Temperature variations will cause an error but it is very small. The AIM
has a table that lists these errors. At 200' AGL (ILS minimum), you will
get a 20' error if the temperature is -20 C. It becomes 60' when the
temperature is -50C (not a common temperature in the US except may be
Alaska). The error is larger at higher altitudes (as much as 300' at 1000
AGL when the temperature is -50C).





Judah wrote in
:

It begs an interesting question, though...

If an IAP is based on object clearance (in some cases, as low as 100'
clearance, right?), but does not account for temperature variations
and does not actually measure geometric altitude above the ground, is
it not possible that one would find oneself at an altitude that from a
barometric standpoint is legal and correct, but from a geometric
altitude is within the bounds of that tower that would have been 100'
lower if it also changed altitude with the temperature?


Just a thought...

"John R. Copeland" wrote in
:


"smf" wrote in message
news:jpblc.12364$bS1.6242@okepread02...
My Altimeter is accurate when on the ground or at lower alt. When I
climb up to 7k and above and check it against GPS and approach
controls read out it is about 300+ feet off.

Is it the altimeter I'm having problems with?

Steve

It's possible you are not having any problems at all.
Barometric altimetry simply does not measure geometric height.
And most GPSs ignore barometric settings completely.

I'm unsure what you mean by "approach controls read out".
The controller can see only what your Mode C transponder sends.
Usually that reading is to the nearest 100 feet, and it derives from
a barometric sensor in your airplane, permanently set to 29.92 in.hg.

With a correct barometric setting, your altimeter should show the
relevant airport's elevation at its true height, but normally *not*
any other elevation. If your altimeter passes its 24-month 91.411
check, it's surely OK. If it fails its check, though, you do have an
altimeter problem. ---JRC---