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Author Topic: Easting & Northing UK  (Read 3598 times)

ian H

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Easting & Northing UK
« on: April 07, 2020, 07:55:04 PM »
Hi, I'm new to Metashape and need advise on converting from Lon/Lat to UK Easting & Northing.

We have already surveyed a plot of land via GPS RTK survey and took a dronedeploy mission of photographs (with a Mavic Pro), so we have Northing, Easting, Elevation of the site.
I am looking to use Metashape to create some GIS output, but I'm stuck at the add ground points stage.
 
The DJI photos come in with Lon & Lat not Easting / Northing and I can't see how to convert them to UK Easting/ Northing. I've identified some points from the GPS survey to transpose GCP's to the photos, however this is in Easting /Northing and not Log/Lat that Metashape is expecting.

Am I missing a processing step or conversion process I should be doing?
Thanks


Alexey Pasumansky

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Re: Easting & Northing UK
« Reply #1 on: April 07, 2020, 11:40:24 PM »
Hello Ian,

In what coordinate system the coordinates of the GCPs have been measured?
Best regards,
Alexey Pasumansky,
Agisoft LLC

ian H

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Re: Easting & Northing UK
« Reply #2 on: April 08, 2020, 12:51:13 AM »

They have been recorded in Coordinate system used :- (ETRF89 Referenced OSGB (ETRF89<-->OSGB via OSTN02 / OSTN15), Ordnance datum (ODN) Newlyn)

I managed to pick a conversion EPSG::27700 which when I created the orthomasic was accurate for X&Y but incorrect Z. I had noticed that when the conversion to Easting & Northing was completing the elevation in the reference table view changed by about -50m. See attached screen shot.

This has made the DEM very incorrect.
I'd appreciate some guidance.
Thanks
Ian 

Alexey Pasumansky

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Re: Easting & Northing UK
« Reply #3 on: April 08, 2020, 03:33:25 PM »
Hello Ian,

I think you need to use EPSG::7405 system, providing that GeoTIFF corresponding to OSGM15 geoid model has been downloaded from the following page and put to the /geoids/ subfolder in the Metashape Pro installation directory: https://www.agisoft.com/downloads/geoids/

Then in Reference pane settings set chunk's and markers' system to EPSG::7405 and cameras to EPSG::4326. Note that by default for ESPG::4326 the altitude values are treated as height above ellipsoid, so make sure that input values corresponds to this assumption.
Best regards,
Alexey Pasumansky,
Agisoft LLC

ian H

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Re: Easting & Northing UK
« Reply #4 on: April 08, 2020, 04:51:39 PM »
Thanks for your help Alexey, please stay with me as I'm having an issue?
I have tried the EPSG::7045 conversion and get the error message in the attached screen shot. I have downloaded and installed the two British Geoid Models in the folder.

I am in the UK.

Please let me know what to do next?
Thanks
Ian

Alexey Pasumansky

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Re: Easting & Northing UK
« Reply #5 on: April 08, 2020, 05:15:22 PM »
Hello Ian,

At first put the GeoTIFF that I have mentioned in the previous post to /geids/ subdirectory in the Metashape Pro installation folder, then re-start Metashape Pro and repeat the action.

If you are planning to use the Convert option I recommend to use it before starting the processing: add images, load the coordinates in WGS84 (or another system), convert the coordinates to EPSG::7405, then proceed with the common processing workflow.
Best regards,
Alexey Pasumansky,
Agisoft LLC

ian H

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Re: Easting & Northing UK
« Reply #6 on: April 08, 2020, 06:21:07 PM »
Thank you for the continuing help... After the restart the height error message is no longer displayed, however the elevation now chnages again to approx -50 meters.
This is before I have started any processing, only loading the images.

Is there a UK setting I should be using?  Is there a UK user who is using this software who can share their settings?
Ian

Alexey Pasumansky

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Re: Easting & Northing UK
« Reply #7 on: April 08, 2020, 09:10:41 PM »
Hello Ian,

50 meters difference seems to be adequate value, at least the online converter from Ordnance Survey shows similar result:
https://www.ordnancesurvey.co.uk/gps/transformation/

What is the average altitude value for the ground control points that you have for this project? And what was the average flight height above the ground?
Best regards,
Alexey Pasumansky,
Agisoft LLC

ian H

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Re: Easting & Northing UK
« Reply #8 on: April 08, 2020, 09:32:36 PM »
Alexey,
 One of the GCP's I'm referencing is 170.26 as captured on survey grade Trimble R10 GNSS reciver. The drone mission was flown at 70m and the raw exif information from the photos import to Metashape at approx 231.30 meters.

I'm not understanding why there is such a high difference, do you think this is an issue with my software installation? We are considering purchacinga DJI RTK drone, but want to use Metasghape to process the data, that's why we are so keen to get this accuracy issue understood.
Thanks
Ian
 

Alexey Pasumansky

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Re: Easting & Northing UK
« Reply #9 on: April 09, 2020, 11:25:18 AM »
Hello Ian,

Are you sure that the altitude values recorded by the drone and saved in EXIF are above WGS84 ellipsoid?

If the altitude is estimated from geoid, then you need to select WGS84+EGM96 height in the Reference pane settings dialog after loading images to the project and only after that perform the conversion operation.

