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    Datums: why a place has more than one lat & long
    From: David Pike
    Date: 2015 Mar 6, 13:30 -0800

    The question was:
    I have pondered how it can be that Google Maps seems to know where my very DRIVEWAY is on my cellphone, but nautical charts are scores of yards off.

    As we know, the Earth is not a perfect sphere. It isn't even a perfect ellipsoid. Greatly exaggerated, it's more the shape of an ugly grapefruit. For most people, spherical geometry is something you revise before you complete a job and forget straight afterwards until the next job. Ellipsoidal geometry is 'bagpipe music' and is something you have to follow a previous example for and forget as you're doing it. The geometry of an ugly grapefruit would be quite impossible. Therefore, cartographers have traditionally selected an ellipsoid which best fits the Earth in the area they are making a map of, and they projected their results onto that idealised ellipsoid. The coordinates based on such idealised ellipsoids are known as datums. If you go to the navigation page of a hand held GPS set and select datums, you'll find many tens of them. This worked fine until the advent of global navigation systems such as GPS. Then an ellipsoid had to be selected which best fitted the shape of the entire Earth. The datum chosen for GPS was World Geographical Survey 84 (WGS 84). In many parts of the world, the difference between the local datum and WGS 84 is very small, but there are times when it could be quite important e.g. a super tanker navigating a narrow channel, or a group of walkers navigating the edge of a precipice in poor visibility. Therefore, there has been a move in recent years to change nautical charts to WGS 84 comparable datums.

    You don't really have to redraw the data on the chart; you just need to move the lat and long lines on the chart and the values around the edge a little bit. During the interim period, you'll find that nautical charts which don't already have a WGS 84 comparable datum will have a small panel somewhere which gives a set of corrections e.g. 0.01' southwards, 0.10' eastwards to be applied to WGS 84 positions obtained from satellite navigation systems. You might say "well if I can change the datum on my GPS set, why not select the same datum as the chart I'm using". The reason, as I understand it, is because the algorithms in a GPS set work best in WGS 84. I believe Google Earth/maps use the WGS 84 datum, so it's little wonder the GPS position of your drive comes in the right place on Google maps. Your nautical chart might well be in a different datum possibly, NAD 83 or NAD27 Canada. If you apply any corrections listed on your chart to the GPS position measured in your drive, you may well find that its position on your chart is now correct. Alternatively, read the datum used for your chart, and apply that to your GPS set. The position obtained should also come very close to the position of your drive on Google maps.

    In the United Kingdom inland maps known as Ordnance Survey maps, and until recently, Admiralty charts close to the coast used OSGB 36 datum based upon the Airy Spheroid. For general walking, it's easiest by far to simply change the GPS set to that datum. The worst case I ever found was whilst transiting the Gota Canal in Sweden. I was moored at a place called Linkoping, and I bought a local walking map. When plotted on the local map, my GPS position from a Garmin 12 set to WGS 84 placed my yacht Tiki about 1 km south of the marina.

    Hope this helps

    Dave

       
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