Northstar
EXPLORER 550/550i
Installation and Operation Manual
44
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 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
NMEA output:
Latitude and longitude coordinates
displayed on any NMEA repeater do not
match the coordinates on the EXPLORER.
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. 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 chart display to
where they should be.
The positions of the boat, waypoints, tracks,
and lines of latitude and longitude on the
EXPLORER 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
chart display.
1 In the Chart setup menu, select
Map
shift.
2 Press
and select Clear.
3 Press
.
!
WARNING
When you change to a chart with a different
datum, change the EXPLORER map datum
again.
!
WARNING
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.
