[Tagging] Topographic Prominence for Peaks

Michael Reichert osm-ml at michreichert.de
Sun Sep 23 13:33:34 UTC 2018


Hi Joseph,

Am 23.09.18 um 02:00 schrieb Joseph Eisenberg:
> Elevation and prominence can both be calculated from SRTM data, eg by
> using Opentopomap tiles and finding the highest contour lines around a
> peak, and the lowest near a saddle.
> 
> Prominence and elevation can be calculated by computer with good data,
> but for my part of the world the SRTM data is not high enough quality
> to get good results without cross-checking against aerial imagery.
> Also the calculations are not simple, and are not precise for sharply
> pointed peaks or deeply carved saddles, therefore I believe it will be
> useful to include this data directly in tags.
>
> I also find that calculation the prominence of peaks has encouraged me
> to add more ridge lines and saddle points (with elevations), which
> should make the database more useful in mountainous areas.
> 
> Do you think I should write up a formal proposal for this tag?

I don't think that there is any need for such a tag in OSM for the
following reasons:

(1) If you assume the earth to be a plane, just order the peaks by their
elevation. It's so simple that I don't give the necessary SQL query
here. If we used a prominence=* key, it would have to be the distance to
the next higher peak. If the value were not a number but a term like
"most_important", "very_important", "neighbour_of_highest" etc. it would
not comply with the verifiability requirement of OSM and invite people
to have edit wars.

(2) If you assume the earth to be an ellipsoid, it is more difficult but
still achievable with ele=* tags in OSM and elevation raster data (SRTM
etc.) only. There is an open source solution to do so.
https://gitlab.com/nuntius35/extremal_peaks/blob/master/README.md
(mentioned in WeeklyOSM issue 423)

(3) If we use a tag like prominence=* for peaks two mappers will have a
different opinion on the value to choose. For example, the Matterhorn is
4478 m high but Monte Rosa is 4634 m high and not that far away.
However, the Matterhorn is more famous and people expect it to appear on
maps at the same or lower zoom scales than Monte Rosa. Whose opinion is
correct? From my point of view, the mapper using the elevation only
because that is the only verifiable fact while the prominence in
advertisement is something we don't record in OSM for a very good
reason. (Please replace "peak" by "city" and "height" by "population"
and read this paragraph again)

Best regards

Michael

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