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Map Overlays (Range Rings; Lat/Long Grid)
The map screen can be customized with four range rings and/or grids
that divide the plotter into equal segments of latitude and longitude.
Range rings are handy for visually estimating distances on the map.
The ring diameters are based on the current zoom range. For example:
at the 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 diameter, the 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 move around the map to see the third and
fourth rings. In this example, the distance 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
RANGE RINGS.
With the range rings highlighted, press
ENT to turn on (check) or turn
off (uncheck) the option.
To set Lat/Long Grid: From the Map Data menu, press ↓ to
LAT/LON
GRID LINES. With Lat/Lon Grid Lines highlighted, press ENT to turn on
(check) or turn off (uncheck) the option. After the option is set, press
EXIT|EXIT to return to the map 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 also may cover the entire world or just a
small portion of it.