Usually, you can find your altitude at your local planning commission or zoning office, on a webpage about your town or city, or contact your local Cooperative Extension Office. Here is an on-line tool:
US Geological Survey elevation search tool at http://geonames.usgs.gov/pls/gnispublic
A service of the U.S. Geological Survey, U.S. Dept. of the Interior.
From their FAQs: Elevation figures are not official and do not represent precisely measured or surveyed values. The data are extracted from digital elevation models of the National Elevation Dataset for the given coordinates and may differ from elevations cited in other sources, including those published on USGS topographic maps….The elevation figures are sufficiently accurate for most purposes of general information. Efforts are continuously being made to improve the accuracy and resolution of both GNIS and NED data, the results of which will be reflected at both sites.
From their Disclaimer: While USGS makes every effort to provide accurate and complete information, various data such as names, telephone numbers, etc. may change prior to updating. USGS welcomes suggestions on how to improve our home page and correct errors. USGS provides no warranty, expressed or implied, as to the accuracy, reliability or completeness of furnished data. (Full Disclaimer at http://www.usgs.gov/laws/disclaimer.html)
National Center for Home Food Preservation, June 27, 2008.