Does NOAA use NAVD88 or GPS (Ellipsoid referenced) Height for AS Determination?

[ch...@chrismaness.com]

I am nerding out calibrating my barometer today.  I have a handheld
precision barometer that I can take co a vertical survey marker close to
a nearby major airport.  I can then call the automated message at the
tower and get real time AS (altimeter settings) from the airport
barometer.  I just am not sure what height these AS settings are
referenced to.  There are two listed in the marker's datasheet.  Ortho
and Ellipsoid.  Ortho is based on spirit leveling and a better reference
to a hypothetical equipotential surface that is tied to one tide station
in Quebec (NAVD88) or the GPS measured Ellipsoid (a simple oblate sphere
that is supposed to approximate said equipotential surface).  The nice
thing about doing this is that I can precisely know the height of my WX
station at home.  I know it within about 5 feet, but not certain until I
take use an average over time of this AS from the local airport.  I
don't have a super expensive Surveyor GPS to determine it.  I do have a
surveyors tansit, but I am thinking doing a closed lip with it from a
local vertical control is way overkill even though this stuff is fun to
me :D

I hope there is someone that is into surveying nerdiness that can answer
this question.  If not, I will have to email the local skywarn contact
meteorologist and bug him.

Regards,

Chris Maness

[vince]

It's hard to tell from your wording, but I'm guessing you perhaps are wanting to set your station altitude super-precisely (?)

I'd suggest that being off from perfect just a few feet either way isn't going to make any significant difference within the accuracy of a consumer weather station's sensors.

[Garry A Lockyer]

Altimeter settings (aka QNH) are referenced to mean sea level. It has nothing to do with GPS altitude. If a properly set and calibrated barometric altimeter agrees with a GPS it is just a coincidence, some might say a rare coincidence.

Regards,

Garry Lockyer
C: +1.250.689.0686
E: Ga...@Lockyer.ca


> On Dec 24, 2020, at 11:06, ch...@chrismaness.com wrote:
>
> I am nerding out calibrating my barometer today. I have a handheld precision barometer that I can take co a vertical survey marker close to a nearby major airport. I can then call the automated message at the tower and get real time AS (altimeter settings) from the airport barometer. I just am not sure what height these AS settings are referenced to. There are two listed in the marker's datasheet. Ortho and Ellipsoid. Ortho is based on spirit leveling and a better reference to a hypothetical equipotential surface that is tied to one tide station in Quebec (NAVD88) or the GPS measured Ellipsoid (a simple oblate sphere that is supposed to approximate said equipotential surface). The nice thing about doing this is that I can precisely know the height of my WX station at home. I know it within about 5 feet, but not certain until I take use an average over time of this AS from the local airport. I don't have a super expensive Surveyor GPS to determine it. I do have a surveyors tansit, but I am thinking doing a closed lip with it from a local vertical control is way overkill even though this stuff is fun to me :D

>
> I hope there is someone that is into surveying nerdiness that can answer this question. If not, I will have to email the local skywarn contact meteorologist and bug him.
>
> Regards,
>
> Chris Maness
>

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[Greg Troxel]

ch...@chrismaness.com writes:

> I am nerding out calibrating my barometer today.  I have a handheld
> precision barometer that I can take co a vertical survey marker close
> to a nearby major airport.  I can then call the automated message at
> the tower and get real time AS (altimeter settings) from the airport
> barometer.  I just am not sure what height these AS settings are
> referenced to.  There are two listed in the marker's datasheet.  Ortho
> and Ellipsoid.  Ortho is based on spirit leveling and a better
> reference to a hypothetical equipotential surface that is tied to one
> tide station in Quebec (NAVD88) or the GPS measured Ellipsoid (a
> simple oblate sphere that is supposed to approximate said
> equipotential surface).

(I am assuming you are in the US and further in CONUS, since you are
talking about NOAA. While this message is US-centric, almost all of it
has analogs for other countries.)


Glad to see someone being extra nerdy about this! I have been slowly
working on a long-term project to calibrate my barometer. I have a
Davis VP2 and a friend about 40km west has one too. I look at both
pressure values and can see approaching waves, but overall they are
close to zero mean. Each has been calibrated by setting approximate
altitude and then by setting to mostly read the same as local airports.

As you figure out the altitude precisely, you'll end up haing to adjust
the pressure sensor itself, in order to get pressure reduced to Sea
Level to be accurate. However, having elevation close results in a
better situation over time than assuming you are at 0.

In the US there are 4 different (current) notions of height, plus
superceded past notions and future notions.

The official notion of height in the National Sprtial Reference System
is "orthometric height" as NAVD88. That means distance along the plumb
line to a reference surface.. Very loosely, distance above Mean Sea
Level. NAVD88 is not 100% accurately defined vs gravity, as there are
errors in definition across the continent, but the standard approach in
the US is to refer heights to the NAVD88 reference (=0 m) surface. You are correct
that running leveling loops is the way to transfer NAVD88.

Because barometric pressure is supposed to be reduced to Mean Sea Level,
almost certainly in the US, the pressure reference means 0 m NAVD88.

The next notion is ellipsoidal height (Height Above Ellipsoid or HAE) in
NAD83. NAD83 is a 3-dimensional datum fixed to the North American
Plate. There is GEOID18 which is a model relating NAD83 HAE to NAVD88.

