84
Map Overlays (Range Rings, Lat/Long Grid)
The map screen can be customized with range rings and/or grids that
divide the plotter into equal segments of latitude and longitude. Range
rings are handy for estimating distances on the map. Ring diameters
are based on the current zoom range. At a 100-mile zoom, the screen
will show two rings with your current position in the center. The large
ring touching the left and right sides of the screen is 100 miles in di-
ameter (same as the zoom range). The second, smaller ring is 50 miles
in diameter (always 1/2 the zoom range).
The distance from your current position to the smaller ring (the ring's
radius) is 25 miles (always 1/4 the zoom range). With the arrow keys
and map cursor, you can scroll the map to see the third and fourth
rings. In this example, the distance from your current position to the
third ring is 75 miles and distance to the fourth ring is 100 miles.
To set range rings:
From the Map Data menu, press
↓
to
R
ANGE
R
INGS
.
With the option highlighted, press
ENT
to turn it on (check) or turn it
off (uncheck).
To set Lat/Lon Grid Lines:
From the Map Data menu, press
↓
to
L
AT
/L
ON
G
RID
L
INES
. With the option highlighted, press
ENT
to turn it on
(check) or turn it off (uncheck). After the option is set, press
EXIT
|
EXIT
to return to the main page display.
Map Datum Selection
Maps and charts are based on a survey of the area covered by the map
or chart. These surveys are called Datums. Maps created using differ-
ent datums will show the same latitude/longitude in slightly different
locations.
The GPS system is based on the WGS-84 datum, which covers the en-
tire world. Other datums may also cover the entire world or just a
small portion of it. By default, your position is shown using the WGS-
84 datum. It, however, can show your position using any of 191 differ-
ent datums.
Datums can be selected only in Advanced Mode.
To change the datum:
1. Press
MENU
|
MENU
|
↓
to
GPS
S
ETUP
|
ENT
|
↓
to
D
ATUM
S
ELECTION
|
ENT
.
2. Scroll
↓
or
↑
to select the desired datum, then press
ENT
.
3. To return to the main page display, press
EXIT
|
EXIT
.
A list of datums supported by this unit is in the back of this manual.
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