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Incorrect altimetery
John Denker (talk | contribs) mNo edit summary |
John Denker (talk | contribs) (Incorrect altimetery) |
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This section contains known and recorded bugs. It makes no claim to be a complete list, or even to be particularly accurate. | This section contains known and recorded bugs. It makes no claim to be a complete list, or even to be particularly accurate. | ||
==== Incorrect altimetry. ==== | |||
There is evidently at least one serious misconception in the | |||
code that calculates atmospheric pressure, altimeter settings, | |||
et cetera. This can be easily demonstrated: | |||
Park at or near the threshold of runway 33 at Aspen (KASE). | |||
Under standard conditions, observe that the altimeter reads | |||
7820 feet MSL, as it should. | |||
Use the Weather : Weather Conditions dialog popup to | |||
look at the ground-level altimeter setting (QNH). | |||
This is bottom row in the "Alt (inHg)" column of the popup. | |||
Verify that it is 29.92. | |||
Now use the dialog box to change this property. Change it | |||
to 30.92, a one-inch difference. | |||
Go to your altimeter instrument and enter the new altimeter | |||
setting in the Kollsman window. Ideally, this '''should''' | |||
cause the instrument to read the correct altitude once | |||
again, namely 7820 feet MSL. | |||
However, due to bugs, it is likely that you will observe | |||
something closer to 8120. This means the airplane is 300 | |||
feet lower than the altimeter says it is. | |||
In case it wasn't obvious, when flying around near Aspen, | |||
being 300 feet lower than you think you are can be very | |||
bad for your health. | |||
The absolutely correct formula for computing the altimeter setting | |||
can be found in the [http://www.av8n.com/physics/barometry.html jsd barometry page] -- look where it mentions Field Pressure. | |||
==== Altimetry misnomers and misconceptions. ==== | |||
Both the Weather Conditions popup and the atis.cxx code | |||
rely on the "pressure-sea-level-inhg" property and use | |||
it in ways that the altimeter setting should be used. | |||
This is at least a misnomer, and probably a misconception. | |||
The altimeter setting is '''not''' the same thing as the | |||
sea-level pressure. The altimeter setting is something | |||
else; it is properly called the altimeter setting. It | |||
is also properly called the QNH, although private pilots | |||
who fly only in the US may be unfamiliar with the QNH | |||
terminology. | |||
This variable needs to be expunged and replaced with | |||
something else, something with a correct name and | |||
with correct semantics. | |||
==== Z-buffer burn-through. ==== | ==== Z-buffer burn-through. ==== | ||