NASA updated their SRTM data in June 2005. This data is now corrected for water bodies are which were previously very noisy.
I use three resolutions of data here depending upon availability these are 1 arcsecond, 3 arcsecond and 30 arcsecond. I do not understand the Datum for the 30 arcsecond data since the heights do not seem to be as I understand them. Specifically in Ghana there is a river delta that is at sea level or very close. The W020N40.DEM file at 5.83165N 0.6101E shows the height as 42 meters whereas it is actually 0 meters as shown by N05E000.hgt.
I am sure this is because of the datum reference which is shown as WGS84 with units of DD. Does anybody know what this (DD) stands for?