I am trying to delineate some watersheds using the SAGA GIS Tools in QGIS. The DEM I am using is the SRTM DEM. When I try to generate the channels and drainage basins using the SAGA>Terrain Analysis - Channels> Channel Network and Drainage Basins tool, I get the following outputs:

Generated Channels

Generated Drainage Basins

What am I doing incorrectly?

I never did any reprojection and just used the DEM files as is.

Those large orange drainage basins are bodies of water.


Here is the gdalwarp code that I used to transform my SRTM dataset:

gdalwarp -s_srs "+proj=longlat +datum=WGS84 +no_def" -t_srs "+proj=utm +zone=51 +datum=WGS84 +units=m +no_defs + geoidgrids =egm08_25.gtx" Lag_SRTM.tif Lag_SRTM_Reproj.tif

Here are the resulting channels after I used the above:


Interestingly, here are the results when I just reproject SRTM to EPSG:32651 without using gdalwarp:

Reprojection Only

What did I miss or do incorrectly?

1 Answer 1


It sounds like you have a vertical datum problem to start with. SRTM elevation values are referenced to the WGS84 vertical datum. The WGS84 vertical datum is a poor choice for hydrography. See this link.

Use an application like vDatum to convert your elevation values to a gravitational based vertical datum for your area of interest. Rerun your analysis and report back if the results are not as expected.

Here is an example of using vDatum to convert a WGS 84 point location/elevation to EGM2008 for some random area in the Philippines. While vDatum will not do conversions on many raster data models it will do a conversion on a geotiff. Convert your surface to a geotiff and try using that against vDatum.

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  • Are there other methods for vertical datum transformation which you would recommend? I'm using the DEM for an area in the philippines
    – Orca
    Feb 7, 2022 at 2:13
  • Have a look at Proj: proj.org/operations/transformations/vgridshift.html#
    – GBG
    Feb 7, 2022 at 5:15
  • May I ask how I can transform the SRTM dataset from its WGS84 datum to that of EGM2008?
    – Orca
    Feb 8, 2022 at 3:37
  • I tried applying proj with gdalwarp, but the result improves only by a tiny bit. The area near the water bodies still aren't delineated properly, as opposed to the mountainous areas.
    – Orca
    Feb 8, 2022 at 7:58
  • Can you update your post to see the code you used with gdal warp?
    – GBG
    Feb 8, 2022 at 17:12

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