where did the custom of putting "elevation" on signs for city names come from?

Answer 1

1. Suggested answer from St. Petersburg
a) It has its origin in the Cold War and civil defense from nuclear attack. Exactly how elevation knowledge helps is not explained.
b) Its use continued for other emergency management purposes, especially evacuation from hurricanes / sea surges. That might be appropriate in Florida, but not elsewhere.
c) A lot of the stated elevations are incorrect. That may make a nonsense of the entire custom.
http://www.sptimes.com/2004/09/24/Southpinellas/Elevation_On_signs__i.shtml

2. This webpage suggests why there is so much variation between the stated elevation, and what is perceived to be the "correct" elevation.
http://www.straightdope.com/classics/a960503b.html

3. There may be some connection with the National Geodetic Survey and its benchmarks, which have hunters (well, it makes a change from train spotting, I suppose).
http://www.ngs.noaa.gov/
http://www.geocaching.com/mark/

4. Maybe you should just go to this webpage, and scroll a short way down till you find "Cuyama town info sign", and then you will see the real relevance of the elevation figure. It all adds up, you see...
http://www.thisisbroken.com/b/signs/index.html