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5-8 SL520/530/631 PLUS Chartplotter Displays
Variation Source
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 display obtains the value of variation
automatically, normally from received data. The variation value that is used
dependson thedata available, and isselected inthefollowing order ofpriority:
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
available, then variation is taken from SeaTalk
• If heading is taken from SeaTalk, but no SeaTalkvariation is available,
then variation is taken from NMEA
3. A calculated variation value, using position data, if no SeaTalk or NMEA
valueisavailable
4. The current manual variation value, if no SeaTalk or NMEA value and no
position 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 variation is not available from an external source.
Bridge NMEA Heading
The display bridges NMEA input data to the SeaTalk bus (see Section 6.8).
The Bridge NMEA Heading option can be used to prevent NMEA heading
data being bridged onto the SeaTalk bus.