On Mon, May 4, 2020 at 9:16 AM Greg Troxel <g...@lexort.com> wrote: > It is a good guess that signs you see are in your > national vertical datum. But some (most?) places have multiple datums, > and it seems very likely that values people have known have been copied > forward on signs for who knows how long, and there's no way to tell > which one is meant. This is true on the US for things like mountains > and "highest point on the masschusetts turnpike" signs -- which lacks a > datum.
They're also likely obtained by looking at contour lines on a topo, and are likely to be 10 m away from the actual value, whatever datum is supposedly in use. (Or they were simply obtained by division of levels, long before satellite geodesy). In that case, the choice of datum is of no significance at all. (But using the ellipsoid is still Just Plain Wrong.) I'd wager that virtually all `ele=*` values in OSM are tabulated only to that level of accuracy. and as long as we say that _some_ modern vertical datum (say, NAVD 29 or newer) is used rather than the ellipsoid, we should be Just Fine. Many peaks in the lists near me (such as Adirondack 46, Catskill 3500) have their elevations quoted as the highest closed contour line on the old USGS topos. Since the summits were used only as vertical controls, their altitudes weren't measured that precisely. Many were used as horizontal controls, and some of the trig points there are used as part of a network that measures continental drift, since we have 150 years of data. The state survey of the 1870s was surprisingly good, and the placement of first-order controls included astrometric measurement of the deflection of the vertical. Obligatory Colvin drawing from the state survey: https://upload.wikimedia.org/wikipedia/commons/9/93/Division_of_Levels_-_Measurement_of_Whiteface_Mountain.jpg Obligatory photo of Kevin's boots at a trig point https://flickr.com/photos/ke9tv/9514568039 -- 73 de ke9tv/2, Kevin _______________________________________________ Tagging mailing list Tagging@openstreetmap.org https://lists.openstreetmap.org/listinfo/tagging
