exporting point cloud - coordinate system

[Rennie]

I'm a new user of Agisoft and am having issues with the export of a point cloud and the correct coordinate system.

The point cloud and model (of a building) have processed fine as far as I can see, including adding 4 control points measured using the OSGB coordinate system. The images were taken by drone using the WSG84 coordinate system.

The point cloud needs to be analysed in CAD based on an OSGB coordinate system, however if I export to OSGB the result is a meaningless straight line rather than the building; if i export to the local coordinate system it doesn't tie in to OSGB (I don't have access to the CAD as the model is being analysed by someone else).

Is there anything I should be doing to avoid this?

Thanks in advance

[Alexey Pasumansky]

Hello Rennie,

Please specify, if you are interested in exporting the data in British National Grid system with ODN height? If so, then you should download OSGM15 geoid model from the following page and put it to teh /geoids/ sub-directory in Metashape Pro installation folder. Then (after re-starting the application) use the corresponding coordinate system for export - with EPSG code =  7405.

According to your description you are selecting Geographic coordinate system with the code 4277 that is using Latitude/Longitude (both in degrees) for X and Y coordinates definition. And it appears that the CAD software that you are using to open the exported results is interpreting all X, Y and Z values as similar units, while they are not.

[Rennie]

thanks - is the page and if so which version should i download?
https://www.ordnancesurvey.co.uk/business-and-government/help-and-support/navigation-technology/os-net/formats-for-developers.html

[Alexey Pasumansky]

Hello Rennie,

I have meant this page: https://www.agisoft.com/downloads/geoids/ (sorry for not posting the link before).

Check the OSGM15 geoid model in the bottom-left corner of the page.

[Rennie]

thanks, got the file and exported

are there settings in the CAD software i can tell my colleagues to use when opening the file which avoid the problem of the results interpreting X, Y, Z values as similar units - or does exporting in code 7405 fix this?

thanks

[Alexey Pasumansky]

Hello Rennie,

When you use EPSG::7405 system for export (OSGB 1936 / British National Grid + ODN height) the units in every direction are the same, so there should not be the effect that you have observed with degrees, when all the points are laying along Z axis.

[Rennie]

thanks for the speedy response, will try this

[Rennie]

Hi Alexey

I'm still having issues with the exported point cloud I'm afraid

The x/y cords are still around a metre out even though control points have been used and the z values are nowhere correct - the exported ground values are approximately -60m, whereas the correct OS z value should be around 30m

any advice appreciated
thanks

[Alexey Pasumansky]

Hello Rennie,

Are the altitude values that you have for the camera locations related to the height above WGS84 ellipsoid?

[Rennie]

the camera coordinate system in the settings is WGS84 (EPSG::4326), with the long, lat and altitude imported with the images - the altitude is recorded as 44m although the flight was conducted at 60m above surface

[Alexey Pasumansky]

Hello Rennie,

If you are using DJI drone, you may need to select WGS84+EGM96 coordinate system instead of WGS84 in the Reference pane settings dialog, if the altitude information is recorded above the mean sea level instead of above ellipsoid expected by default.
In case the altitude values are recorded above the take-off point, all the height information should be corrected according to the take-off point offset above the ellipsoid.

[Magda]

Hi there, I also have issue with exporting data in OSGB36 + ODN.

My project is in WGS84 ellipsoid (coordinates of images and coordinates of GCP) i would like to export data in OSGB36 + ODN. I already download geoid model but my exported data are clearly in wrong place by giving me Altitude ~ -500m.

I also try to convert coordinates inside Metashape from WGS84 to OSGB36+ODN before exporting but it also giving me wrong coordinates (see attached images).

Any ideas what possibly could be wrong?

Thanks.

[Alexey Pasumansky]

Hello Magda,

I have converted a few cameras and markers from your list from WGS84 to EPSG::7405 and have got about 101 meter for cameras and about 65 meters for markers. So there might be something wrong with the steps that you are following or with the file/system that you are using.

[Magda]

That's weird.

So you just open metashape, import images and  GCP in WGS84 and in export dialog choose OSG36+ODN? That is how you get correct heights?
I might try create from scratch a new project one again.

Thank for quick reply!

[Alexey Pasumansky]

Hello Magda,

I have created the markers, input the coordinates in WGS84 from your screenshots and then used Convert option to transform the coordinates to OSGB1936/British Grid + ODN Height (EPSG::7405). The geoid file used is OSGM15 geoid model downloaded from https://www.agisoft.com/downloads/geoids/

Make sure that you do not have other geoid files related to the same system definition which your affect the data interpretation.