Author Topic: Determining Heights for PWSs  (Read 1931 times)

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Offline kbellis

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Determining Heights for PWSs
« on: January 01, 2019, 11:12:50 PM »
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Who wants to know the height of what exactly, and in which datum?

Many online weather services want to have a little box filled in that's related to height when registering a weather station, but these sites use inconsistent language site to site, sometimes even what seems like inappropriate terminology, and in general, give little in terms of an explanation of exactly where on what bit of hardware they're needing an elevation; e.g., individual sensor's height, grade, etc., and then there's the totally neglected subject of datum, something which we all need to be paying attention to in advance of the coming purely gravimetric modeling of the geoid.

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Offline Mattk

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Re: Determining Heights for PWSs
« Reply #1 on: January 02, 2019, 03:57:40 AM »
So what are you trying to say? So far a lot of words without really saying anything, especially when we have been working with geoids and modelling gravity in relation to ellipsoidal and orthometric heights for many years.

Offline kbellis

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Re: Determining Heights for PWSs
« Reply #2 on: January 02, 2019, 04:56:42 AM »
I'm curious what you think about determining height values for the purposes of weather stations, and in particular regard to online services that you report.

Specific vertical datum, including the next height system for the US, should also be considered whenever talking about heights.


Offline kbellis

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Re: Determining Heights for PWSs
« Reply #3 on: January 03, 2019, 01:11:13 AM »
In a nutshell, a discussion of any measurement, not from the perspective of meteorology, but rather from that of a metrology, I believe is worthwhile. I also think it worth talking about in the same discussion; the resolution of the measurement, or the instrument's least count. Back to the matter of what height value any online weather service is actually asking the user to provide, I'm still scratching my head. :?

Offline Weatheroger

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Re: Determining Heights for PWSs
« Reply #4 on: January 03, 2019, 02:19:59 AM »
Uh, far out man  :shock:
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Offline Mattk

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Re: Determining Heights for PWSs
« Reply #5 on: January 03, 2019, 02:52:39 AM »
Ok so what type of heights do weather providers request or specify, relative to what?

Offline kbellis

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Re: Determining Heights for PWSs
« Reply #6 on: January 04, 2019, 09:13:08 AM »
Ok so what type of heights do weather providers request or specify, relative to what?

Hi Matt,

That's what I've been wondering too. Starting this thread is a first blush at trying to understand this subject. With the now 2-week long shutdown of about 1/4 of the federal government, this may not have been the best time to try digging into what standards various entities, including NOAA, have pertaining to height related metadata on their equipment. My inquiries to NWS were responded to with a muted, "Due to the government shut-down we are only allowed to answer mission critical questions.  Please get back to us once the shut-down has ended.". Writing to the FAA concerning our local AWOS so far has only yielded a null response.

In a response to my writing to a friend of a friend, and who is retired meteorologist, Rick wrote back in part:

"To get halfway decent barometric pressure measurements,  one needs to know the altitude of the pressure sensor.  Probably to the nearest 5 ft to 10 ft depending on the quality of the weather station.   I generally use a USGS topo map to determine that.  Some libraries have topo maps, but suspect many donít.   One can also get a fair idea using a hand held GPS, but the vertical component of GPS isnít great unless the device is of very high quality therefore one reason I rely on the topo maps.   There are other sources besides the topo maps.  For weather stations I generally want the pressure to be reduced to mean sea level and I use millibars or hecto pascals for units.  Depending how on far the station is going to be from Trenton Airport, you can use the latest pressure reported by the Bar Harbor FAA weather station in either inches of HG or millibars.  It is better to use that if it isnít windy Ė slacker pressure gradient.  It can be found at:

https://w1.weather.gov/data/obhistory/KBHB.html

You sound like a scientist or engineer with your comments/map about the orthometric height change.  In all honesty,  for these type of simple weather stations, I wouldnít worry about that level accuracy.  It is an interesting point however and probably I should be a bit more rigorous and take into consideration such changes for my network of weather stations.

