On 02/15/2007 04:55 PM, Alex Perry wrote:
> More generally: It is always very important to distinguish between the facts
> that arise from the
> simulation of the planet (such as SLP and variation), and the facts that
> arise from simulation of
> the airspace (such as QNH and VOR alignment).
Yes.
> There tends to be fairly good correlation between
> the two, because that makes engineering sense, but the differences are
> routinely enough to kill
> people.
That's 100% true.
One slight suggestion: I have recently started avoiding the term SLP.
In most cases it is possible to substitute
Psl := Pressure at Sea Level
and thereby reduce the potential for confusion.
The reason is that in some places (such as the Remarks section of
a METAR) it is possible to find the term SLP used as an abbreviation
for Reduced Sea Level Pressure. (You might think that would be
abbreviated as RSLP, but I guess the R is silent.) In fact the
METAR is referring to the "field pressure reduced to sea level
along the ISA contour". Under ISA standard-day conditions, the
METAR SLP is equal to the Psl ... but this is a special case not
representative of the general case.
For details, see
http://www.av8n.com/physics/altimetry.htm#sec-psl-slp
=============================
By way of status report, I'm moderately far along in moving the
atmosphere-modeling code out of altimeter.cxx and into its own
FGatmodel object that can be used by other instruments.
I totally agree that we need to maintain the conceptual distinction
between modeling the planet and modeling the instrument. However,
there is a huge amount of commonality between the two tasks, so
it looks like the altimeter class and the atmosphere-model class
will be /derived/ from a common ancestor.
I'd like to tell you I am XX% of the way along in this task, but
the problem is going to be teaching environment.cxx to use a
model of the air-column that does not violate Newton's laws.
I've got the model, but retrofitting it into the FG structure may
not be smooth sailing. The problem is that environment.cxx offers
a baroque interface. It even has a function to set an arbitrary pressure
at an arbitrary point in the air column. In almost all cases, this
would contravene the laws of physics. This /particular/ function
is easy to remove, since AFAICT it has never been called ... but
if certain of the other interface functions are actually being used,
things could get ugly.
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