I don’t know what they do if one doesn’t exist, but in most cases the city’s elevation is measured at the local airport. For everything else you can fudge it by twenty or thirty, but an airport must know exactly how high it is.
And this altitude measure is at a standard temperature and pressure, because changing those actually alters what an altimeter will read. When flying, before a plane gets near the airport they check the weather data radio and adjust the plane’s altimeter to the proper setting for the local air pressure.
The one who determines the exact altitude of a city, then, is the Federal Aviation Administration, and they measure it at the airport. (Which is, by nature, mostly flat!)
Welcome to the Straight Dope, Keenath. It’s considered polite in the Comments fora here to post a link to the column you’re discussing, so that everyone on the thread can reference it. Don’t worry, almost everyone here has left it off on one of their early threads.
Here’s the link: How is the city elevation shown on road signs and maps determined?
I think that the method you describe would certainly be the way elevation is marked on aviation charts, and is probably as accurate (and probably more so) than any other method. But I think Cecil makes it clear that the highway signs might get their data from less reliable sources, and there is no real standard.
Hope you stick around and join more of the palaver hereabouts.
Shot version: what Saltire said.
Long Version:
Welcome to the SDMB, and thank you for posting your comment.
Please include a link to Cecil’s column if it’s on the straight dope web site.
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Cecil’s column can be found on-line at the link kindly provided by our valued SDMB member Saltire.
moderator, «Comments on Cecil’s Columns»
I would say that they pick some point in the city and go with it. It’s not hard to find the elevation, just use the USGS marker and measure from that. The other problem is that there are a number of different Vertical datums and everyone uses the one that they want. The most recent Vert Datum is from 88, but there are others and that can accout for a large difference in height.
I’ve never heard of a city having it’s own datum, there is a lot of work going into a datum and I don’t see a real reason to do one, the state datum is normally what is used.
Just one more quick thing, airports do not need to be flat, I work for the FAA making aero charts and just today shortened a runway by 800’ with an elevation change of 20 some feet for those 800’.
- The old grass strip at Eagle Nest NM was a lot of things, but level was not one of them. :wally
2)Datum does depend on the end user. Mapping in Las Vegas NV is done on several different datums, same for Fayetteville AR.
- A lot of local maps are made on a local datum supplied by the customer, usually an engineering firm that is requesting the work. With all the compter/digital stuff now a days, it can be converted to most anything you want in the blink of an eye.
What datums are you talking about?