In regard to "exact" accuracy, spotted this article a few weeks ago in regard to the Japanese earthquake:
https://japannews.yomiuri.co.jp/society/noto-peninsula-earthquake/20240111-161375/ 70cm / 28" to 2m / 6'6" horizontal & 3cm / 1.25" to 1.3m / 4'4" vertical movement! :-( Thanks Graeme On Mon, 29 Jan 2024 at 11:39, Kevin Kenny <kevin.b.ke...@gmail.com> wrote: > > > On Sat, Jan 27, 2024 at 7:06 PM Greg Troxel <g...@lexort.com> wrote: > >> As someone not happy about the deprecation of mailinglists, a few brief >> comments here: >> >> First, I think this proposal is fine, as documenting widespread >> practice. Regardless of my further comments, I think it's solidly >> progress to adopt it. >> >> While yonur comments about survey feet are valid, modern elevations >> (NAVD88) are as far as I can tell actually in meters, and when >> expressed in feet, in international feet. Elevations are small enough >> that 2 ppm is less than the errors in the values. >> >> I would expect the proposal to give an example. It seems that one >> would have a tag >> ele=6288 ft >> for Mount Washington (showing my East Coast bias). >> >> It would be good to explicitly state that in keeping with convention, >> ft means international feet, perhaps with a parenthetical comment that >> if someone meant US Survey Feet they would have written ftUS. Maybe >> this is already documented. >> > > As far as I know, Survey Feet are used only for horizontal measurements, > mostly in state plane coordinates. And I don't think that there are yet any > elevations, anywhere, for which precision to 2 parts per million matters. > > >> Further, WGS84's first height definition is ellipsoidal height, and >> that simply is not elevation. Obviously elevation should be in "WGS84 >> Orthometric Height", which is what GPS receivers provide as elevation. >> But elevations are not published in WGS Orthometric Height; they are >> published in a national or regional datum which is pretty close, as >> all datums at least roughly target a similar origin. >> > > In newer datums than WGS84, even horizontal position is reported relative > to an epoch, with corrections for phenomena like continental drift. I've > hiked to one of the stations used for such geodesy - a copper bolt placed > in 1870 in truly ancient and stable rock (Hawkeye Gneiss, ~1.15 Ga) with > accuracy that would have been considered first-order even a century after > its placement. > > The difference between survey feet and international feet for horizontal > coordinates is significant, because in UTM coordinates, the 2 > part-per-million error adds up to 20 metres in the polar regions (or 10 or > so in the mid-latitudes). It's somewhat less significant over the smaller > range of state plane coordinates, which is why most states don't plan to do > anything about it until 2025 at the earliest. In a few 'bad' planes like > Nevada North and Michigan East, the origins are far enough outside the > state that the errors can be about 15 m. > > Many "authoritative, official" data sets that I've worked with appear to > suffer from mixed horizontal datums, with NAD27 coordinates mistakenly > transferred over into WGS84 uncorrected. When working with county tax maps, > for instance, I always try to spot-check things like street intersections > or monumented corners, to make sure what datum I'm working with, because > it's often not what the map claims to be. Sometimes the error is backward - > the county has never made the conversion of its systems off of NAD27, but > WGS84 data have crept in! > > But for vertical coordinates, I'm willing to wager that no mapper has the > technology to measure absolute elevation to less than a mm. In fact, I > doubt that it's even meaningful to discuss that sort of precision in > elevation. The geoid isn't defined to that accuracy. So I can easily live > with the ambiguity in which 'ft' are used. (Moreover, I can't think of any > OSM tag at the moment in which a measure in feet would need a few parts in > 10**-6 precision.) > > I'd certainly approve of a proposal optionally to tag elevations with the > vertical datum used - but since the differences are typically on the order > of 1-2 metres, I surely don't insist on such a tag! I understand that most > GPS units use GRS80+EGM1984, but I'm sure that NAVD88 has crept in, even > among objects that I've mapped. And I betcha there's a ton of uncorrected > NGVD1929 in TIGER. For a metre or two, who cares (yet?). > > But I'll roundly condemn anyone who confuses height-above-ellipsoid with > elevation! (I'm looking at you, Android Location Services!) > > Changes in vertical datum together with LIDAR data have wrought confusion > among some of the hiking clubs around here, who maintain lists like the > Adirondack 46'ers (the 46 summits in the Adirondacks thought at the time of > the list's compilation to exceed 4000 feet elevation) or the Catskill > 3500's (the 34 summits thought at the time of compilation to exceed 3500 > feet, plus Leavitt Peak - which had escaped the notice of the people who > compiled the list, and minus Doiubletop and Mt Graham - which are now > off-limits to hikers). The uniform decision of the clubs was to ignore the > problem and simply say that the lists are by now traditional - it's _these_ > summits, and no others. Most Adirondack 46'rs also climb Mt MacNaughton, > which was not on the list, but was revealed in a 1953 survey to exceed 4000 > feet. But then the change of vertical datums to NAVD88 put it below the > 4000 foot threshold again, and it's listed as 3,983 feet. Four of the > original list also fall short of 4000 feet but are still required for the > award. > > -- > 73 de ke9tv/2, Kevin > _______________________________________________ > Tagging mailing list > Tagging@openstreetmap.org > https://lists.openstreetmap.org/listinfo/tagging >
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