In case the altitude is estimated from the take-off point, you need to perform preliminary correction to fit either ellipsoidal or geoidal height.
Best regards,
Alexey Pasumansky,
Agisoft LLC

ian H

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Re: Easting & Northing UK
« Reply #10 on: April 09, 2020, 12:23:28 PM »
Hi Alexey, thanks for staying with me on this issue.  Below is the text from the first few lines of information exported from the exif information.

CoordinateSystem: GEOGCS["WGS 84",DATUM["World Geodetic System 1984",SPHEROID["WGS 84",6378137,298.257223563,AUTHORITY["EPSG","7030"]],TOWGS84[0,0,0,0,0,0,0],AUTHORITY["EPSG","6326"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degree",0.01745329251994328,AUTHORITY["EPSG","9102"]],AUTHORITY["EPSG","4326"]]
#Label   X/Longitude   Y/Latitude   Z/Altitude   Error_(m)   X_error   Y_error   Z_error   X_est   Y_est   Z_est
DJI_0001.JPG   -2.013795   52.343490   231.177000                     
DJI_0002.JPG   -2.012605   52.344135   231.377000

In a previous reply you mentioned that the 50m difference looked about right between the geoids. Is the solution to find the elevation difference between the drone reference datum and the reference used to pick up the GCP's (OSTN15), and put this figure in the elevation offset? Is that how users are doing it?

Looking at the drone photo information and true UK datum height this is the breakdown:

The drone took a photo before take off and reported an altitude of 161.45m, the measured GNSS height was 167.20
The drone recorded it's photos at an exif reported altitude of 231.30, and the drone mission flight was to fly at 70m

The drone reported data of 161.45m(take off) + 70M(flight height) = 231.30(approx). These numbers make sense in relative reference to the drone data, but i'm not understanding how to match the elevation in the real world.   
Ian 

Thanks
Ian                   
« Last Edit: April 09, 2020, 01:00:54 PM by ian H »

Alexey Pasumansky

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Re: Easting & Northing UK
« Reply #11 on: April 09, 2020, 02:00:45 PM »
Hello Ian,

Quote
The drone took a photo before take off and reported an altitude of 161.45m, the measured GNSS height was 167.20
Were these measurements taken in the same coordinate system?

My recommendation is the following:
- add photos, load coordinate information from EXIF,
- in the Reference pane settings dialog switch the coordinate system from WGS84 (EPSG::4326) to WGS84 + EGM96 height,
- use Convert button on the Reference pane toolbar to transform the coordinates to EPSG::7405 system.
Best regards,
Alexey Pasumansky,
Agisoft LLC

ian H

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Re: Easting & Northing UK
« Reply #12 on: April 09, 2020, 02:14:15 PM »
Thanks Alexey.
I'm coming to the conclusion that the Drone & the GCP's are different coordinate systems. However I can now work out the relative height difference between them by recording the drone takeoff point with my survey grade GNSS and subtracting it from the exif data in the first photo the drone takes, which is just prior to take off.
When I have this offset figure, where do I put it in Metashape so the produced DEM will match the recorded CGP's?

Is this the workflow that users are generally doing?

Thanks
Ian     

James

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Re: Easting & Northing UK
« Reply #13 on: April 09, 2020, 02:49:06 PM »
Hi Ian,

I don't know if this is helpful, as i'm relatively new to it myself, but this is how i understand it. I'm sure you probably get this already as you seem to be asking more about how to fix it than how to understand it, so am partly posting this for my own benefit so anyone can correct me if I'm wrong!

The Drone and your Trimble R10 are using the same coordinate system that all GPS systems use, which is WGS84 (EPSG::4326) which gives height relative to an 'ellipsoid'. An ellipsoid is an ellipse which is rotated around it's short axis to give a 3D shape like a squashed sphere, only squashed in one direction so still very sphere like. The ellipsoid is 'fit' to the earth's shape to overall give a minimal deviation from 'real' sea level which is not ellipsoidal.

The coordinate system that UK Eastings and Northings correspond to is EPSG::7405. This is a coordinate system local to the UK, and gives heights as relative to the OSGM15 'Geoid' which is not an 'ellipsoid'. It is a much more detailed shape, described in the TIFF file you downloaded, which represents where sea level would 'actually' be if the sea was able to flow through hypothetical channels cut through the land.

The difference between using a 'geoid' and 'ellipsoid' explains the ~50m difference in levels that you get after converting from one system to the other. The 6m difference between Drone and Trimble is more likely down to the accuracy of the Drone. I've never used drones but believe the GPS altitude is not particularly accurate and is sometimes combined with Barometric sensors, which would need to be calibrated prior to take-off.

As far as workflow goes (bearing in mind that I've never done a drone survey!) one thing you could do is set the altitude accuracy of your drone image coordinates to a very low value so that height is only really considered from your more reliably surveyed GCPs.

ian H

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Re: Easting & Northing UK
« Reply #14 on: April 09, 2020, 03:43:06 PM »
Thank you both James & Alexey.
James, thank you for your input it was very helpful. We are about to purchase a DJI Phantom 4 RTK drone and this is why I'm investigation the production of DEM's and GIS output, and we're evaluating this software.

I'm very confused with the various reference files and ellipsoids however it is good to hear/surmise that the variance we are seeing could be in our current drone (Mavic Pro), as it it isn't really survey quality.

However I though that with good quality GCP's the software could be 'adjusted' to compensate for the variance, and this is the missing piece of the jigsaw for me at the moment.

Cheers
Ian