I think you need to decide which parameters are the most important for you to obtain good measurements from and perhaps what  the data is going to be used for.  For example, if you want temperature data which can be compared to other stations including those operated by the NOAA or the FAA, then you need to try to follow the exposure standards they use.  That isnít too easy to find in this heavily forested state.  For e.g., you want the sensors out in the open and not under a tree or next to a building or pavement.  The same applies  for wind and precipitation, but if oneís instruments are pretty simple and the data requirements arenít too rigorous, then sensor exposure isnít too critical.   I know quite a lot of sensor exposure and can share some of the standards if it is needed.
"



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The NWS shows the local AWOS at our county airport displaying its elevation as 82 ft. The airnav listing shows it to be 83.2 ft. Both of these values; i.e., 82 and 83.2, 1) don't identify to which vertical datum these elevations relate; 2) and are spatially higher than the little building housing the AWOS. Grade near the building is about 71.5 ft NAVD88. Given that wind speed and direction are critical to pilots, these heights might therefore pertain more likely to the anemometer than the barometer.

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A couple of days ago, I brought the Ambient Weather WS-2902A display console down to the airport to check its barometric reading against the nearby AWOS located about 3200' to the north of the public air terminal.

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The day and time turned out to be perfect with a large steady mass of high pressure which only rose 0.01" over the course of the 90 minutes of observations.

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Offline Mattk

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Re: Determining Heights for PWSs
« Reply #7 on: January 04, 2019, 06:39:45 PM »
The actual orthometric height of the anemometer is irrelevant, the main weather criteria with this being the height above ground level and relativity to surrounding obstructions.

Barometric height is typically relative to Sea Level pressure or Station height pressure, this being typically referenced to/from mean sea level. NAVD29 was essentially referenced to sea level and this could be an adjustment of multiple sea level recording stations or as determined from even a single station, levels were then determined physically by geodetic levelling, level and staff along thousands of miles of network then adjusted. As more and more physical levelling was completed over the years new adjustments were possible but the mean sea level origin remained basically the same even though in more recent times GPS methods has been able to highlight anomalies in the levelling and even in the mean sea level determination due to a whole range of things way way beyond the space to include here. Mean sea level heights between warm and cold waters across a country can account for differences of up to 3 feet.

With regard the difference (1-2 feet) between say NAVD29 and NAVD88 then this is no big deal in relation to the height setting of a Barometer, essentially it is still a relationship relative to mean sea level but one that has become more refined over the years with additional data available today.

As for GPS derived heights , just be a little careful with these as this is where quite large differences can become confusing as GPS (ellipsoid) height can be quite different to orthometric type heights by +/- 100 metres or so depending on your location on the globe. Geodetic GPS receivers can determine ellipsoid heights at the mm level as the ellipsoid is a pure mathematically figure where as the local orthometric heights are rather undulating for quite a number of reasons relative the ellipsoid and this is where determining local orthometric type heights from GPS ellipsoid heights  is quite complicated but then most of the mathematics and modelling is generally already done and knowing the location (Lat/Long) the difference between ellipsoid and Geoid can be determined by model interpolation and quite reasonable orthometric heights can be determined within 3cm in some cases but dependent on the data behind the geoid model.

But these GPS derived heights can only ever be as good as the equipment being used and general civil handheld type GPS is 2-3 times less accurate in the vertical than the horizontal as well as using a generic global type geoid model that is more in the ball park when compared to map type approx. heights and contours etc.  A handheld GPS doesn't have the means to determine what height datum is what, better to use an official map or chart for heights relating to barometer settings, that is of course unless you have access to $25,000 GNSS receivers                                 
« Last Edit: January 04, 2019, 06:44:37 PM by Mattk »

