[email protected] 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 In the US there are 4 different (current) notions of height, plus superceded past notions and future notions.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. -- You received this message because you are subscribed to the Google Groups "weewx-user" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/weewx-user/rmia6u2hco1.fsf%40s1.lexort.com.
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