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Thread: Pro Quo Mapping Software

  1. #1
    Widdler Iain McK's Avatar
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    Have any users of Pro Quo had issues with altitude readout vs spot heights on maps (1:50000 OS)? I have a misalignment whereby higher altitude is shown against a lower contour on the map. Example as follows:

    Grid refererence NY 1970 2489:

    Map spot height = 525m

    Quo Pro readout and route card = 493.8m

    The readout actually shows a higher altitude against a lower contour line: NY 1979 2497 = 507.7m

    Comments appreciated.

  2. #2
    Super Moderator captain paranoia's Avatar
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    Spot heights are details added to a drawn map.

    Electronic mapping systems (at least ones that use raster mapping images) have no knowledge of what the map shows (i.e. contours or spot heights), other than the grid ref; it's just a picture.

    Height data is provided by another database, which is of a lower resolution than the mapping data, often a 50m or so grid.

    As such, this spot height probably doesn't fall exactly on a grid point, and it has made an estimate from the surrounding four grid heights. If it's clever, it will have applied a filter to look at the surrounding landscape, and try to decide what the landscape is doing, so it should realise that this is a local maxima and adjust its estimate to allow for the fact that it nature, things are usually rounded, and not abrupt, flat surfaces (although this can happen...)

    It's a common problem with electronic mapping.

  3. #3
    Super Moderator captain paranoia's Avatar
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    For instance, I've just fired up my copy of LandSerf, and, using the Shuttle Radar Topography Mission data, it suggests the height at the spot height (319700, 524885)should be about 508m. Looking around the general area of the peak, it finds the highest point is about 511m.

    That's using an interpolated terrain data with a 25m resolution, pulled out of the SRTM data at 3 arcseconds resolution and then re-projected from angular lat/long to OSGB.

  4. #4
    Widdler Iain McK's Avatar
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    Thanks for comments, the filter may explain why Anquet is more map representative, the spot height of NY19702489 showing 521m which is 4 off the map.

    Both mapping systems have their advantages/disadvantages, route length is however almost identical.

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