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Northstar Explorer 538i/538 Installation and Operation Manual
41
Setting map datum
1 In the Chart setup menu, select
Map
datum.
2 Select the map datum for the paper chart you
are using.
3 If you select a datum other than WGS 84 the
Explorer 538/538i asks if you want to apply
the NMEA datum offset (see below).
NMEA datum off set
If you select a map datum other than WGS 84,
the map datum offset can be applied to latitude
and longitude coordinates sent on the Explorer
538/538i NMEA output:
Latitude and longitude coordinates
displayed on any NMEA repeater do not
match the coordinates on the Explorer
538/538i. Latitude and longitude coordinates
broadcast on any NMEA VHF transmitter will
be the same as the coordinates on a WGS 84
chart.
Latitude and longitude coordinates
displayed on any NMEA repeaters match
the coordinates on the Explorer 538/538i.
However, latitude and longitude coordinates
broadcast on any NMEA VHF transmitter will
be slightly offset from coordinates on a WGS
84 chart.
Map shift
Some charts have consistent position errors. To
correct this, apply a map shift. After a map shift:
The positions of cartographic features (such
as land, rocks, buoys and depth contours)
move on the Explorer 538/538i chart display
to where they should be.
The positions of the boat, waypoints, tracks,
and lines of latitude and longitude on the
Explorer 538/538i chart display remain
unchanged.
Applying a map shift
1 Move the boat to a known point on the chart,
for example a marina berth.
2 In the Chart setup menu, select
Map shift.
3 Move the cursor to the position on the chart
where the boat actually is.
4 Press
and select Set.
5 Press to set the new map shift. The
boat will now be displayed at its actual
location.
Clearing the map shift
Clearing the map shift removes any map shift
from the cartographic features on the Explorer
538/538i chart display.
1 In the Chart setup menu, select
Map
shift.
2 Press and select Clear.
3 Press
.
When you change to a chart with a different
datum, change the Explorer 538/538i map
datum again.
Map shift is for eliminating minor offsets.
It should not be used if the correct datum
is available. Use map shift with caution:
incorrect application will cause incorrect
boat positions.