# Why are the formats of these coordinates different?

I'm really sorry if this is a silly question (and I'm sure it is) but I have tried searching but I am a complete newbie to coordinates and maps and I don't know what I'm looking for so am not finding my answer!

I have a gpx file which has coordinates like as follows:

51 47.322,-2 47.218

51 47.340,-2 47.071

I need to add some new coordinates but I am taking these from google maps,but the coordinates are in a different format.

ie:

51.804979,-2.738793

I know there is a logical reason for this as I've read about different coordinate types, but I don't understand why and what types they are and what I need to do to convert from one to the other?

Thanks for any help!

Bex

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## 1 Answer

All the number you have are in degrees, but they're just formatted differently. The first set of numbers you have are "degrees, decimal minutes" whereas the second coordinate pair are in decimal degrees.

To convert from the first to the second, you just need to divide the second part (e.g. 47.218) by 60, which is the number of minutes in a degree and then just add it on to the first part.

So the latitude of your first coordinate pair is 51 degrees 47.322 minutes (often written 51d 47.322'). Dividing 47.322 by 60 gives 0.7887 Adding that on to 51 gives a decimal degrees value of 51.7887 The same can be applied to the other values you have.

Now, not all coordinate pairs of the same value will point to the same place on the earth, it all depends on the coordinate system, the ellipsoid, and a few other numbers. But I would say that if you don't know what coordinate system they're in, you can expect it to be WGS84 which is the system that GPS devices use, but I've been bitten by that assumption in the past so if you can double-check it, do so.

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ahhh.. that makes sense! Thank you! Is there somewhere explaining the coordinate systems? – Bex May 5 '11 at 11:01
A quick look over this Wikipedia article: en.wikipedia.org/wiki/World_Geodetic_System seems to indicate its telling the truth (always be careful of Wikipedia articles!), and this page: geography.about.com/od/geographyintern/a/datums.htm covers some interesting stuff. I expect there are lists of good sites in this SE forum, just look around. And don't worry, it wasn't a silly question! – MerseyViking May 5 '11 at 17:01