Offline CW2274

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Re: Determining Heights for PWSs
« Reply #8 on: January 04, 2019, 07:11:39 PM »
Ok so what type of heights do weather providers request or specify, relative to what?
A couple of days ago, I brought the Ambient Weather WS-2902A display console down to the airport to check its barometric reading against the nearby AWOS located about 3200' to the north of the public air terminal.
The day and time turned out to be perfect with a large steady mass of high pressure which only rose 0.01" over the course of the 90 minutes of observations.
As I stated in another thread for another member, you're comparing your non-altimeter capable console against an altimeter setting. Nothing "wrong" with that, just don't expect the accuracy to continue, especially since you set it way up at 30.30". By the time you get to "standard" pressure at 29.92", and then even lower, I'm betting it'll be considerably off and getting worse as the pressure drops further. 

Offline Old Tele man

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Re: Determining Heights for PWSs
« Reply #9 on: January 04, 2019, 07:24:34 PM »
REITERATION:  Elevation is your 'ground level' above mean sea level, so a console sitting on a shelf 5' above your 'ground level' is their combination which is what you enter for barometric height, because the barometer sensor is located inside the console, NOT up in the anemometer.
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Offline kbellis

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Re: Determining Heights for PWSs
« Reply #10 on: January 05, 2019, 11:21:15 PM »
As I stated in another thread for another member, you're comparing your non-altimeter capable console against an altimeter setting. Nothing "wrong" with that, just don't expect the accuracy to continue, especially since you set it way up at 30.30". By the time you get to "standard" pressure at 29.92", and then even lower, I'm betting it'll be considerably off and getting worse as the pressure drops further.
Thanks for letting me know. I want to understand this further and what you mean by my non-altimeter capable console. I honestly thought I was following the instructions from the WS-2902A users manual, but now must give pause and study what you said. Which thread were you referring to in regards to the other member?

Kind regards,

Kelly

Offline kbellis

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Re: Determining Heights for PWSs
« Reply #11 on: January 05, 2019, 11:35:00 PM »
REITERATION:  Elevation is your 'ground level' above mean sea level, so a console sitting on a shelf 5' above your 'ground level' is their combination which is what you enter for barometric height, because the barometer sensor is located inside the console, NOT up in the anemometer.

Thanks for your reply.

As a land surveyor, I'm more accustomed to the term "elevation" being applicable to every thing which has a vertical spatial component; e.g., elevation at grade, finished floor, height of instrument, etc., I think maybe that's why I was asking in the first place plus the vaguely worded sites asking for a number. More on that in the morning.

Kind regards,

Kelly

Offline CW2274

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Re: Determining Heights for PWSs
« Reply #12 on: January 06, 2019, 12:00:22 AM »
As I stated in another thread for another member, you're comparing your non-altimeter capable console against an altimeter setting. Nothing "wrong" with that, just don't expect the accuracy to continue, especially since you set it way up at 30.30". By the time you get to "standard" pressure at 29.92", and then even lower, I'm betting it'll be considerably off and getting worse as the pressure drops further.
I want to understand this further and what you mean by my non-altimeter capable console. I honestly thought I was following the instructions from the WS-2902A users manual
I'm sure you are. The thing is, the altimeter vs SLP are different, not vastly, but different. If you'll look at my Meso site, you'll see three pressures, station, SLP, and altimeter.
https://mesowest.utah.edu/cgi-bin/droman/meso_base.cgi?stn=C2274&unit=0&time=LOCAL&product=&year1=2018&month1=2&day1=15&hour1=12&hours=24&graph=0&past=0

Your console is meant to calculate SLP, not the altimeter. You can use the altimeter just fine (kinda), but set it closer to 29.92". As the pressure gets further from that, the error of SLP vs altimeter becomes more evident, obviously because you've set the altimeter from the airport, not SLP.

Offline Mattk

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Re: Determining Heights for PWSs
« Reply #13 on: January 06, 2019, 02:18:58 AM »
.... obviously because you've set the altimeter from the airport, not SLP.

Then you would be more referring to QNH?

