Wikipedia:Reference desk/Archives/Miscellaneous/2024 July 18
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July 18
editLake Lats and Longs
editIs there an accepted, standard location when finding latitude and longitude for ponds and lakes? Or, is wherever the pond lake first encounter? It is probably changed with GPS and Sats. DMc75771 (talk) 19:47, 18 July 2024 (UTC)
- If there were one, I believe it would be the same for any type of geographic feature occupying an area with a somewhat defined shape, such as an island, a swamp, a salt flat, and so on. A plausible candidate, if one doesn't want to single out a specific feature of the area, is its geographical centre. As stated in a United States Geological Survey document quoted in our article, "There is no generally accepted definition of geographic center, and no completely satisfactory method for determining it." For most purposes, the centroid of the area will usually be satisfactory in practice. I suppose one will want the location to fall inside the area, but if the area is not even roughly convex, for example C-shaped, its centroid may fall outside the area. --Lambiam 09:04, 19 July 2024 (UTC)
- Agreed; the United States Geological Survey used "locations of lake centers in latitude and longitude" in its Digital Data Base of Lakes on the North Slope, Alaska (1986) p. 1. Alansplodge (talk) 11:18, 19 July 2024 (UTC)
- Surprisingly hard to find a definitive statement from a major cartographic agency, but the Natural History Museum, London guidelines for recording the location of species says "It is best to use the geographic centre (the centroid/midpoint of both the latitude and longitude extremes) for the coordinates of named places". [1] Alansplodge (talk) 11:48, 19 July 2024 (UTC)
- This definition means that – in theory – two disjoint lakes can share their centres; look here. --Lambiam 23:50, 19 July 2024 (UTC)
- Your long and curved lake is better suited to the advice for rivers: "Make a straight line from the mouth of the river to the head of the river. Calculate the centre of this line, and place the coordinates closest to the centre of the line on the river itself". Alansplodge (talk) 11:21, 20 July 2024 (UTC)
- Another twist here is that for feature geometries polygons can have holes, and water body features are often modeled with multipoloygons, that is a set of polygons each of which can have zero or more holes. So even for a convex polygon there is no guarantee the centroid or the center of the bounding box are inside the feature. fiveby(zero) 02:03, 20 July 2024 (UTC)
- From Centroid § Properties: "The geometric centroid of a convex object always lies in the object." It is not hard to show that this also holds for the centre of its bounding box. --Lambiam 09:11, 20 July 2024 (UTC)
- A convex polygon with holes is not a convex set, but if there are holes i guess it's not a simple polygon and you can't really call it convex. fiveby(zero) 13:42, 20 July 2024 (UTC)
- From Centroid § Properties: "The geometric centroid of a convex object always lies in the object." It is not hard to show that this also holds for the centre of its bounding box. --Lambiam 09:11, 20 July 2024 (UTC)
- This definition means that – in theory – two disjoint lakes can share their centres; look here. --Lambiam 23:50, 19 July 2024 (UTC)
- Surprisingly hard to find a definitive statement from a major cartographic agency, but the Natural History Museum, London guidelines for recording the location of species says "It is best to use the geographic centre (the centroid/midpoint of both the latitude and longitude extremes) for the coordinates of named places". [1] Alansplodge (talk) 11:48, 19 July 2024 (UTC)
- Agreed; the United States Geological Survey used "locations of lake centers in latitude and longitude" in its Digital Data Base of Lakes on the North Slope, Alaska (1986) p. 1. Alansplodge (talk) 11:18, 19 July 2024 (UTC)
- I'm not sure that there's an actual requirement for perfect mathematical exactitude; if the co-ordinates indicate a point somewhere in the middle of a particular lake, then the job is done. Alansplodge (talk) 11:36, 20 July 2024 (UTC)
- As I wrote, "I suppose one will want the location to fall inside the area", but if one is writing a program for assigning locations to geological features of a known extent, one needs some more precise definition. For example, if the centroid is not inside the area, the algorithm could select the location inside the area as far away as possible from its boundary, and if there are several such locations (for example for the C-shaped lake in my illustration), one as close as possible to the centroid. --Lambiam 13:17, 20 July 2024 (UTC)
- Some rounding to avoid false precision may be appropriate. Giving the position of Lake Superior to arcsecond precision may be excessive, as degree precision is good enough. PiusImpavidus (talk) 08:57, 21 July 2024 (UTC)
- It may be worthwhile to point to WikiProject Geographical coordinates's guidelines on the matter. Elsewhere on that page one finds "For geographical features with an area, such as lakes, reservoirs, and islands, use a point reasonably in the center of the feature. (Remember not to specify too much precision; see Precision guidelines below.)" Deor (talk) 13:55, 21 July 2024 (UTC)