marin.faure@boeing.com writes:
But I was curious if there is a simple point-and-read instrument for
determining distances that didn't cost a bunch.
Another accurate method besides radar is your chartplotter.

The yellow east coast refrence book, by Eldridge, has a simple range finding
technique discussed. It relates a 45 degree angle from a shore point...versus
the perpendicular view angle can anyone post that, its only a paragraph or two
as I recall. And well worded after likely a 100 years.
It relates your distance traveled at a known speed parallel to shore (from
perppendicular to 45 degrees, or 45 degrees to perp) and takes advantage of the
fact that legs opposite the 45 degree angles in a triangle are equal length.
So your distance travelled equals the distance offshore.

I try to practice estimating distances by comparing visual objects with my
chart plotter. First look at,say a buoy, then guess how far away it is and
compare to your chartplotter distance..or radar. After a while you get a lot closer
than your initial guesses, things on the water LOOK closer than they usually
are. When in season, I can often get with 15%, not so at the start of the
season when 25% is about my best...

Rob Brueckner
1972 Hatteras Yachtfisherman

Tuesday, December 6, 2005, 4:54:20 AM, GYMKIDD319 wrote:

Gac> The yellow east coast refrence book, by Eldridge, has a simple range finding
Gac> technique discussed. It relates a 45 degree angle from a shore point...versus
Gac> the perpendicular view angle can anyone post that, its only a paragraph or two
Gac> as I recall. And well worded after likely a 100 years.
Gac> It relates your distance traveled at a known speed parallel to shore (from
Gac> perppendicular to 45 degrees, or 45 degrees to perp) and takes advantage of the
Gac> fact that legs opposite the 45 degree angles in a triangle are equal length.
Gac> So your distance travelled equals the distance offshore.

This is a special case of the more general "Doubling the angle on the bow".  If you take two bearings on the same object, with the second bearing twice the first, the distance run between bearings equals the distance off at the time of the second bearing.

There are a couple of special cases: if you take bearings at 22.5 and 45 degrees, the distance off when abeam will be 7/10 of the distance run.  If you use 26.5 and 45 degrees, the distance run between bearings equals the distance off when the object is abeam.

Bowditch Table 7 covers this using a wide range of angles.

--
Peter Bennett, VE7CEI    Vancouver, B.C., Canada
Lien Hwa 28 (AKA Polaris 30) "Sea Spray"
GPS and NMEA info: http://vancouver-webpages.com/peter
Vancouver Power Squadron: http://vancouver.powersquadron.ca