Looks like you've got the elevation for your location nailed down if you've
gotten it down to 10cm. For us ordinary folks we have to rely on topo maps
or google earth ( or similar map reference tools) to get down to 1 meter
accuacy. Not all countries have gone digital for surveys quite yet.
Here is some general concepts to consider when you are calibrating/setting
your barometer:
1. *Absolute pressure **(A**bs**)* is the current air pressure/atmospheric
pressure measured at your elevation.
2. *R**elative pressure **(R**el**)* represents what the atmospheric
pressure would indicate if your station was theoretically located at sea
level. This is done because it provides a common barometric reference for
all weather stations regardless of their elevation.
For our weather stations we usually assume station pressure = atmospheric
pressure (raw pressure) measured/observed at the elevation of sensor.
The term "relative pressure" is not used so much these days but this is the
SLP you see in METAR reports. Basically, The station pressure is
theoretically "reduced" down to sea level elevation. Note: it sis easy to
get confused here with the terminology. You are not reducing pressure you
are reducing elevation to mean sea level. Yes, high tides, low tide is
taken in account.
The calibration process is usually two steps.
1. correct for sensor error
2. set your barometer to your elevation.
For a Davis VP2, SLP is already calculated for you in the console so all
you need is the elevation of your sensor. Hwever you still need to check if
your barometric sensor is accurate or not, The BMP280, BME680 are accurate
to 1 hPa and so is the VP2 barometric sensor. The newer Bosch barometric
sensor (BMP390) is accrate to 0.5 hPa.
You could calibrate your VP2 with the higher spec BMP390. Just match the
pressures and you are done! or alternatively you can match Altimeter with a
close-by METAR.
So no, we will not be achieving 0.05 to .10 hPa accuracy WMO standards with
our consumer grade personal weather stations!
For Ambient Weather/Ecowitt weather stations, SLP or Altimeter (setting) is
*estimated* by using a fixed offset ( a fixed amount correction) to
Absolute pressure/station pressure.
Keep in mind that different countries use different pressures. Most
countries don't use SLP or Altimeter - they use QNH in whole integer
amounts which makes matching difficult as you would have to match 1013,
1014, 1015, etc. If you luck out, some countries publish decimal QNH like
the BOM in Australia.
I couldn't answeryour questions regarding digital survey/elevation but none
of the sensors we're talking about come even close to 10 cm accuracy. as
these sensors have "noise", drift, , etc. You can only do so much with a
$10 sensor. but yeah, it can be a rabbit hole for sure.
On Saturday, May 28, 2022 at 10:37:02 AM UTC-4 Greg Troxel wrote:
>
> tl;dr: What elevation is barometric reduced to? 0 meters in NAVD88?
> Some other height datum? What about other countries, WMO, etc?
>
> I'm going down the rabbit-hole of barometer calibration. I'm trying to
> address it in two parts: calibrating station pressure, and then using
> the correct altitude. This is part trip report and part question.
>
> I realize that my question has close to zero practical value. But nerds
> like to measure things correctly anyway.
>
> For station pressure, I have a VP2, a Brunton "Atmospheric Data Center"
> that measures pressure (also T/H), a BME280, and a BME680.
> Provisionally treating the Brunton as a correct reference (because it's
> portable, so I can move it to my various sensors and at some point take
> it to visit other people's stations), I find the following offsets:
>
> VP2 +0.1 hPa
> BME680 +2.5 hPa
> BME280 +1.5 hPa
>
> It's encouraging that the Brunton and VP2 are so close (resolution 0.1
> and 0.01 hPa respectively), but of course they could be off the same
> amount. It is clear that that BME280 family isn't that accurate, but
> that's no surprise given the cost and intent. However the offsets are
> pretty stable.
>
> For reference
>
> - a height change of 1 meter corresponds to 0.12 hPa. (That's 0.09
> mils of Hg per foot :-)
>
> - WMO seems to specify 0.1 hPa as the required uncertainty for field
> instruments.
>
> For altitude, I borrowed some 25-year old survey-grade GPS equipment,
> and am in the process of measuring from that mark to where my console
> is. The data is natively in "NAD83(2011) epoch 2010.0 Height above
> Ellipsoid", but I can transform to NAVD88 or to "WGS84 Orthometric
> Height". I expect to end up with a height value that is accurate to
> about 10 cm in HAE and not too much more in orthometric heights.
>
> My questions are:
>
> - When people talk about reducing a station pressure reading to "sea
> level", what do they mean?
>
> - Specifically, what does NWS mean? Given the overall US government
> approach, I would guess they mean "0m elevation in NAVD88".
>
> - What is the situation with other countries' national weather
> organizations?
>
> - What is the situation with the WMO? It seems there is an idea that
> each country might specify a height reference more precisely and is
> to report it. There is a hint of EGM96 in CWOP-WM08 -- but EGM96
> has long since been superceded. Hence mentioning "WGS84 Orthometric
> Height" above.
>
> I would appreciate not only an answer but a pointer to where NWS, other
> national agencies, and WMO specify this.
>
> (As a side note for people who find my question boggling, "sea level" is
> a concept that really only applies to a particular tide gauge station,
> and the elevations of different stations' zero points are different.
> Referring to "sea level" in a general way introduces fuzz of around a
> meter.)
>
> Greg
>
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