I'm not sure just how accurate this system is.
I checked 5 sites of family houses/properties and using this site, they are out by 2- 3 meters (to high)?
This is measured against satellite surveys we had carried out by a company based in BK, using a land based system, that receives signals from 5 Geo stationary satellites, way back in 2006.
That system is accurate to within 20mm
I suppose, as a very rough guide, this site is OK but I would not put my trust in it, if I was going to buy land that may, later be prone to flooding. 3 meters, is a hell of a lot of water to find outside your front door after heavy rain.