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WORKING WITH COORDINATES AND UNITS
This chapter covers the various grids,
datums, and other measuring items in a Garmin unit. Much of
this information is mathematical but I have attempted to simplify
this wherever possible and just provide an overview of the information.
This simplification can lead to errors or misunderstandings
so please let me know if you find any of these or areas that
simply need to be beefed up to make them clearer. The
datums and grid preferences can be reached from the main menu
by selecting the navigation setup options. You can set the
navigation datum, the navigation grid, the user units, and
if your unit support cdi, this can also be set here.
What is a datum? First, what it is
not. It is not a singular form of the word data which some
of you may have heard. Data is a collective noun and the plural
of datum is datums when used in a geophysical sense. For GPS,
navigation, and geophysical use a datum represents a reference
point from which you can measure things. At one time this
was a location on the earth and could have actually been just
a single point of reference. In modern usage it defines not
only the point for reference, it also defines the model of
the earth used to support those measurements. The model is
a three dimensional one that describes mathematically the
shape of the earth, the location of the center, and the location
on the surface that represents the starting point for measuring.
Some older datums only defined a horizontal measuring model
or, in some cases, only a vertical one. Some datums permit
measurements to be made worldwide while others are only defined
to support a local grid measuring system. If you were to use
a different datum to measure the same place you will typically
get different results and is some cases drastically different
results. Internally Garmin units always store information
and compute using the WGS-84 datum but will display the answers
in the datum of your choice.
Typical Garmin handhelds let you define
any of over 100 datums for use anywhere in the world but there
is no check that you used an appropriate datum for the area
you are currently located in. The question is: "Why would
you want to define a datum?" Since a gps unit makes all measurements
using the WGS-84 datum this might seem enough to just display
the answer using that datum. In fact this may be enough for
some folks and some gps units only support this one datum.
However, surveying and map making have been going on for centuries.
If you have a map created before the modern age of gps your
map probably was not made using the WGS-84 datum. If you want
your position data to agree with the map or survey marker
on other source of geophysical location information you will
need to ensure that your gps datum agrees with the one used
to generate the data. Generally on a map this information
is given in the legend of the map itself. In addition some
maps use a local grid system (covered later) that requires
the use of a specific datum to maintain the accuracy of the
grid system.
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