Offline CW2274

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Re: Determining Heights for PWSs
« Reply #14 on: January 06, 2019, 03:27:34 AM »
.... obviously because you've set the altimeter from the airport, not SLP.
Then you would be more referring to QNH?
More reference, no. Same thing, just semantics.

Offline Mattk

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Re: Determining Heights for PWSs
« Reply #15 on: January 06, 2019, 04:12:49 AM »
.... obviously because you've set the altimeter from the airport, not SLP.
Then you would be more referring to QNH?
More reference, no. Same thing, just semantics.

Pretty much why I think some are going just a tad overboard in regard barometric pressure as far as weather is concerned, some of the semantics are irrelevant for the purpose.

Offline CW2274

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Re: Determining Heights for PWSs
« Reply #16 on: January 06, 2019, 05:03:02 AM »
.... obviously because you've set the altimeter from the airport, not SLP.
Then you would be more referring to QNH?
More reference, no. Same thing, just semantics.

Pretty much why I think some are going just a tad overboard in regard barometric pressure as far as weather is concerned, some of the semantics are irrelevant for the purpose.
Your opinion.
Just to be clear, the semantic is QNH and the altimeter are the exact same thing, SLP and the altimeter are not.

Offline Mattk

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Re: Determining Heights for PWSs
« Reply #17 on: January 06, 2019, 05:16:19 AM »
.... obviously because you've set the altimeter from the airport, not SLP.
Then you would be more referring to QNH?
More reference, no. Same thing, just semantics.

Pretty much why I think some are going just a tad overboard in regard barometric pressure as far as weather is concerned, some of the semantics are irrelevant for the purpose.
Your opinion.
Just to be clear, the semantic is QNH and the altimeter are the exact same thing, SLP and the altimeter are not.

Which is why it would be handy for you to explain why there is any real need to have any interest in altimeter etc as far as weather observations is concerned?

Offline CW2274

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Re: Determining Heights for PWSs
« Reply #18 on: January 06, 2019, 05:42:35 AM »
Simple. The altimeter setting is what every media outlet in the country uses as it comes from airport ASOS/AWOS, ect., which are practically all used as official measurements, let alone the fact it's what I issued as a professional for 30 years. Makes comparisons very easy.

Offline Mattk

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Re: Determining Heights for PWSs
« Reply #19 on: January 06, 2019, 06:12:07 AM »
Gee some days some really try and confuse the crap out of something that should be really simple, now you are saying media outlets are interested in QNH? NO wonder some get a tad confused with barometric pressure which for most purposes all people want to know is , it is steady or it is rising/falling and would that be fast or slow, everyday people listening to the media don't need the QNH area forecast, they ain't interested and wouldn't know what to do with it in any case

Offline WeatherHost

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Re: Determining Heights for PWSs
« Reply #20 on: January 06, 2019, 07:25:15 AM »
I put mine up yonder a bit.

And I don't embed 18 quotes in my posts.

Banners, tags, widgets, etc. are why I have Signatures turned off.

Offline kbellis

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Re: Determining Heights for PWSs
« Reply #21 on: January 06, 2019, 09:04:29 AM »
I put mine up yonder a bit.
:lol:
yonder_a_bit ft?

Offline kbellis

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Re: Determining Heights for PWSs
« Reply #22 on: January 06, 2019, 09:08:33 AM »
Then you would be more referring to QNH?

Cool! An altimetry reference! Query: Nautical Height - maybe good for a different thread? Though it does sound interesting to this newbie! Thanks Matt.

 

Offline kbellis

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Re: Determining Heights for PWSs
« Reply #23 on: January 06, 2019, 09:10:15 AM »
More reference, no. Same thing, just semantics.

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Offline kbellis

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Re: Determining Heights for PWSs
« Reply #24 on: January 06, 2019, 09:16:53 AM »
If you'll look at my Meso site, you'll see three pressures, station, SLP, and altimeter.
Hmmm, I'm seeing four:
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anything