Datums, Datums and Datums
This is one of the biggest sources of confusion for beginners to GPS.
"Why do the coordinates on my GPS not match those on my map?"
Well the answer is most likely because the coordinates are referred to different datums.
To explain this lets start with an analogy regarding time:
Most people understand that, if the time is 10:00 in New York, then the time in London is 15:00.
It is the SAME time, even though the number is DIFFERENT.
When talking about 'global' times, it is normal to attach the Time Zone to the number.
10:00(EST) is the SAME time as 15:00(GMT)
Similarly, even though the number is the same:
10:00(EST) is a DIFFERENT time from 10:00(GMT)
So whenever you specify a time value you should always specify the referenced timezone
otherwise confusion is likely to arize.
The timezone is a time datum.
It is the same with coordinates, unless you know the datum
they refer to, they have no meaning. For example, saying
I am 5 kilometres west
has no meaning without reference to another position. However, you could say:
I am 5 kilometres west of the Church.
or you might say
I am 2 kilometers west of the Tree.
Both statements define the same position - the first statement is in Church datum and the second
statement is in Tree datum. A GPS receiver may store the position internally in some other datum,
for example relative to the Pentagon. However, it knows that the Church is 5 kilometres west of the Pentagon
and the Tree is 8 kilometers west of the Pentagon, so it can do any conversions that are needed.
If you tell your GPS I am 5 kilometres west of the Church, the GPS will store a
position of 10 kilometres west of the Pentagon.
If your friend wants to know where you are but can only understand the Tree datum,
he can query the GPS and obtain the response 2 kilometres west of the Tree.
YOU TREE CHURCH PENTAGON
Your position (coordinate) can be expressed as any of the following:
- W 2 (Tree datum)
- W 5 (Church datum)
- W 10 (Penatagon datum)
In other words a single position will be expressed in different datums using different values
for the coordinate. You don't need to know what datum the GPS uses internally, but you DO need
to tell the GPS which datum to use when it is talking to you.
A coordinate referenced to WGS84 datum has a different value from the same
coordinate referenced to another datum. In the case of the datum used in Great Britain
(Ord Srvy Grt Britn) the difference in coordinate values between this and WGS84 datum
is approximately 0.08' in longitude (or 100 metres in Easting),
0.01' in Latitude (or 70 metres in Northing).
The difference does vary depending on the location.
GPS Utility takes care of datum changes, but if you are using coordinates from an
external source (e.g. a map) then you need to make sure that you are using the correct datum.
When calibrating a map, check the small print on the map to identify the datum being
used and set GPSU accordingly.
A Practical Example
- N50°00.0000' W001°00.0000' (WGS84)
is exactly the same position as
- N49°59.9647' W000°59.9126' (Ord Srvy Grt Britn)
which is the same position as
- N50°00.0553' W000°59.9167' (European 1950)
So when dealing with coordinates is VERY important to know which datum they are referenced to.
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