27 to WGS 84 used.

[Dragline]

Alexey,

The results that I get from this website are also the same as what I get from QGIS. I assume the transformation being used is the same. Possibly this can lead in the right direction?

[Alexey Pasumansky]

Hello Dragline,

In ESPG-registry (http://epsg-registry.org/) there are about 35 transformations from NAD27 to WGS 84.

But I'll try to check, if it's possible to extract the one that is used by QGIS.

[Dragline]

Hello Alexey,

I have tried altering the TOWGS84 in proportion to the errors I am seeing, however this does not seem to solve the problem. (Perhaps this is not what you meant for me to do?) The errors do not change in proportion the the change in these values.

I have also tried loading a projection file that I know works for this transformation, however Photoscan will not load it (Error message: Selected transformation is not supported). I have attached it to this post. Could you please take a look and advise?

[Alexey Pasumansky]

Hello Dragline,

In your PRJ file there are no TOWGS84 parameters defined. That's the reason of "unsupported transformation" if you are trying to convert the systems that are based on different datums.

If you select EPSG:26734 system from the list, you'll have the access to the Datum Transformation Parameters to select the most appropriate transformation. Which transformation is used in the online converter?

[Dragline]

Hello Alexey,

Thank you for taking the time to work through this with me.

This is the official utility for converting from NAD27: https://www.ngs.noaa.gov/NCAT/. I believe that it uses NADgrids, as NAD27 is quite an old reference system and there are some distortions built into. The grids can be found under the download tab of this website. I believe that this may mean that the TOWGS84 parameters change across the area of my projection? However I am not sure?

Perhaps the proper approach is to try and calculate the proper TOWGS84 parameters for here? I could easily provide a list of EPSG4326 and EPSG26734 coordinates for this.

[Alexey Pasumansky]

Hello Dragline,

Can you please provide the list of coordinates for the same points in WGS84 and EPSG::26734 systems, so that we could try to find out, if it can be performed via any available transformations available in EPSG registry (you can send them to support@agisoft.com)? Also is in every case the altitude defined above the corresponding ellipsoid?