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#1 2013-11-08 01:16:20

openzzz
Member
Registered: 2013-10-03
Posts: 215

Irish Mountains

Hi,

did you notice that on the OSM map the elevations of Irish mountains
do not match the usual "above mean sea level" (MSL) values?
In Wikipedia you will find the MSL values for elevation.
But on the OSM map there are "WGS84 elliptical heigths" values
on the mountain peaks, which differ a lot from MSL.

In the "development" forum I asked about how we should
interpret the definition of the "ele" Tag in the Wiki.
I think many mappers don't know about the difference
between the WGS84 geoidal and ellipsoidal heights.

At the moment, Irish mountain peaks are mapped in a different
coordinate system as in other countries.
I hope we could harmonize "ele" for the OSM globally.

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#2 2018-05-29 11:25:09

CharlieJeit
Member
Registered: 2018-05-29
Posts: 3

Re: Irish Mountains

Just came across the same issue today. The heights rendered in OpenStreetmap are 50m / 160 ft different from the generally know Irish Sea Level based heights. I have been unable to find an explanation online for doing it this way and I can't think of an advantage over with making the default ele tag based on sea level as the OSM Wiki says it should be.

Can anyone help?

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#3 2018-05-29 11:58:24

Nop
Moderator
Registered: 2009-01-26
Posts: 2,226

Re: Irish Mountains

But on the OSM map there are "WGS84 elliptical heigths" values
on the mountain peaks, which differ a lot from MSL.

It's not quite as simple.

You also need to look at the history of the wiki page. Originally, elevation was given in the usual above sea level. Then at some time, a single person with no documentation of a discussion or decision changed it to WGS84 and the change went unnoticed for years. He was probably thinking it was for the good, but IMHO it was a very stupid thing to do. Almost nobody checks the wiki before adding something as common and intuitive as an elevation. So now you have a total mess of mixed values in the data. Your guess on how many people simply put in above sea level and how many looked it up in the Wiki and pulled out the calculator is as good as mine. :-)

But if I remember correctly, the difference between the systems should be much smaller than 50m. Also because of the mess in the wiki, this would be rather random and not a systematical error for all elevations. So I would assume the reason for your discrepancy is somewhere else. I would rather suspect a systematic programming error - were those elevation values maybe imported?

bye, Nop


Nothing is too difficult for the man who does not have to do it himself...
Projekte: Reit- und Wanderkarte mit Navigation    Kartengenerator Map Composer

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#4 2018-05-29 12:35:08

CharlieJeit
Member
Registered: 2018-05-29
Posts: 3

Re: Irish Mountains

Thanks Nop

However I did look at the history and unless I am reading it incorrectly this data arrived this way in the original import 9 years ago from iemv_import - ie. the big original bulk summit data import from mountainviews.ie source

Edited over 9 years ago by iemv_import
Version #1 · Changeset #797943
Location: 54.9853066, -7.0807528

ele 449.2
ele:local 396  (OSNI 1:50,000 confirms as 396)

BTW this in the summit of Loughermore Mountain.

Am I missing something?

Thanks

Charlie

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#5 2018-05-29 13:41:35

Nop
Moderator
Registered: 2009-01-26
Posts: 2,226

Re: Irish Mountains

So it was an import which explains the uniform discrepancy, and it was also pretty well done, giving the correct value in ele:local. The ele value is also repeated in ele:wgs84, so there is your explanation: An import done properly but according to an unrealistic and impractical statement in the wiki.

However renderers usually show ele values as they are given that most of the ele values are truly given in meters above sea level regardless of what the wiki says. This is visible on the main mapnik map. Also, in my map I display ele values for cities as they are and I never got any feedback or complaint about wrong elevations in the 10 years it is online.

So the only thing map renderers can improve with the current mess is use an ele:local value if it is present and assume ele is given in meters above sea level otherwise.

The reasonable thing in my eyes would be to try to reduce the mess in the data by changing the wiki back to local  above sea level as default in ele and other coordinate systems in special tags. Then automatically change all documented wgs84 values like in the irish import back to local values in the data. Then all maps would show the values expected by their users and the only remaining problem would be a few undocumented wgs84 values to clean up.

bye, Nop


Nothing is too difficult for the man who does not have to do it himself...
Projekte: Reit- und Wanderkarte mit Navigation    Kartengenerator Map Composer

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#6 2018-05-29 17:29:32

CharlieJeit
Member
Registered: 2018-05-29
Posts: 3

Re: Irish Mountains

I don't think this is right - the OSM wiki specifies using the EGM96 geoid model as reference - not the simple GPS WGS84 ellipsoid. The data set uploaded uses the WGS84 which being based on a one simple ellipsoid representing the surface of the whole earth will be prone to massive error (or variation) in relation to national mean sea levels and EGM96 (which is a complex bumpy approximation to real surface shape). I think EGM96 height Datum and Irish Mean Sea Level should be within a couple of metres so - are pretty interchangeable for practical hill height.

I would like to contact the original uploader - the user had the following contact details:

Queries to IrlJidel or mackerski or email talk-ie mailing list, talk-ie@openstreetmap.org

irc://irc.oftc.net #osm-ie : Irish OSM channel

Any idea how I could do this?

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