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Thread: WGS84 to OS grid converter needed

  1. #1

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    Nov 2009
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    Default WGS84 to OS grid converter needed

    Not geocaching, but I'm trying to produce a map-driven website using Open Streetmap (OSM) tiles. Because distances between points are fundamental to my application, I'm using coordinates based on the OS grid internally because OSM tiles are based on angle of lat/lon rather than distance across the ground. This means that I need to know the OS grid ref of each corner of each of the OSM tiles I'm using.

    I have GSAK, which has done the trick for all but the bottom row of my map. While GSAK gives me the OS grid ref for the top of those tiles, it won't for some of the bottom edge. For example, I need the OS grid ref for (49.837982, -2.109375) to within a few metres.

    Can someone recommend a (preferably free) WSG84 to OS grid converter that can do this?

    TIA,

    Geoff

  2. #2

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  3. #3

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    That does the trick. Many thanks.

  4. #4

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    If you want to do batches of coordinates the Ordnance Survey offer this service, which has downloadable software if you plan doing sizeable files.

  5. #5

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    Nov 2009
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    Quote Originally Posted by agentmancuso View Post
    If you want to do batches of coordinates the Ordnance Survey offer this service, which has downloadable software if you plan doing sizeable files.
    Many thanks. However, that seems to work with ETR89 rather than WSG84, and the lat/lon coords I've got are to the latter.

    FWIW, I'd spotted that service and also the downloadable Grid InQuest software to which you refer (and that I'd already installed). In tests, both the online and downloadable versions returned results that were not in agreement with GSAK and so I discounted those converters. I'd also downloaded and tried GPS Babel but couldn't get my head around it. Since GSAK uses GPS Babel under the hood, I suspect that it wouldn't have converted the bottom edge of my map anyway.

    Again, many thanks.

    Geoff

  6. #6

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    Quote Originally Posted by Pajaholic View Post
    FWIW, I'd spotted that service and also the downloadable Grid InQuest software to which you refer (and that I'd already installed). In tests, both the online and downloadable versions returned results that were not in agreement with GSAK and so I discounted those converters. I'd also downloaded and tried GPS Babel but couldn't get my head around it
    The divergence between GSAK and OS conversions is almost certainly down to the fact that the former uses a simplified mathematical formula which creates rounding errors of <7m, whereas the OS site uses the vastly more precise OSTN02 transformation.

    The divergence between ETR89 and WSG84 is now about 50cm, so using the ETR89 system would still result in a markedly closer approximation of the original coordinates.

    But then, given that GPSr accuracy wobbles are generally large enough to obliterate these small measurement differences, you are probably right to stick to the system that seems easiest to handle.

    I couldn't make any sense out of GPSBabel either.

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