you’ll be able to read the correct elevation right from the GPS results
without further adjustment.
4. With the correct altitude/elevation in hand you will need to determine the
correct offset. To be added to the absolute pressure reading in order to
compute relative pressure (sea level equivalent). Correction tables can be
found on-line in many places. One example is the table found at the web
site at https://novalynx.com/manuals/bp-elevation-correction-tables.pdf.
Locate your elevation in the first column and read the correction in the
third column. This table, however is rather coarse, making it hard to be
precise. An alternative is an on-line calculator such as the one found here:
http://www.csgnetwork.com/barcorrecthcalc.html
For our example of 42 ft. above sea level we input 42 ft. of elevation and
a standard pressure of 1013.25 hPa/mb and press calculate. We find an
“absolute barometer value” that should be -1.5626061222588443 hPa
lower than at sea level. The inverse (because relative pressure is higher
than absolute pressure) of this number will be our “REL PRESS
OFFSET” value. Use the settings procedure to increase REL by +1.6
(nearest rounded value we can input).
5. Now we need a reliable reference for pressure at sea level. Locate the
official identifier for the nearest airport. Refer to “World Airport Codes”
at https://www.world-airport-codes.com or a similar reference. Enter
your location or nearby airport name, and press “Search.” Select the
correct airport from your search results and click on it. For example,
search for “Mountain View” and click on “Moffet Field.”
6. From the resulting page find the ICAO code, if listed. Otherwise use the
IATA code. For the example above, you would find IATA code “NUQ.”
7. Now go to a web site like AVIATION WEATHER CENTER (for US
locations) at https://www.aviationweather.gov/metar?gis=off and enter
the code you found in step 2, and click “Decoded” (to make the next step
easier) before requesting the METAR information. For the example we
would enter “KNUQ” and find a result output like: “30.09 inches Hg
(1019.0 mb) [Sea level pressure: 1019.1 mb]”
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