30 DSM300 Operation with PLUS Series Displays
Variation Source
The variation value is the difference between True and Magnetic direction data for
heading or bearing values. The Variation Source option provides soft keys for
selecting Auto or Manual variation mode, displays the current variation value for
each and highlights the currently selected mode.
Auto Mode
If you select Auto mode, the unit obtains the value of variation automatically,
normally from received data. The variation value that is used depends on the data
available and is selected in the following order of priority:
1. Variation value from the same source as the heading data:
• If heading data is being taken from NMEA, then variation is also taken
from NMEA
• If heading is taken from SeaTalk, then SeaTalk variation is used
2. Variation value from a different source:
• If heading data is being taken from NMEA, but no NMEA variation is avail-
able, then variation is taken from SeaTalk
• If heading is taken from SeaTalk, but no SeaTalk variation is available,
then variation is taken from NMEA
3. A calculated variation value, using position data, if no SeaTalk or NMEA value
is available
4. The current manual variation value, if no SeaTalk or NMEA value and no posi-
tion data is available
Manual Mode
If you select Manual mode, by pressing either of the MANUAL keys, you can
specify the local variation value according to the area in which you are operating.
Press the appropriate MANUAL key to adjust the variation up or down, to a
maximum of 30° East or West.
This value is then transmitted to any other SeaTalk instruments on your system. It
is retained if you turn the display off and on again.
In Manual mode, incoming NMEA variation is ignored. However, if the variation is
changed on another SeaTalk instrument, the new value is used and the manual
value that is displayed is updated.
Note:
The Manual variation value defaults to 0°, so it is important to set up a value if vari-
ation is not available from an external source.
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