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Hi I have been using OSM, and other mapping apps that draw their data from OSM, and have found that most, if not all, of the spots heights for summits are wrong when compared to OSNI maps. An example of this is Mid Hill @ N54.97265 W006.13209 which is shown as 493 metres whereas OSNI show the highest point is 444 metres @ N54.9743 W006.1237. It is obvious when the contours are studied that 444 is the correct height for this summit. I have compared several other spot heights and found on OSM they are all considerably higher than those on OSNI maps. I would welcome any feedback on this problem. Regards |
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Having just looked at the data, there is another issue. The mapper who added this information has chosen to use a WGS84 datum not the standard Irish one (EPSG 29002 or 290003), and has instead added more reasonable elevations in the local dataum as ele_local. In the case of Mid Hill the local elevation provided is 440 m which is significantly closer to the value you expect. It appears much of the data was imported from the mountainviews.ie back in 2009. It is also likely that other spot heights were entered from a GPS device and will be much less accurate than OSNI data. For instance if a GPS altimeter is not calibrated before use the values obtained may be very inaccurate. If using the actual values provided by GPS signals, then these refer to an entirely different datum than that used by OSNI. The only source of altitude data we have are GSGS 3906 maps compiled from early 20th century OSI 6 inch mapping, which will have some altitudes in feet and contour lines at (IIRC) 100 ft intervals. OSNI data are not compatible with the general licence requirements of OSM, so are not in scope as a data source. I think there is a case for showing local datum elevation in the main elevation tag, which is what we do in the rest of the UK (as far as I'm aware). However, such a change is best discussed on the talk-ie mailing list. NOTE: For data consumption of these values, for instance on a Garmin device, simple rules to select ele_local rather than ele will provide the type of data expected. |
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This would be the same question as: The heights for mid hill are: https://www.openstreetmap.org/node/332373222 ele 492.6 ele:local 440 ele:tm75 440 ele:wgs84 492.6 ele_local 440 But the main question is probably, should the main elevation tag be ele:local or ele:wgs84. Based on the tag wiki: https://wiki.openstreetmap.org/wiki/Key:ele It should not be ele:wgs84. |
