B&G Deckman - Manual (Page 53)

Chapter 3: Navigation
3.26
number is variation to the East. Deckman will automatically calculate
variation based on your time and position based upon the world
magnetic model so you should never really need to change this.
Bearings can be set to TRUE or MAGNETIC so that all bearings,
laylines etc are displayed accordingly. This can be extremely useful in
areas of the globe where variation changes rapidly such as the Southern
Ocean.
Tide
You have a choice of different sources of tidal information in Deckman:
Manual
This is simply a manually entered current rate and direction – these are
set in the current rate and direction boxes at the top of the Navigation
options window. This might be a particularly useful if you have just
noticed the current on a buoy as you sailed by.
Measured
Every 2 seconds, Deckman compares the GPS position with the dead
reckoned (DR) position, with the difference being due to current effects
(and, possibly, calibration errors). The difference is then termed the
measured current. This value is then averaged over a time period set in
the Current update time box.
Diamond
Another current which Deckman is constantly updating is referred to as
the diamond current: this is calculated every 10 seconds from the tidal
database — if the database doesn't cover your area then the calculated
values are zero. All the information is controlled by entering times and
heights of high water for ports near to where you are sailing, as
described in Edit High Water (see page 3.14).
These values may be more steady than the measured current, but it is
possible that they are also wrong because of the conditions at any
particular time.