There are two totally different issues involved here, and it is important that you do not confound the two.

The first is the question of what map datum you are using, the second is what notation you are expressing (writing) a location with.

Concerning datums:

A bazillion years ago, when man first started creating paper maps, one of the first things he discovered was that any map that was drawn that had any form of grid system overlaid on it to enable a location to be identified needed to have a map datum defined.

The Romans were probably the first to make widespread use of datums, referencing everything to the distance from Rome. Eventually, other datums were established, primarily when the new area being mapped was separated from the old area by a large expanse of sea, making it impossible to connect to an established datum. Hence, there was a 1841 Basel datum (worked OK for Europe), then an Australian datum when that place got mapped, various Russian datums, a datum specific to Mount Everest, etc. etc.

Having all those different datums was no problem until people started to navigate intercontinentally (i.e. across datums) on a regular basis. At that point in time, the need for a 'worldwide' reference datum arose. This was first met by the WGS (World Geodetic Survey) datum of 1960. As the science of measurement advanced, that datum was refined and updated, resulting in WGS 1966 and WGS 1972.

Finally, the 'worldwide' datum (WGS), the one that most everyone agreed to use, was refined into its present version by the international scientific community and published as WGS 84 (this was done, no prizes for guessing, in 1984).

WGS 84 is sufficient for millimeter-precision definition of location, and it is the datum that is used by the NavStar Global Positioning System that is operated by the United States Military (what we call 'GPS').

Many years ago, when various long range positioning systems were developed (LORAN, OMEGA, etc.), users were still plotting position (latitude and longitude) derived from the positioning system onto paper charts drawn to a multitude of different datums (well over 100 different datums). Hence, early long range positioning systems offered the user a selection of datums that could be used, this to eliminate the need to do complex mathematics to correct from the positioning system's datum to the datum of the paper chart being used.

I remember flying in Africa in the 1980s when we used OMEGA navigation for position determination... some of the paper maps we had were published in the 1950s, and still used datums that were probably established by Stanley and Livingstone when they first charted that continent. Those days are gone now.

Today, just about all the paper maps published in the past 30 years have been referenced to WGS 84. This means you can take a position (lat/long reference) from a long range positioning system and transfer it onto a paper map without any need to convert datums. For that reason, it is becoming less common for consumer level GPS units to offer the ability to select different datums... because everyone uses WGS 84. There are still a limited number of paper maps printed that use a datum other than WGS 84, but in those rare cases, the measurement difference from WGS 82 amounts to only millimeters, which is of no significance for hikers, riders, or drivers.

Naturally, if your GPS system included electronic maps, you can take for granted that the electronic map will be referenced to the WGS 84 datum... you don't even need to think about it.

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Now, getting around to the notation - the method used to express a latitude/longitude in numbers - that is something entirely different that has nothing to do with datums. It is also a lot simpler to comprehend.

Think of how we express time: Hours, minutes, and seconds. It's not a decimal (base 10) system. There are 12 hours (24 if we use a 24 hour clock), with 60 minutes in each hour and 60 seconds in each minute. We can describe this time notation as hh' mm'' ss''', or hh m' ss'', or in other similar ways. However we express it, you get the idea, it means hours, then minutes (1/60 of an hour), then seconds (1/60 of a minute).

If we wanted to use decimal notation for expressing time, we would just need to break the hours into units of 100 minutes each, and the minutes into units of 100 seconds each. Hence "twelve-thirty PM' would be written as 12.50.00 (note the decimal points), rather than 12'30''00'''. Similarly, twelve forty-five and twenty seconds would be written as 12.75.33.

Noting latitude and longitude in hours, minutes, and seconds (with 60 units in the minutes and seconds) was the common system for many, many years. With the widespread adoption of the decimal system and use of computers for doing calculations, it became obvious that it is somewhat simpler, and less prone to error, to use decimal notation rather than base 60 notation.

You can use whatever system you want, just be sure not to mix the two. Most GPS navigation devices give the user the option of choosing the notation system that is used. Most paper maps still use the base 60 system to draw the scales on the edges of the map, which is why the base 60 system persists today.

If you are transferring co-ordinates from a paper map to your GPS device (or vice-versa), it might be easiest to set your GPS device to use hours, minutes, and seconds. If you are not making such transfers, then it doesn't matter at all what notation system your GPS device uses.

Hope this helps.

Michael