How to tranform to Irish Transverse Mercator, with Malin Head Datum

[harrytwomey]

We have tried and tried, downloaded the geoid, making .prj files, and we still have no solution

We have drones that geotag photos using rtk, accuracy less than 20mm, an we would like to transform the coordinates into the correct Irish Transverse Mercator, with Malin Head Datum, before aligning them

Even after the downloading of the goeid model osgb36_osgm15.tif there is no improvement, still the tranformed coordinates are off by maybe 50 meter in height

Can some help us please??

[Dave Martin]

Harry,
Are you perchance using DJI kit? - there's a notorious and long running issue where the 'height' in the EXIF is actually barometric height above the take-off point.
Dave

[harrytwomey]

We have a DJI too, so this information is very useful

But we also have a SenseFly Ebee, with rtkppk, so we get exif cords that are in WGS84 that are sub 25mm accuracy
when we bring them into photoscan and translate to ITM the error in alignment is far worse than in before converted

[Alexey Pasumansky]

Hello harrytwomey,

Do you have the file with the undulation grid for the required vertical datum?

Maybe you can share with support@agisoft.com the list of points that have coordinates defined in both WGS84 and IMT + geoid height systems, so that we could check the conversions?

[harrytwomey]

Hello Alexey Pasumansky

I am unsure what a undulation grid file is, as everywhere I searched it is mentioned in reference to the geoid goetiff. which is what I thought a undulation grid was??

I sending you the raw exif exports, which it the wgs84 from the drone. And a correct version transformed by the official the OSI application. And the Incorrect version transformed by photoscan using the default itm grid.
Also the .prj I tried to create after downloading the osgm14 geoid model from you site

Hopefully you can help

[Paulo]

Harry,

i downloaded the Malin Head geoid from http://www.isgeoid.polimi.it/Geoid/Europe/Great_Britain/Great_Britain_OSGM15.html and from grid file created a tif containing the geoid height or undulation for Malin Head… see attached tif file….

And in Metashape I configured a custom compound  CS for IRENET95 ITM + Malin Head geoid height adding the above mentioned tif file….

Now your photo centers transformed to IRENET95 ITM + geoid Malin Head is in attached text file.

Please check if transformation is correct,

Cheers,

[harrytwomey]

So we checked it against GridInquest, which is the official app for doing transformations between wsg84 and Malin head(OSGM15).
We are getting results that's are very close to correct, yet there seem to be a error of between 10 to 14mm between the two transformations in the height field only, Easting and Northing coords match good


The error seem to be less as the data points are closer to the west of ireland, 5 - 7mm
The error seem to be less as the data points are closer to the east of ireland, 3 - 5mm

Do you have any idea where this error in coming from.

[Paulo]

Harry,

I am not familiar with GridInquest but maybe interpolation used is different or actual original grid file used is not same?

The Malin grid file I downloaded from http://www.isgeoid.polimi.it/Geoid/Europe/Great_Britain/Great_Britain_OSGM15.html has points spaced about every 1.3 km  over Ireland…. so the actual interpolation method matters here….

Attached is Zip file of Ascii grid file downloaded from site as well as corresponding tif generated from Metashape….

Note that Malin Head geoid correction has accuracy of about 2.3 cm over Ireland….

[Paulo]

Harry,

i recalculated the transformed coordinates of your photo set and now I get a different result in height as my OSGM15_Malin.tif file was originally a little off…

The latest tif I sent is correct...

Maybe now it concurs with your results!

[harrytwomey]

Hello Paulo

It works

can you explain how you makes these grid to tiffs, is it in photoscan or something else. it would be helpful for everyone if you did a youtube video

thanks again.

[Paulo]

Hi Harry,

in Metashape, to create the compound CS with provided grid file, just follow the steps 1 to 7 highlighted in attached screen captures….

Once you click OK in step 7, you will have a tif geoid file stored in your users AppData\Local\Agisoft\Metashape Pro\geoids folder….(in case you used install for current user only option). This tif file will be used in your COMPD_CS to transform elipsoidal height to MSL height or ortho HT....

Hope this helps,

[harrytwomey]

Thank paulo, is is a great help

[harrytwomey]

Hello, after a update form photoscan 1.4.4 to metashape 1.5.2, the custom transormation datum not longer works, i get an error saying vertical datum out of range.

I went through all the steps of this walkthrough again and still got the same error. has something change in how to accomplish this??

[Paulo]

Hi HarryTwomey,

make sure you have project where the coordinates of photos  and markers are within the IRENET 95 bounds....

for example I imported your foto coordinates in Meta 1.5.2 in WGS84 datum or Irenet 95 (equivalent)

Code: [Select]

47_EP-11-31756_0022_0107.JPG -7.681338 52.749182 265.181
and when I ranform to Irenet 95/ITM + Malin head head I get

Code: [Select]

47_EP-11-31756_0022_0107.JPG 621513.954020 666505.171191 208.568732
Should work if you have your geoid tif in coorect geoids folder of Metashape and use attached prj file....

[harrytwomey]

it was a single photo way down the list that had null lat long, silly me