GL: Swinging

[KevinR]

> -----Original Message-----
> John, can you recommend a GPS that your method would work with?  Every GPS
> that I have needs more motion than four people twisting a boat to give a
> good directional reading. 

 
Vance,

The COG heading shown by a GPS receiver says nothing about the direction the
bow is pointing (the way a compass does), but rather it shows what the
average direction was that the boat traveled over the ground over the past
xx seconds (x varies with manufacturer). That reading is not useful in
swinging a compass when turning circles. If the boat was pointing North, but
drifting backwards in a southbound current, the GPS would show 180, when the
bow was really pointing 000!

To use a GPS receiver in building a deviation table, the boat must be run in
a straight line with no wind and current affecting the COG. Then the GPS COG
reading should be the same as the direction of the bow, and that will be a
fairly accurate number to use in swinging the compass. Just running the boat
in circles will not give you useful number on the GPS COG reading for use in
swinging the compass.

Kevin