Finding your Latitude and Altitude for Barometer Corrections
The National Map Viewer
offers free access to the US Geoglogical Survey's topographic base map data. US Topo maps are available for free web download from the USGS Store
is free for personal use. You can find your latitude, longitude (not needed for barometer corrections), and ground elevation by entering the address or place name, then zooming in on detailed satellite photos and maps.
You can get your latitude coordinate and ground elevation from many local maps of your area. The most detailed is usually the US Geological Survey 7.5 minute, 1:24,000-scale quadrangle series topo maps, available from local outfitter stores, some bookstores, your local BLM or Forest Service office, and from the USGS Online Store
. Some public libraries may have topo maps available.
To find your ground elevation, look at the wavy never-ending lines on either side of your location. They are lines of constant elevation called contour lines
. Find some with numbers inserted in them. The number is the elevation of that line. In the United States they are mostly in feet, the contour interval
between them may be 10 feet or more, and every fifth perhaps every 50‑foot contour line may be in bold. You may interpolate your ground elevation between the elevations of the contour lines on either side of your location.
Introduction to Topographic Maps
, a tutorial from the GeoSpatial Training and Analysis Cooperative, offers detailed information on understanding and using topo maps.
Commercial GPS (Global Positioning System) devices have typical latitude errors of 10–15 meters which is good enough for the relatively small gravity correction, however the GPS typical elevation errors are much too large for barometry where errors ideally should not exceed one foot.
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Labels: barometric pressure, technical support