I'm a member of mountainviews.ie so familiar with their maps and
information. I do know they have members of the community that have access
to / borrowed high-quality surveying equipment which has resulted in the
comprehensive dataset they have.
I would not be surprised if there have been updates in the past 10 years
regarding elevation, location, etc.
Donal
On Sun, 1 Dec 2019, 13:38 moltonel 3x Combo, wrote:
> On 29/11/2019, Donal Hunt 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.
>
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