On 29/11/2019, Donal Hunt <donal.h...@gmail.com> wrote: > The latest major revision of WGS 84 is also referred to as "Earth > Gravitational Model 1996" (EGM96), first published in 1996, with revisions > as recent as 2004. This model has the same reference ellipsoid as WGS 84, > but has a higher-fidelity geoid (roughly 100 km resolution versus 200 km > for the original WGS 84).
Hopefully the 2004 revision is close enough to the 1996 one, as it seems that osm is defaulting to 1996. > The tl;dr is that you need to reference which model your peak height is > measured against. EGM96 is preferred (I'm assuming) because it has higher > definition and more refined model. AIUI from the wiki and taginfo, in an OSM context, the naked "ele" tag refers to EGM96, and there are subtags like "ele:wgs84" or even "ele:local" for "whatever's printed on the local signpost". I've figured out the overpass query I had in mind: https://overpass-turbo.eu/s/OCb but it finds basically every peak we have with an elevation. See also https://help.openstreetmap.org/questions/15397/elevation-of-irish-mountains-why-the-discrepancy-osmwikipedia which complains the the issue ireland-wide, and notes that the osm data in other countries is fine. All the mountainviews imports basically have ele:local/ele_local/ele:tm75 (3 keys with first value) and ele/ele:wgs84 (two tags with second value). I couldn't find a definition of the "tm75" coordinate system that we got from the mountainview export, I'm guessing this is an irish-specific coordinates system, and I have doubts that it is equivalent to the egm96 that we would like. At this point, I have a few questions: * Are the "tm75" values usable for the naked "ele" tag ? * If not, can we convert tm75 to egm96 ? * As a bonus, I think we should remove "ele_local" where we already have "ele:local" * Is it ok to edit 790 as a single changeset (once we agreed with the changes) or do we want to check each individually ? * Second bonus (probably for a second stage): is there's any point in keeping the iemv:* tags ? Might have been a requirement for the import ? It was done in 2009, maybe it's time to see if they have updated data. Cheers. _______________________________________________ Talk-ie mailing list Talk-ie@openstreetmap.org https://lists.openstreetmap.org/listinfo/talk-ie
