[wxqc] Baraometer quality problems

[Lark McDonald]

Although I really am not worried about this, I have an academic question I
would like to ask just to better my understanding of the elevation and
barometric pressure reporting.

I have a new station CW5332 which is using a Davis VP2 which receives its
data via a wireless signal.

According to the topo map my station should be at an elevation of 6578' (as
verified on the CWOP site). The lat/long of the site on CWOP is correct.

http://weather.gladstonefamily.net/site/C5332?tile=10;temperature_date=2006-
02-09;days=3#Data

According to a detailed elevation survey (1 ft contours) conducted by a
survey company last year, the elevation at the site is actually 6554'. I
reported 6559' as an initial elevation, because the sensor suite was located
on a 5ft pole.  I am making the assumption that this elevation is more
correct than the topo map made about 20 years ago. 

Today I learned courtesy of Gary Oldham (thanks Gary) that the barometric
pressure in my system is captured by the console, not the sensor suite.  I
use an Envoy receiver, but assume that this is the case as well. The Envoy
receiver which reports the data is located 470' away from the sensor suite,
at an elevation of 6570' according to the survey (adjusted for actual height
above ground in the office).  According to the topo map, the receiver would
sit at an elevation of about 6595' (again when adjusted for actual height
above ground).

Admittedly the numbers are not far apart in my case, but they could have
been had I located the station in a slightly different spot where the ground
was steeper, or had the distance between the units had been greater, and
that got me thinking.So given this mini case study, I was hoping that
someone could help me understand what things determine the "correct"
elevation, and why.

Two questions:

1) Actual accuracy vs. relative accuracy?  Since most of the surrounding
stations are probably giving elevation based on the same map, should I be
using the topo map as a base, or is actual elevation more appropriate for
the reporting of data?  What should be my reported elevation be in my case?

2) Is elevation used for calculating anything else beyond barometric
pressure, and if so, is the station or the receiver elevation more
appropriate considering the big picture? 

An expiring mind wants to know. Thanks for indulging me.

Lark McDonald
CW5332

-----Original Message-----
From: wxqc-bounces at lists.gladstonefamily.net
[mailto:wxqc-bounces at lists.gladstonefamily.net] On Behalf Of Keith Miller
Sent: Sunday, March 12, 2006 9:45 AM
To: 'Discussion of weather data quality issues'
Subject: RE: [wxqc] Baraometer quality problems

> -----Original Message-----
> From: wxqc-bounces at lists.gladstonefamily.net
> [mailto:wxqc-bounces at lists.gladstonefamily.net]On Behalf Of John 
> Yurciw
> Sent: Sunday, March 12, 2006 10:54 AM
> To: Discussion of weather data quality issues
> Subject: Re: [wxqc] Baraometer quality problems
> 
> 
> Gary,
> 
> Thanks for the info on the location of the sensor, I was not aware 
> that the sensor is in the console, I just assumed that all the sensors 
> were located out in the ISS.
> Then that will make the sensor even higher since I have the console 
> located on the second floor in my house, which would make it another 
> 15-20 feet higher than the ISS sensors. Could that also make a 
> difference?
> 

As a rough rule, 100' of elevation will be about a .1" Hg
(3 mb) difference. There's other factors, but those are easy numbers to
remember that'll get you close.


Keith
--
CW5250


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