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Thread: GPS and Maps

  1. #1
    Widdler
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    Hi,
    Santa bought me a GPS a very basic one but after years of walking i have come into the tech age. Problem is that if i want to plan a route using the grid ref on a map how do i use a 8 fig from my map to the GPS and still keep it accurate. I tried it today but was a long way out and i find it difficult to give a fine ref. The GPS will not down load from the web.
    Any ideas ?

  2. #2
    Initiate bryan crick's Avatar
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    Generally Puzzled So am I bump!

  3. #3
    Widdler
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    Hi Bryan,
    what i mean is the scale of the map is too small to see a 8 fig grid ref without any aids.And because i am guessing on the last fig it throws the GPS out by about 200 yards. Hope this explains it a bit better.

  4. #4
    ‹bermensch Hamish Fenton's Avatar
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    No I think Bryan was wondering too, your question reads fine (and i don't have an answer for you)

  5. #5
    Goon Windscale's Avatar
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    I find if I use a 1:250000 scale map and the roamer scales on my compass that I can get a fairly accurate 8 figure grid ref. off a map. With 1:50000 maps it's a bit more hit and miss and I usually only figure accurate to 6 digits. Without using some sort of measuring device I find it's difficult to estimate the last digit to any sort of accuracy though.

    The only other things to check if your grid references are out on the ground that I can think of are:

    a) You've set the GPS correctly to use the OSGB grid system.

    b) You've selected the right datum to use with the OSGB grid system. Some GPS seem to select to datum when you select the grid system and others have it as a separate setting.

    c) On the ground, make sure you have a 3D rather than 2D position fix - there should be a way of checking this somehow. From memory a 3D fix requires a lock on four satellites I think. 2D fixes can easily be a couple of hundred metres out since they can't take your elevation into consideration. Try seeing if your GPS unit can give you a reading of EPE - "Estimated Positional Error," that will give you an idea of how accurate the GPS currently thinks it is.

    d) If your GPS automatically averages, then it's reading will improve with time if it remains in the same location. Some GPS automatically average but apparently on some you have to select it (never used one of those though).

    Stephen.

  6. #6
    Widdler
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    Thanks Stephen i am a bit wiser now.

  7. #7
    ‹bermensch Ace High's Avatar
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    Hi Mark,

    What make and model GPS do you have?

    I agree with Stephen's comments above, there's nothing I can add there.

    One trick I can pass on though. If you find that determining the last digits in a 3 or 4 figure groups is difficult, try setting that digit to zero. A grid ref can be thought of as referring to the lower left hand corner of a box, the size of which depends on if you are using two, three, four or more figures. Once you arrive at the gps derived position the location of interest should lie in a box (with sides the size of which are determined by the scale you are using; 10, 100, 1000 metres etc) with you standing at the south west corner.

    In other words the position you are interested in will lie between zero and ninety degrees of your position. This will be sufficient for most purposes, though I wouldn?t recommend navigating off Ben Nevis this way ;-)

    In some respects it?s a little like the off course navigation method of dead reckoning flying over featureless terrain where you ignore wind drift and fly out on a compass heading for the period of time you have calculated, then turn into wind. The destination should appear on the nose eventually!!

  8. #8
    Widdler
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    Hi Dick,
    the make of GPS is a Magellan Explorist 100 which is basic but is all we could aford at the moment.

    Hope this helps

  9. #9
    ‹bermensch Ace High's Avatar
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    Good, I have a Magellan Sportrack, so I may have the answer then!!

    There?s a nice piece of software called GPSUtility available from
    http://www.gpsu.co.uk that should work with your Magellan. You can scan maps into your pc or you can down load OS samples for ?educational purposes? from OS and use them with the software. It is easy to use and I think it is wonderful.

    Once you have you map in the pc you can bring it into the software. After positioning the map and calibrating it you can point the cursor at a point and read off the full map reference. Easy peasy!! You can plan routes and waypoints etc on the map and then up load them into your gps. My Magellan doesn?t have the map facility but it will accept routes and waypoints.

    I?ve been using this software with my basic Magellan for two or three months now and I?m highly delighted with the results. The trial version is free and you can upgrade to the full version for £25. A lead to connect my Magellan to the PC cost £25-£30. There is a very useful and active forum at the web site that I have used a couple of times when I?ve got stuck and the author gives the product full support.

