[wxqc] Baraometer quality problems

[spamfree at pensom.org]

Hi,

Based on calculations I did a while back while writing some code to do barometer conversions  for Vantage Pro stations, I looked at the issue of station pressure vs. sensor pressure and the effect of a difference in elevation between the official station elevation and the elevation of the physical barometer sensor.

To make a long story short, unless the difference is greater than something like 50 or 100 feet, the difference is not consequential. You can account for the elevation difference by calibrating your barometer to a nearby reliable weather station.

Since the barometer values sent to CWOP are already adjusted for elevation, I don't think it's important if your physical barometer is 10 or 20 feet above or below your reported station elevation.

Best regards,
Steve
  
======= At 2006-03-12, 16:42:45 you wrote: =======

>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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