GL: Question

[Ken Bloomfield]

Chuck,
I understand your point, but in this case I think that your "Crow flys 
distance" comment is not strictly correct, especially as it relates to what 
the question was about.  It is too easy to confuse the "distance log" 
function of a GPS and the "Distance to next waypoint" functions.  As regards 
the "distance log" function, a GPS has an internal update repitition rate 
whereby it updates the current position on a timed basis.  In effect, when 
you are moving, this is a whole host of little position points, typically 
some seconds apart.  It measures the distance between these points and keeps 
an internal running sumlog of the distance, so it is pretty accurate, 
especially if the update is frequent.  This, by the way, is not always the 
same repitition rate that the position is "reported to the screen" (i.e. 
your little boat symbol is moved).  So, this is truly integrated Speed Over 
Ground (SOG) and is pretty good.  Some GPS units also will report Velocity 
Made Good (especially common in aircraft units) where your rate of closure 
to your waypoint is shown.  This is not very useful/meaningfull in a boat as 
it changes with every bend in the river.

You  can verify this by example.  If what you say was always the case, then 
if you left Point "A" and went in a big circle to point "B" and it so 
happened that point "B" was 500 feet away from your initial point "A" then 
according to your description the distance logged would be only 0.1 mile, 
and that is not the case.  If you set Point "B" as the first (next) waypoint 
and then steered from "A" to "B" manually in a big circle of some 5 miles 
instead of point to point, your distance log would report pretty close to 5 
miles, even though if just as you started you asked the GPS distance to 
Waypoint "B" it would probably say "Distance to waypoint "B" 0.1 mile".

It is, however true, that if you put in a "GO TO" waypoint and ask the GPS 
the distance to that waypoint, the distance to that waypoint will be shown 
as an "as the crow flys" distance since it has no way of knowing how you 
intend to get there.  However, when you start actually travelling, the 
distance log on any reasonably new (i.e. the past 5 years) GPS will be the 
actual distance travelled.  However, whenever queried, the "Distance To 
Waypoint" will be, as you say "as the crow flys".

Ken Bloomfield
AGLCA# 3529
MTOA# 2062
M/V Tellico Lady, 50' MT-Walkaround



> Your GPS is measuring 'as the crow flys', a straight line from point A
> to
> B, mileage markers actually follow the river path.
>
> Chuck Kersey
> S/V
> Second Wind