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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.
Google Earth 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.
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 too large for barometry where errors ideally should not exceed one foot.
Why don't the elevations on the USGS topographcal maps agree with those provided by my GPS system? Which ones are correct? They both might be correct. The reason that the elevations do not agree might be because they are set on different datums. Most maps use NAD27 and the elevations are based on mean sea level. Your GPS receiver uses WGS84 and the elevations are based on the NAD83 ellipsoid. These datum shifts can result in differences of tens of meters horizontal and hundreds of meters vertically. Source: US Geological Survey
To convert from degrees/minutes/seconds to decimal degrees:
(1) divide the seconds by 60
(2) add this decimal to the minutes
(3) divide this decimal minutes by 60
(4) add this decimal to the degrees
Online Converters
Federal Communications Commission
OnlineConversion.com
To determine your true altitude, refer to any of the above maps to find your ground elevation. To the ground elevation, add or subtract the vertical altitude of your barometer cistern's free surface above or below the ground level (or other precise level indicated on the topo map which can be measured locally).
Instructions, examples, and charts of barometer corrections for temperature, gravity, and elevation can be found in the Instruction Manual for 230-7410 and 230-7420 Series Mercury Barometers. (PDF 6.9MB)
A spreadsheet for automatic barometer corrections is provided by Princo Instruments, Inc. (Requires MS Excel or a compatible spreadsheet program.)