fireflybob Point taken about controlled airspace although not all bases are set on airfield QNHs. I was thinking about safety issues when flying IMC, and I accept that we could probably abandon regional pressure settings without a problem. My main point is that the altimeter is a pressure instrument that measures altitudes (or height) assuming a 'standard atmosphere'. It does not, and cannot, measure true altitudes. GPS makes a reasonable attempt at doing this but has to make assumptions about the shape of the earth which will not always be right. I'm not sure whether B or C is the correct answer to the OP, but I don't think A is the right answer unless, very specifically, the altimeter is set to an airfield QNH. The altimeter should then read a true altitude when, and only when, the aircraft is on the ground at that airfield. At all other times it will give a pressure altitude which is usually going to be different from any true altitude.