Who picks a city's longitude, latitude, & sea level?
February 6, 2012 11:45 AM Subscribe
City latitude, longitude, and elevation. Is there an 'official' method and governmental entity for choosing the point in a city that determines its latitude/longitude and its elevation above sea level? How is/was sea level established?
According to Cecil on Straight Dope: The article goes on to list various conflicting answers: posted by filthy light thief at 11:50 AM on February 6, 2012
oo oo, I asked my civil engineer friend this once. He said most cities pick something important in the city, such as city hall or something like that. There is no set rule for determining what gets picked.
posted by magnetsphere at 12:01 PM on February 6, 2012
posted by magnetsphere at 12:01 PM on February 6, 2012
I thought they just used the GPO for the city - that's where the dot on the map goes, at least. or is that just a commonwealth thing?
posted by russm at 12:15 PM on February 6, 2012
posted by russm at 12:15 PM on February 6, 2012
As this guy can attest after losing Who Wants to be a Millionaire on a question poorly formulated for this exact reason, it's pretty much arbitrary.
This is a very strange man who had a First Person with Errol Morris made about him.
posted by cmoj at 12:24 PM on February 6, 2012 [1 favorite]
This is a very strange man who had a First Person with Errol Morris made about him.
posted by cmoj at 12:24 PM on February 6, 2012 [1 favorite]
Ooh ooh! I know part of this. Sea level is very complicated; it has to be defined locally, because the sea isn't the same level everywhere. The specific word you want is "geoid", and if you forgive the self link I have lots of details on my blog.
Latitude and longitude are also tricky to define, although these days pretty much everything is referenced in terms of WGS84. In particular that's what your GPS reports. Lots of older geographic data is in different systems though and they can vary by quite a bit, like hundreds of feet.
I don't know if there's a standard for defining the actual point that's the lat/lon of a city. I think it's customary to use city hall, the county courthouse, or some other surveyor's reference mark. The centroid would be a natural synthetic center.
posted by Nelson at 12:24 PM on February 6, 2012 [2 favorites]
Latitude and longitude are also tricky to define, although these days pretty much everything is referenced in terms of WGS84. In particular that's what your GPS reports. Lots of older geographic data is in different systems though and they can vary by quite a bit, like hundreds of feet.
I don't know if there's a standard for defining the actual point that's the lat/lon of a city. I think it's customary to use city hall, the county courthouse, or some other surveyor's reference mark. The centroid would be a natural synthetic center.
posted by Nelson at 12:24 PM on February 6, 2012 [2 favorites]
I believe "sea level" as far as GPS altitudes care about is the WGS 84 geodetic datum. However, for surveying, I think they don't use an ellipsoid-based datum like WGS 84, but rather a gravity-based datum. In North America, that is NAVD88, which, quoth Wikipedia:
posted by Fortran at 12:25 PM on February 6, 2012 [1 favorite]
It held fixed the height of the primary tidal bench mark, referenced to the International Great Lakes Datum of 1985 local mean sea level height value, at Rimouski, Quebec, Canada.So perhaps, in North America, one can consider the mean sea level in Rimouski in 1985 as "sea level".
posted by Fortran at 12:25 PM on February 6, 2012 [1 favorite]
I thought they just used the GPO for the city - that's where the dot on the map goes, at least. or is that just a commonwealth thing?
I understand that post offices are used by default for measuring the distances between towns, so that might make a default point for specifying the city's latitude & longitude as well.
It's not always the post office or GPO, though. In Sydney, for example, "kilometre zero" is a small obelisk that dates back to 1818, well before the GPO opened in 1874.
posted by UbuRoivas at 1:43 PM on February 6, 2012
I understand that post offices are used by default for measuring the distances between towns, so that might make a default point for specifying the city's latitude & longitude as well.
It's not always the post office or GPO, though. In Sydney, for example, "kilometre zero" is a small obelisk that dates back to 1818, well before the GPO opened in 1874.
posted by UbuRoivas at 1:43 PM on February 6, 2012
Correction: the GPO is used now: "They are such an intrinsic part of our capital cities that all official distances are measured from their doorstep."
posted by UbuRoivas at 1:59 PM on February 6, 2012 [1 favorite]
posted by UbuRoivas at 1:59 PM on February 6, 2012 [1 favorite]
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