Using titiler with GEDTM30 Dem dataset to serve webmercator terrain tiles

[tdurand]

Hello,

I am trying to serve webmercator DEM tiles in the terrarium encoding for this dataset: https://github.com/openlandmap/GEDTM30 (COG file), but I am struggling to find the right params.

Here is an example tile using the Copernicus DEM dataset (titiler instance is deployed on a lambda)

https://dg15ujrf.execute-api.us-east-1.amazonaws.com/copernicus/tiles/WebMercatorQuad/14/8272/5808.png?algorithm=terrarium&resampling=bilinear&reproject=bilinear

And here is the same tile I try to serve from the GEDTM30 dataset COG file

https://titiler.xyz/cog/tiles/WebMercatorQuad/14/8272/5808?url=https://s3.opengeohub.org/global/edtm/gedtm_rf_m_30m_s_20060101_20151231_go_epsg.4326.3855_v20250611.tif&algorithm=terrarium&resampling=bilinear&reproject=bilinear

The altitude values of this tile are not correct when I try to display this in a javascript map client.

When I use the hillshade encoding, they look pretty similar "visually":

CopernicusDEM:

GEDTM30

I am pretty sure I am missing some configuration param but couldn't find which one, here is how it looks when I open this COG in QGIS:

Thanks a lot for the help 🙏

[vincentsarago]

The GEDTM30 COG is of type In int32, with scale set to 0.1.

curl https://titiler.xyz/cog/info\?url\=https://s3.opengeohub.org/global/edtm/gedtm_rf_m_30m_s_20060101_20151231_go_epsg.4326.3855_v20250611.tif | jq
{
  "bounds": [
    -180.00125,
    -65.00124999999787,
    180.00124999999488,
    85.00125
  ],
  "crs": "http://www.opengis.net/def/crs/EPSG/0/4326",
  "band_metadata": [
    [
      "b1",
      {}
    ]
  ],
  "band_descriptions": [
    [
      "b1",
      ""
    ]
  ],
  "dtype": "int32",
  "nodata_type": "Nodata",
  "colorinterp": [
    "gray"
  ],
  "scales": [
    0.1
  ],
  "offsets": [
    0.0
  ],
  "driver": "GTiff",
  "count": 1,
  "width": 1440010,
  "height": 600010,
  "overviews": [
    2,
    4,
    8,
    16,
    32,
    64,
    128,
    256,
    512,
    1024
  ],
  "nodata_value": -2147483648.0
}

TiTiler does not apply this scale by default so you need to set unscale=True

https://titiler.xyz/cog/tiles/WebMercatorQuad/14/8272/5808?url=https://s3.opengeohub.org/global/edtm/gedtm_rf_m_30m_s_20060101_20151231_go_epsg.4326.3855_v20250611.tif&algorithm=terrarium&resampling=bilinear&reproject=bilinear&unscale=true

[tdurand]

VincentGPT strikes again with incredible speed !! 🙏🙏

Thanks a lot for the answer and the explanation, each time I learn something new about COG files 🍾

[tdurand]

FYI, also with a very good timing, the maintainers of this dataset have just published another version of it: openlandmap/GEDTM30#9 (comment) that seem to work out of the box:

Here is a version of GEDTM30 version 1.2 in EPSG:3857. This version I have done some few configuration to make it easier to interpret. https://s3.opengeohub.org/global/dtm/v1.2/gedtm_rf_m_30m_s_20060101_20151231_go_epsg.3857.3855_v1.2.tif

data type is in float32, and scaling has been removed (unit meter)

https://titiler.xyz/cog/tiles/WebMercatorQuad/14/8272/5808?url=https://s3.opengeohub.org/global/dtm/v1.2/gedtm_rf_m_30m_s_20060101_20151231_go_epsg.3857.3855_v1.2.tif&algorithm=terrarium&resampling=bilinear&reproject=bilinear