QGIS Watershed Delineation - generated channels and drainage basins are incorrect

Question

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:

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.

Edit:

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:

What did I miss or do incorrectly?

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.