Coordinate Systems and GPS Intro

[Ben Low]

There are myriad "how to use a compass" type guides around the place, but few that explain the concepts behind things like why your map has three Norths (and that there is at least one more), what a grid reference actually is, why a GPS gives 12-digit grid references (and how to convert to a common 6 digit version), what the mysterious "56H" and "LG" that sometimes appear in map references mean, and why a six digit GR on it's own is relatively useless.

Attached is a short doc explaining the above, and as a bonus, how GPS works (basically the same concept of triangulation that navigators have use for centuries!).

PS: an activity to play with Scouts to introduce the idea of how GPS works: preparation: mark sets (one set for each Patrol) of three locations on the floor of the hall (e.g. masking tape), these will be the "satellites". Pick some unmarked location within the set of three satellites, this is the target the Scouts need to find later. Measure out (e.g. knot) three lengths of rope from each of the three satellites to the target, and label/colour code each (or simply measure the distances from each satellite to the target and have the Scouts measure out the required rope lengths). To "play": place a Scout at each location (satellites). Give each "satellite" a piece of rope and the pre-measured lengths of rope/distances. The non-satellite Scouts should be able to swing the taught ropes around in a circle - with two satellites they'll have zero, one or two intersections (should be two, of you did the prep correctly :-), with the third Scout they'll have one unique position fix.

PPS: also attached is a simple cheat-sheet guide to the common (cheap!) Garmin etrex GPS units (the older model). These would have to win an award for user-unfriendliness - the cheat-sheet gives the basics to get a "where the heck am I" fix (GR); intended to be printed out, cut, each copy folded in half, laminated and attached to the GPS unit.

Yours in Scouting,

Ben Low

Venturer Leader
1st Balgownie Scout Group
0407 17 25 21
ben...@balgowniescouts.org.au