WORKING WITH COORDINATES AND UNITS

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UPS and special zones

As can be imagined from the drawing above the tips of the points have very little area associated with them and a great big gap. UTM solved this problem by drawing a line at 84 degrees North Latitude and 80 degrees south latitude. The areas above and below those lines are rearranged into two circles like pieces of a pie. These new zones use a different pie shaped grid system called UPS. The southern pie is a bit larger since it needs to cover Antarctica. Two of the letters on each end of the alphabet are used to reference zones in this area. A and B divide the southern pie into two hemispheres while Y and Z do the same with the northern. The map picture for this area would be a polar projection.

A few of areas below the 84 degree separation also have been modified. This has been down to permit an island to be fully contained within one zone. At these latitudes you won't have any problem overflowing the 50,000 false easting even with the wider zones. Zone 32 has been widened to 9° (at the expense of zone 31) between latitudes 56° and 64° to accommodate southwest Norway. Similarly, between 72° and 84°, zones 33 and 35 have been widened to 12° to accommodate Svalbard. To compensate for these 12° wide zones, zones 31 and 37 are widened to 9° and zones 32, 34, and 36 are eliminated.

GPS helps UTM

One of the biggest problems in using UTM is the handling of the zone splits. Within a zone it is very easy to find coordinates and to measure and compute any distance on the map. When you hit the boundary you are suddenly confronted with a different number with no relationship at all to the one you just left in the east/west direction. When a map has a populated area that crosses a zone boundary this is usually handled by placing a secondary grid in the new area using the old zone grid data. This permits measuring and locating objects on a map based on a projection from a location in the other zone. Thus in areas near a zone boundary you will see both zones extended into the other zone area. These other zones will not be aligned with the edge of the map which makes them easy to spot. Zone boundary lines are usually drawn right on the map to help you with this.

Your Garmin unit automatically knows when you leave one zone and enter another. Further if you project a waypoint into another zone it will be automatically recalculated into the correct value for that zone changing the zone number and letter as needed. In addition you needn't enter the zone letter for any waypoint you need to enter except that, because of the false northing for southern hemisphere you will need to be sure the letter code is in the correct half of the alphabet. In addition you will need to use a UPS zone letter to let the gps know when you want a grid in this zone.

UTM is the most precise measurement system on your gps. It reports your position to 1 meter precision. None of the other grid systems can achieve this level of precision.

MGRS

Some Garmin gps receivers also support the Military Grid Reference System. This system is just another form of UTM so if your unit doesn't support this it is easy to translate to MGRS from the UTM numbers. MGRS replaces the most significant digits of the UTM coordinate with two letters. If you use the example shown above for UTM 0392000E and 3382000W you would see UQ 9200082000. The two letters UQ are in addition to the normal zone number and letter code. The two letter codes of MGRS are not unique and may repeat at other points in the globe but they are unique enough that for tactical use within a few thousand miles the zone number and letter code are not needed. The two letter codes are clearly indicated on the map and are used as the main reference locator. These two letter local square designations permit rapid orientation and rough position indications.

In the same way that was described for UTM you can use less digits for less accuracy when desired. For rapid orientation to a given area something like UQ 920820 could be useful. For gps use you will need to enter the full digit code down to the 1 meter level.

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