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PCF OFFSET
The GPS navigation system relies on complex mathematical calculations
to determine your position based on satellite data and other factors. One
factor is the Earth's shape. Since the Earth is not a true sphere, variations
in the calculations have to be made to accommodate deviations. To make
matters more complex, not everyone uses the same data to determine
what the deviations are. The size and shape of the ellipsoids that are used
to approximate the earth's surface are improved often. This can lead to
errors if your navigation device uses one ellipsoid, while your chart uses
a different one. The term used for these ellipsoids is "Datum."
To reduce the error factor between datum, this unit gives you the capability
to move or "offset" the position shown on the display to match the one on
the chart. The unit will add this offset to all position displays at all times.
For example, suppose you are anchored at a location that is accurately
marked on a chart. Your unit shows a longitude position that is .010
degrees less than the one on the chart. Using the Position Correction
Factor (PCF) Offset feature, you make the unit read the same as the chart.
If you raise anchor and move, the unit will continuously add the change to
all position displays. This makes it more closely match the datum used by
the chart. For this reason, you should be careful when entering the PCF
offset. This offset is saved in memory. It doesn't change when the unit is
turned off. However, a Preset does erase the PCF factor.
To set the PCF factor, first
press the MENU key, then
press the key next to the
"Change GPS Settings" label.
Now press the key next to the
"PCF Offset" label. The screen
shown at right appears.
Now press the key adjacent to
the "Change Offset" label to
enter the correction factor for
your location. The screen
shown at the top of the next page appears. Remember, this is the
difference between the location shown on the present position display and
the position shown on the chart.
Use the arrow keys to move the black box to the number that you wish to
change in the latitude, then enter the numbers. Press the key next to the
"Latitude N S" to change the latitude from north to south, if necessary.