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103
Coordinate System
Introduction to Coordinate Systems and Datums
The MobileMapper system uses standard coordinate systems
and datums used by surveyors and cartographers around the
world. MobileMapper offers the capability to define your
own coordinate systems and datums. This involves selecting
your own map projections, coordinate systems, and datums
- all of which are defined below.
A map is developed using a projection that is a mathematical
translator between the roughly spherical Earth and the flat
map. For this reason, any map is inherently inaccurate be-
cause it must “stretch” to fit over a sphere (see comment op-
posite). This is actually quite complicated, because a map is
flat and the Earth is not. You can demonstrate this yourself
by taking any spherical object such as an orange and trying
to wrap a sheet of paper around it while creating the mini-
mum of folds and wrinkles. The only way to get the paper to
wrap evenly is to cut some sections out and stretch others.
That is what a map projection does, but in reverse. It takes
the somewhat spherical surface of a portion of the Earth and
flattens it while trying to avoid distortion along the way. The
challenge is to make a projection that fits optimally through-
out the space it covers with the least distortion possible.
There are many map projections available to the mapmaker,
but for the most part there is only a handful in practical use
today. MobileMapper Office software supports most of
these common projections.
Coordinate systems describe where you are in a map projec-
tion. Some people use “grid systems” using northings and
eastings - the distances, typically meters or feet, to the north
and east of an agreed-upon starting point. Others work in
geodetic coordinate systems using latitude and longitude
numbers that divide the Earth into the degrees, minutes, and
seconds that most people are familiar with.
The Earth is really not a
sphere but a “spheroid”
because its rotation
causes the equator to
bulge out slightly so that
the Earth's circumfer-
ence is greater around
the equator than it is
through the poles. When
looking at the Earth's
surface, however, you
are really considering
just sections of the
spheroid. And, if you
remember your geome-
try correctly, the name
for a section (slice)
taken through a spheroid
is an "ellipsoid." An
ellipsoid is to a spher-
oid as a circle is to a
sphere. Mathematician-
cartographers have his-
torically attempted to
write equations for ellip-
soids that accurately
describe the Earth's
geometry. For example,
the ellipsoid the most
commonly used today
was developed by
Clarke, in 1866.