Around me (Boston), HAE and NAVD88 are about 29m different which is
huge.

The third notion is HAE in WGS84(G1762), which is essentially the same
as ITRF2014. This HAE is different from NAD83 HAE because the
ellipsoids have a different origin, but all things NAD83/WGS84 are
within about 2m, about 1m horizontally. Compared to anything you can
measure with a barometer, that's identical.

The fourth notion is "WGS84 orthometric height", also called "EGM2008
height', which is distance along the plumb line to the WGS84 reference
equipotential surface, as determined by HAE measured by GPS and the
EGM2008 gravity model. Some object to this because WGS84 is a US
military thing. (While ITRF2014 is not a US thing, I am unaware of ITRF
definining gravity-based height.)

Because there is a notion of international comparison of pressure data,
it's a really good question if NOAA's altitude reference is
WGS84/EGM2008, or NAVD88. However, my basic take is that NAVD88 and
WGS84 orthometric height are within a meter or so -- both intending to
model some average sea level -- and indivdual barometers have
calibration errors, so some sort of calibration/norming is needed and
this absorbs any definitional error.

Put another way, can you measure pressure well enough to establish
absolute heights that are accurate to better than 1m? If so, let me
know how!

Your use of a barometric altimeter to transfer elevation from a
benchmark to your station makes sense. I would measure at home, go to
the mark, meausure, and come home and measure, so you can compare the
before/after average with the mid measurement.

My impression is that the standard approach is to wait until there is a
persistent high pressure with little variance and then compared your
(corrected to sea level) barometer with that of nearby official stations
and then basically apply an offset. I did that and my configured
station height is something that I know believe is off. But I think the
combination of aboslute pressure measurement error and height error is
close to zero.

Only recently have do I have the ability to measure height accurately.
A friend brought over a 25-year-old survey-grade L1/L2 GPS receiver. We
set it up and measured for 30h and processed the data vai the NGS. Now,
one can do RTK with the u-blox F9P, and be similar. So I continue on my
quest for barometer calibration.

[ch...@chrismaness.com]

Great response!

> The third notion is HAE in WGS84(G1762), which is essentially the same
> as ITRF2014. This HAE is different from NAD83 HAE because the
> ellipsoids have a different origin, but all things NAD83/WGS84 are
> within about 2m, about 1m horizontally. Compared to anything you can
> measure with a barometer, that's identical.

I thought WGS84 was a gravity field model.  Not sure -- I just assumed.

>
> Your use of a barometric altimeter to transfer elevation from a
> benchmark to your station makes sense. I would measure at home, go to
> the mark, meausure, and come home and measure, so you can compare the
> before/after average with the mid measurement.

Yes, I made a loop with two benchmarks and my hand held barometer while
comparing to my station's AS (barometric pressure reading). The loop
closed perfectly (within a foot), and arriving back home the altimeter
read 1078ft (328.6m), so I am very confident I now have the correct
ortho height of my WX station.  I held the unit right to the sensor, so
not ground level but sensor level.  I also set the handheld barometer
directly on top of the bench marks and checked my home pressure settings
since I was not further than a quarter mile returning quickly back home.

>
> My impression is that the standard approach is to wait until there is a
> persistent high pressure with little variance and then compared your
> (corrected to sea level) barometer with that of nearby official stations
> and then basically apply an offset. I did that and my configured
> station height is something that I know believe is off. But I think the
> combination of aboslute pressure measurement error and height error is
> close to zero.

I had the same notion, but changed my mind.  I do believe that zeroing
my station to the QC feedback I am getting from Gladstone (CWOP) is
probably the better way to go.  I am going to keep tweaking my station
pressure calibration until my signal is right in the middle of their
signal.  My new WeeWx config goes off (and corrects) station pressure. 
It ignores my AS (barometer) reading. That is probably a good thing
because it looks like my WG1000 just adds a Y intercept to the data
instead of doing the correct altitude adjusted equation.  I am sure the
output is close, but that is not just a linear equation or simple
addition.  I am just going to ignore the airports for now, and when I am
done tweaking to CWOP I am confident that on a high pressure day where
all the airports agree, my station will match those as well.

>
> Only recently have do I have the ability to measure height accurately.
> A friend brought over a 25-year-old survey-grade L1/L2 GPS receiver. We
> set it up and measured for 30h and processed the data vai the NGS. Now,
> one can do RTK with the u-blox F9P, and be similar. So I continue on my
> quest for barometer calibration.

A man after my own heart.  I purchased a spirit level (surveyor's
tansit) so that I could play with benchmarks a few years back.  I also
have a solar filter for it, so that I can measure off of the Sun's limbs
in my reticle.  I learned playing with that thing a compass (even when
corrected for magnetic declination) is a very inaccurate instrument.  I
wondered why the azimuth control benchmarks were always off by wildly
varying readings.  I thought the surveyor's that monumented those
benchmarks were sloppy.  That was until I used the Sun to find true
North.  Those benchmarks were accurate right down to the resolving power
of the vernier on my transit.  Hats off to those guys.  Geodesy is
serious business, and they know what they are doing.  Fascinating
discipline.  I am a physics professor, and that stuff is right up my alley.

Regards,

Chris Maness -- in SoCal.

>