Manifold pressure gauge?

[Alflight]

I was reading that manifold pressure gauges in piston engine aircraft are temprature and alt compensated. Is the reason behind this to allow power settings to be stated in hg in the p charts without have to explain the variation or is there some other reasoning I had not considered?
:) after thought, and if it is temp compensated what is the effect of carb heat for instance? or alternate air? or for example with custom aerobatic aircraft I have seen unfiltered engine airlet's just behind the prop?

[MarkerInbound]

Some of the new FADEC stuff on piston planes may be but from C-182 to CV-440, you climb 1000 feet, you lose an inch of MAP and have to push the throttle a bit to make it up.

Looking at it the other way, what you see is the MAP the engine gets. Doesn't matter if the air was 5 degrees or 45, QNH 950 or 1050, if the gauge reads 25 inches, that's what the engine thinks it's getting. So you could say the reading compensates for the state of the air going in. But without some sort of auto-throttle/auto-wastegate system, it will not correct for variations.