Eagle Electronics 250i manual Map Datum Selection, Map Overlays Range Rings Lat/Long Grid

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 EXITEXIT 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.

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