Am Fr., 8. Mai 2020 um 03:26 Uhr schrieb Greg Troxel <g...@lexort.com>:
> The notion of "local" has the same problem, and it is also a poor choice > of words in that in surveying, "local", refers to coordinate systems > established for particular projects or surveys that have no lasting > significance. > the "definition" for "ele:local" (in German language on the English talk page of the tag) seems to be about this: a local datum based on a local reference (i.e. not the same as ele:regional). I am not in any way involved in ele:local, just wanted to point it out. > Basically, either you know what datum an elevation is in, in which case > you can 1) transform it to WGS84 height above geoid or 2) label it > correctly, or you don't know, in which case you should simply *admit > that you do not know*. > ele:regional is about admitting that you don't know > I would also ask: if this is reasonable for height, why isn't it also > reasonable for horizontal coordinates? because we typically have much more horizontal positional information with which we can compare. Errors in horizontal position are more evident because the relation to the surrounding (alignment, reference, angles, axis, proportions, etc.) won't fit. Up to a certain degree (e.g. when other information around is missing, and when accurate positional information is missing) we also do have and tolerate very approximate horizontal spatial information. Greg, you wrote we should convert locally signed regional datum elevation information to the WGS84:geoid reference. In other context, we do not do unit conversions, for example we do not convert speed limits in mph to kph, nor do we convert weight information or maxheight information. I could imagine doing a "reasonable" conversion (precision the same as expected from the original value) if there would be support from the tools (e.g. mobile editors, iD or Josm), but otherwise I would not expect the vast majority of mappers doing this conversion at all in case of a value read off a sign, and even if they did I would expect this conversion step to be error prone (e.g. the mapper won't know which is the original datum). Cheers Martin
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