I have three DEMs of our country which are spot height DEM, SRTM DEM and ASTER DEM.
SRTM has 90m resolution and ASTER has 30m resolution.
My supervisor told me to delineate watershed using these three DEMs separately, and also using merged DEMs.
What is the main differences between the three types of DEM that I mentioned above? How do I merge these three DEMs? What is the most suitable DEM to delineate watershed?