Terrain elevation offset

[denisdeville]

Hi,

I'm using mago-3d-terrainer from the docker image. My terrains are well generated (which was not the case using https://github.com/tum-gis/cesium-terrain-builder-docker). But, i have an offset, and my terrains seem to be ~30 meters underneath the expected level.

SPECS

I use ol-cesium. I create the scene and add my data like this:

`

// Cesium terrain from ion.cesium
// scene.setTerrain(new Cesium.Terrain(Cesium.CesiumTerrainProvider.fromIonAssetId(1)));

// Built terrain from mago-3d-terrainer
scene.setTerrain(new Cesium.Terrain(Cesium.CesiumTerrainProvider.fromUrl('http://localhost:8099/OUTPUTS/NAMUR_MAGO_OPENTOPO')));

// Cesium world building
scene.primitives.add(await Cesium.Cesium3DTileset.fromIonAssetId(96188));`

INPUTS

I tried with 2 tif files:

• One from ODWB, city center of Namur (https://carto.ville.namur.be/data/MNT_CLIP/Namur_MNT_1.tif). But it's in EPSG:31370, so I need to reproject it to WGS84, which I did with FME
• The reprojected one from FME (NAMUR_ODWB_WGS84_FME.tif) (I uploaded it to Google drive: https://drive.google.com/drive/folders/1li8FEecFmMNwD85JcN_q5Ieu149bLtoZ?usp=sharing )
• A tif in WGS84, so that i dont have to do a reprojection and maybe break the data: NAMUR_OPENTOPO.tif (in Google Drive as well)

I then run the following commands:

docker run --rm -v "./:/workspace" gaia3d/mago-3d-terrainer -input /workspace/INPUTS/Namur_MNT_1.tif -output /workspace/OUTPUTS/NAMUR_MAGO_TERRAIN

OUTPUTS

The terrain is well generated, and waaaay faster than with ctb. But i can see an offset from the expected terrain altitude, which can be seen here with the Cesium World Building layer:

Which is NOT the case when I load the cesium terrain instead of my generated one:

Do you guys have any idea of what I'm doing wrong here ? Thanks in advance !

[znkim]

Hello @denisdeville,

From what I can see, this issue seems to be caused by a height difference related to the Height Reference.

In the case of GeoTIFF data in the EPSG:31370 coordinate system, the height values are likely already corrected by an applied geoid.
You can find a brief explanation of geoids here: (link)

By default, data converted with Cesium ion generates tiles using Mean Sea Level (EGM96) as the height reference.
On the other hand, data converted with mago-3d-terrainer generates tiles based on Ellipsoidal Height.

Because of this difference, the height offset you see in the image is most likely equal to the geoid separation.

Unfortunately, at the moment we only support generating tiles using ellipsoidal height as the default height reference.
As a workaround, you can modify the GeoTIFF height values in tools such as FME or QGIS before conversion.

Orthometric height + Geoid height = Ellipsoidal height

We are also planning to support additional height references, such as Mean Sea Level, in the near future to improve usability.

Thank you for your interest and for reporting this issue.

[denisdeville]

Hi, thanks for you response.

I'm sorry but I don't have any clue on how I can modify my tiff to apply the geoid offset. Would you have any further information about that ?

[znkim]

@denisdeville, sorry for the late reply.

Here is a short step-by-step:

1. Download a geoid raster (for example EGM96 15')
   https://www.agisoft.com/downloads/geoids/

2. Open both the DEM and the geoid GeoTIFF in QGIS.

3. Align the geoid raster to the DEM:

   • Raster → Projections → Warp (Reproject)
   • Same CRS, resolution, and extent as the DEM.

   ref docs: https://docs.qgis.org/3.40/en/docs/user_manual/processing_algs/gdal/rasterprojections.html#warp-reproject

4. Use Raster → Raster Calculator:

   DEM_ellipsoidal = DEM_orthometric + Geoid
   

   ref docs: https://docs.qgis.org/3.40/en/docs/user_manual/working_with_raster/raster_analysis.html#raster-calculator

5. Use this converted DEM as input for mago-3d-terrainer.

This should fix the ~30m height offset.