*** UTM System Explained ***

>Message : #25066101    From: tony saint
>Address : tsaint@eve.adam.com.au
>Group   : Usenet.sci.geo.satellite-nav
>Subject : UTM display
>Org.  : ADAM Pty Ltd
>
>I would appreciate help from somebody able to decipher Garmin 75 UTM
>positional display reading:
>54H0273948
>UTM 6132563
>I can see that the last 5 digits in each row are grid coordinates, but the
>54H02 in the top line and the 61 in the bottom are meaningless to me.
>Thanks in advance
>Tony Saint

It's hard to explain without complex visual aids but here goes.


54H     = UTM zone
0273948 = Easting in meters
6132563 = Northing in meters


The UTM (Universal Transverse Mercator) grid system was devised to make the
math required when calculating artillery trajectories for lobbing shells at
your enemies easier.

The world is divided-up into a grid where the vertical grid lines
(X coordinates) are drawn every 6 degrees of longitude starting at 180 deg.
W and counted as columns numbered from 1 to 60 moving east.

   |     |     |     |
   |  1  |  2  |  3  |
   |     |     |     |...
  180W              162W deg.

There is a vertical line drawn down the middle of each column and given an
easting (offset from the west boundary) of 500,000m. This is not a true
offset as the west limit of the zones will never reach 0; always positive,
and at the east side, will always be less that 1,000,000m.  And of course as you move further North or South, the width of the zones becomes less as
lines of longitude come closer to converging, but the vertical line found
half-way across each zone is always called an easting of 500,000m.



The horizontal lines of the grid system  (Y coords.) start at 80 deg.S
latitude and drawn every 8 deg.  moving north with the exception of the row
from 72deg.N - 84deg.N which is 12 deg. (there are lots of exceptions in
this system, must have been devised by an Englishman, partial to exceptions
they are) These rows are labeled with a letter starting with C through to X, and exclude I and O.

16N  ----------------
            P
8N   ----------------
            N
0deg.---------------- Equator
            M
8S   ----------------
            L
16S  ----------------

The polar zones are mapped with different grids, called the UPS Grid where
zones Y and Z can be found at the North Pole and A and B at the South.


54H would put you between 138deg E and 144deg E Long., and between 32deg S
and 40deg S Latitude.

The 0 in the easting 0273948 (in meters) is redundant and in some uses of
UTM left off altogether as this will always be 0.  You can sort of figure
roughly where the location is if 141deg.E is an easting of 500,000m, it is
to the west of there.

The northing of 6132563 is an offset from the equator, in meters.  The
equator has a 0 northing for zones in the northern hemisphere, and a
northing of 10,000,000m for zones in the southern hemisphere.  In both
hemispheres the northing gets bigger as you move north.  54H is in the
southern hemisphere.


And that, in a very basic form, is the UTM system.  If you really want to
get confused, read on.

Now here's where it becomes complicated.  The military, in an effort to
simplify things, added more to the system.  They have imposed a flat
two-dimensional grid onto the zones.  But as everyone knows, you can't draw
an X-Y grid onto a sphere, so the military did.  They have subdivided each
UTM zone into 100,000m squares.  Since lines of longitude aren't parallel,
there are wedge shaped "squares" adjacent to the edges of the zones, and
fewer squares as you move farther north and south.  Each of these squares is given a two-letter name, the first letter indicating sub-divided column, and 
the second designationg a sub-divided row.  Just to comlicate things, these
names do repeat:
ie there is a square called BA in zone 1N as well as zone 1Q.

You don't need to be familiar with the military zones to use UTM on a GPS
unit.  I won't get any further into this as it will take some time to
explain.  And yes I did consult a text for this: "Introduction to Map
Projections", Porter W. McDonnell, Jr.,  1979 Marcel Dekker, Inc.  Look it
up if you want to know more.


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