What is The difference between Decision Altitude and Decision Height?

Answer 1

Altitude is measured above sea level, Height is measured above ground level.

An example would be an airfield that is on a hill would have and altitude of however many feet it is above sea level, where as it would have a height of 0ft, no matter how far above the sea level it is.

Now a Decision Altitude is used for a precision approach ie an ILS or MLS. So for example the standard minimum Decision Height (ie above the airfield) for a Cat I approach is 200ft. Say for example that airfield was itself on a hill that was 500ft above sea level the DH would remain 200ft but the DA would be 500ft + 200ft = 700ft.

If you had two altimeters on the flight deck one set to QNH ( the pressures setting at sea level) and one set to QFE (the pressure setting at the airfield) when you arrived at the Cat 1 minima the QFE altimeter would read 200ft and the QNH altimeter would read 700ft.

Now if the question is actually what is the difference between Decision Altitude (DA) and Minimum Decision Altitude (MDA) the answer is sightly different. A DA is as answered above and is used on a Precision Approach (ie ILS) and the MDA is used on a Non Precision Approach (ie Loc, GP, VOR, NDB)