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Northstar M121/M84 Installation and Operation Manual 73
Map datum
Instrument GPS positions are based on a
worldwide reference (datum) known as WGS
84. Most paper charts are based on WGS
84. However, some paper charts are based
on other datums In these cases, the latitude
and longitude coordinates of objects on the
Instrument chart window are different to the
latitude and longitude coordinates of these
objects on the paper chart. This applies to all
objects, such as the boat, waypoints, tracks,
lines of latitude and longitude and cartographic
features such as land, rocks, buoys and depth
contours.
Use Map datum to select the Instrument map
datum to match the datum of the paper chart.
Then, the latitude and longitude coordinates
of objects displayed on the Instrument
will change to match the corresponding
coordinates on the paper chart.
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 Instrument asks if you want to apply the
NMEA datum offset (see below).
NMEA datum offset
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
Instrument NMEA output:
Latitude and longitude coordinates
displayed on any NMEA repeater do not
match the coordinates on the Instrument.
Latitude and longitude coordinates
broadcast on any NMEA VHF transmitter will
be the same as the coordinates on a WGS 84
chart.
!
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.
!
WARNING
When you change to a chart with a different
datum, change the Instrument map datum
again.
Latitude and longitude coordinates
displayed on any NMEA repeaters match the
coordinates on the Instrument. 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 Instrument chart window to
where they should be.
The positions of the boat, waypoints,
tracks, and lines of latitude and longitude
on the Instrument chart window 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
MENU
and select Set.
5 Press
ESC
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
Instrument chart window.
1 In the Chart setup menu, select
Map shift.
2 Press
MENU
and select Clear.
3 Press
ESC
.