How is the city elevation shown on road signs and maps determined?

Dear Cecil: You’re familiar with the green freeway signs that announce when you enter a new city. They always show the population, which we understand, and the elevation, which we don’t. Is the elevation measured at the sign itself (no way), some standard place like city hall (improbable), or is it the average elevation of the town (if so, how does the state of California perform this calculation)? John and Mary Schelling, Orinda, California

Cecil replies:

This is not the scientific process you might think. As far as I can tell nobody publishes an official list of elevations for cities in North America. Highway departments, mapmakers, almanac compilers, and what all come up with numbers for their own purposes, but they use different sources, and their figures don’t always agree.

For example, Denver, the mile-high city, is listed in the World Almanac as having an elevation of 5,280 feet. But if you tap into the Geographic Names Information System maintained by the U.S. Geological Survey (http://geonames.usgs.gov/), you find Denver has subsided to just 5,260 feet.

Town elevations may be the altitude at some prominent public place (Caltrans uses city hall) or they may be an average for the downtown area. Either way they’re often just estimates. Caltrans got its numbers from the U.S. Geological Survey, but the USGS got them by eyeballing the contour lines on maps. OK, it’s not like they grabbed the maps off the rack at the gas station, but jeez, doesn’t anybody do original research anymore?

It’s only when we get into the no-nonsense world of engineering that we start to get some precision. Many big towns have established a “city datum,” a standard elevation pegged to some known point, which is used in blueprints for major construction projects. For instance, on a drawing for an office building, the elevation of the sidewalk in front of the entrance may be marked as “+15′ Podunk city datum.”

Even that’s no guarantee things won’t get screwed up. In 1989 officials in Chicago were trying to dig a pedestrian tunnel connecting city hall with the state office building across the street. The city started at one end, and the state started at the other — and when they met in the middle they found they were nine inches off. (They fudged it with a ramp.)

Turns out ground zero on the city hall blueprint wasn’t Chicago city datum, it was just the bottom of city hall. When people say things are crooked in Chicago, little do they know.

Cecil Adams

Send questions to Cecil via cecil@straightdope.com.