[wxqc] Baraometer quality problems

[Gerry Creager N5JXS]

I can launch into the long explanation of determination and reference of 
vertical, but there are a couple of points.

1.  What's the most accurate (not necessarily the most precise) survey 
data available to you?  You should use that.
2.  Which one is referenced to NAVD 1988 for the vertical datum?  That 
should have better data than an NGVD 1929 survey.  It's possible to 
transform an NAVD88 height above ellipsoid (plus geoid separation) to an 
MSL altitude.

If anyone's interested in the various issues associated with NGVD 29 and 
NAVD 88, I can give that lecture and talk about selection of geoids and 
ellipsoids.

Altitude is used to calculate "altimeter pressure" which is the station 
pressure corrected to show the sea level pressure when an aircraft 
altimeter is adjusted to the known station height.  It does not account 
for the effect of temperature in the correction of station pressure to 
sea level pressure.

To the best of my knowledge Dave, Philip, Russ, et al, do not use 
station altitude to calculate other parameters.

gerry

Lark McDonald 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
> 
> 
> _______________________________________________
> wxqc mailing list
> wxqc at lists.gladstonefamily.net
> http://pond1.gladstonefamily.net:8080/mailman/listinfo/wxqc
> 
> The contents of this message are the responsibility of the author.
> 
> _______________________________________________
> wxqc mailing list
> wxqc at lists.gladstonefamily.net
> http://pond1.gladstonefamily.net:8080/mailman/listinfo/wxqc
> 
> The contents of this message are the responsibility of the author.

-- 
Gerry Creager -- gerry.creager at tamu.edu
Texas Mesonet -- AATLT, Texas A&M University	
Cell: 979.229.5301 Office: 979.458.4020 FAX: 979.862.3983
Office: 1700 Research Parkway Ste 160, TAMU, College Station, TX 77843