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In my experience, for most non-GIS folks, the most commonly used and understood coordinate system is good old Latitude and Longitude, displayed as degrees, minutes, and seconds (DDMMSS.ss). In GIS, it's of course much easier to use decimal degrees (DD.dd), and with the growth of everyday GIS apps like Google Maps, the non-GIS user is much more familiar with decimal degrees these days than they were 10 years ago.

But every now and again, someone brings me a set of coordinates in degrees and decimal minutes. This seems like the worst of both worlds: it can't be used in GIS without a little work, and the format is not immediately obvious to most non-GIS people (many think that it's DDMMSS at first). So why does it exist when we already have decimal degrees, which is easily understood by computers, and degrees-minutes-seconds, which is easily understood by humans?

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experiment-resources.com/babylonian-mathematics.html. BTW, have you ever had the pleasure of receiving positional data in gradians? And don't even ask about mils! –  whuber Jul 13 '12 at 19:54
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Thanks! I guess I wasn't 100% clear in the question though, what I was getting at was "why use decimal minutes when you already have decimal degrees and DDMMSS"? –  Dan C Jul 13 '12 at 20:10
    
Or gons. I've used survey levels that have bezels measured in gons. It's a Japanese unit of angle, in which there are 400 gons in a circle; this means that if you're not careful, you read an angle is almost but not quite in degrees. Well blow me, I just followed the link given by @whuber, and gradians == gons, just to add to the confusion... –  MerseyViking Jul 14 '12 at 10:41

2 Answers 2

up vote 5 down vote accepted

It is used in marine navigation to make the maths simpler, as well as instrumentation like sextants.

Take the example of a navigator who wants to find the angle of a celestial body with the horizon, he'll first make coarse adjustments on the sextant to get to the nearest degree, then turn the vernier to make fine adjustments. The vernier is marked off in minutes so it can be easily read to 1/10th of a minute.

His next task will be to adjust for various errors by adding and subtracting minutes and tenths based on altitude, temperature, sextant inaccuracies and so on. Doing this in decimal degrees would be an excercise in frustration because 1 minute of arc isn't a nice round decimal, so there would be lots of accumulated error in the calculations after a while. If he used DMS, there would be lots of extraneous calculation for not much benefit - on a ship you could never accurately measure to within a second of arc. Nautical calculations only go to 1/10th of a minute, and you need to make at least three observations before you can be sure of your position.

So in this case at least, it makes sense to have GIS data in DDM if it's nautical in origin (or generated by someone with a nautical background), or if it's been derived from manual surveying.

I must admit, I was like you a while ago - I couldn't really see the point. But since I've been studying navigation, it does seem like a natural and simple way of expressing angles.

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That jives with the recent experience that caused me to ask this question: most of the data that's been brought in in this format has come from observations taken on the water. –  Dan C Jul 14 '12 at 15:34

I'd guess it is because of a nautical reference too.

One of the "tricks" to rough navigation is that a Nautical Mile is (roughly) 1 minute of latitude, and also (roughly) 1 minute of longitude at the equator.

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