    After raving about it like this I should declare that I have no connections with the author and I don?t get a commission; I just love the software and think that it represents outstanding value for money.

  10. #10
    ‹bermensch
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    First check your map datum as recommended by Stephen Butler

    I assume that you are referring to the figures (eg 8634/2071) and not the letters that denote the 100-km square.

    It is not too difficult to key in a reasonably accurate 8-figure grid reference providing you use an Explorer map and a romer that measures the eastings from 10 to 0 rather than the more conventional 0 to 10. This allows you to place the edge of the romer at 0 on the feature whose GR you wish to calculate and read off the result on the blue grid line of the map. It is then easy to estimate the number of tenths and this will give you an 8-figure GR accurate to within 10 metres.

    My Garmin Geko 201 requires a 10-figure GR so, as it is impossible to calculate this with the naked eye, I always key the last figure as 5 being, on the balance of probabilities, the least inaccurate figure.

    I have been experimenting with GPS receivers for several years and believe that their usefulness has been exaggerated. They seem to serve only two worthwhile purposes:
    1 to establish your present position
    2 when walking off-path it will take you unneringly to a destination on the far side of broken terrain when you are unable to follow a compass bearing accurately because of the need to make numerous diversions to avoid bogs, rocks etc.

    They serve no useful purpose in lowland countryside.

  11. #11
    Widdler aibokicker's Avatar
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    I think they are a useful tool when navigating, however as mentioned I think people have on the whole come to expect too much.

    You get the most out of them when using with Anquet/Memory map or similar software, as you can click and plan your route, get the info on distance, height, etc then download to your GPS.

    I find having a grid reference in a "you are here" kind of way handy to back up my navigation. (Particuarly if foggy!) But they are designed to be used in combination with a map and compass.

    I also use mine as a trip computer when out cycling, so mine serves a perpose in the lowlands (well the South Downs) :-P the most expensive cycle computer ever!

    Anyway, with the Magellan i'd suggest substituting the last number with a 0 as mentioned to solve the origional problem. Not sure why that Geko needs a 10 figure GR...have you double checked the settings? My Garmin (ok its an etrex Legend) uses 8 fig.

  12. #12
    ‹bermensch
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    In response to Albokicker, I can confim that the Garmin Geko 201 requires a 10-figure grid reference. I suspect that it is because it is enabled for both WAAS (Wide Area Augmentation System) that covers North America and, of more immediate consequence, EGNOS (European Geostationary Navigation Service). The latter is currently being tested and, if all goes to plan, will be working in 2005. it will give EGNOS-enabled GPS receivers an accuracy as good as three metres.

    I am a Dirty Macintosh man and as far as I know there is not yet any Apple-compatible software that wil allow me to download maps into my receiver, so I have no experience of this particular technology. However, try as I might, I find it difficult to understand what advantage there is in investing quite heavily in a technology for so little real advantage. After all, you have to be a competent map-reader before you can use the technology. I 'm not a luddite - just getting old!

    Thanks to Stephen Butler for pointing out the advantage of using a zero for the final figure in a long grid reference. I'm not too old to learn!

  13. #13
    ‹bermensch
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    Apologies to Dick Barrett!

    The acknowledement of the value of entering a zero as the final figure in a grid reference should have been mde to him.

  14. #14
    Ultra King Parky Again's Avatar
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    i sympathise with you hugh. gps is a useful emergency "where am i?" and also an emergency get me out of here with the trackback facility.
    they really come ino their own when coupled with digital mapping, although perhaps a bit more nerdy, where you can plan routes and upload/download where you're going/been. some mapping allows you to attach pictures to points which is a nice feature. again, it is an invaluable tool if you plan/have routes for sharing.

  15. #15
    Super Moderator Jon Doran's Avatar
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    Hugh, I have it on good authority that at least two of the mapping software companies is currently porting their programmes to Mac, so you (and I) may not be excluded from the electronic mapping game for too much longer. Which'll be nice.

    You can run PC programs with an emulator, but my experience on my ancient, mouldering iMac G3 is that it's all too slow to be useful. Might be better with a fast G5 though...

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