Wednesday, June 2, 2010

How exact do you need to be when specifying longitude and latitude

If you want to follow the advice of my previous blog post and advertise the position of your home or business via latitude and longitude coordinates, you might wonder how exact you need to be with your measurements. Obviously if you are more precise when you measure the position you lessen the chance that your friends will call to your neighbour's house instead of yours, but on the other hand a long number is harder to remember than a short one.

The level of precision needed depends slightly upon how big is the place is that you are advertising and where exactly on the globe you are, but the short answer is that 4 decimal places are often enough and 5 decimal places is definitely accurate enough.

If you want to learn how to accurately convert lat/long cooridnates into distances you should read this web page (be warned the maths gets complicated) but a rough rule of thumb is that if you need to move one meter north or south to increment the fifth decimal place of your latitude i.e. 0.00001 degrees north is one meter away from the equator. The same rule of thumb can be used when conversing between longitude and distance when you are at the equator, but as you move away from the equator there is a decrease in the distance represented by a given number of degrees. For example, in Ireland if you move 0.00001 degrees east or west this is a distance of only 60cm on the ground. It is hard to imagine a situation where you would need to specify your location with greater accuracy than this and in any case most consumer GPS devices would not even be this accurate.

If you look at the page listing IBM locations in Ireland you will see that some of the GPS co-ordinates are given to an accuracy of 6 decimal places which specifies the location of the facility to a precision of 10cm, which is obviously much to much detail.

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