[wxqc] Baraometer quality problems

[Claude Schoch]

Lark:
USGS topo maps are not going to be 25 feet off.  Your new survey may have 1 
foot contour lines, but unless it was accurately tied back to the nearest 
USGS bench mark, it is not going to show accurate elevations relative to sea 
level.  Your USGS topo map will show bench marks as "BM" followed by an 
elevation. Look around your topo map and you'll see them. At the actual 
bench mark site, will be a concrete monument with a 2 inch brass disc with 
the station name.   These bench marks were extremely accurately set by the 
USGS many years ago.  Everyone of them is part of the US geodetic survey, 
and each one links all the way back to the nearest ocean coast to establish 
the elevation above sea level.
The modern topo maps were made by taking high altitude stereo photos of a 
region.  The photos were calibrated to the bench marks, and using stereo 
viewers, the contour lines were then drawn.

I really doubt the surveyors that did the new survey bothered to run the 
survey several miles back to the nearest bench mark.  The footnotes on the 
new survey should either show the USGS benchmark used, or they didn't use 
one.  Often these new topo's just pick an elevation out of a hat that's 
close and use it since the expense of running a traverse all the way to the 
nearest benchmark isn't worth it.  If the new topo was also done using 
stereo aerial photos (most are these days), then the photo used needed to be 
large enough to cover at least three benchmarks in order to get the photo 
calibrated and "flat" in order to draw the contours.

I would check your new survey carefully before deciding to use it to 
overrule a USGS topo.

Claude,
----- Original Message ----- 
From: "Lark McDonald" <LMcDonald at MCSolutions.com>
To: "'Discussion of weather data quality issues'" 
<wxqc at lists.gladstonefamily.net>
Sent: Sunday, March 12, 2006 6:42 PM
Subject: RE: [wxqc] Baraometer quality problems


> 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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