Kaptin M-
I believe it's the other way around. If you are reading 1400 and the crossing altitude is published as 1500 then you're 100 ft. to the good (safe side). Assuming the same 100 foot error exists at minimums, then when you're reading 200, you are actually at 300.
The barometric (pressure) altimeter is only calibrated for ISA. If the temperature is colder, then the altimeter will read higher than actual. Bad. That is why Jepp publishes an Altimeter Correction chart. You enter it with two values, the reported temp. and the altitude above the reporting station (airport). I don't have the chart handy, but the magnitude of the error is signifigant. Approximatly 6000' above a -30C airport, the altimeter may be reading about 1000' lower than actual. The PEC pales in comparison.
BTW, I think an Outer Marker is much too inaccurate to see a 20-30 (or even 100') error. If you're using the swing of a Compass Locater, or a DME it will be more accurate.
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