dips wrote:
hi all,

iam facing a problem regarding the realtion between latitude,longitude
and (x,y) pixcel.The basic idea is to plot the tracks given by gps
reading ,to the map .

The "realtion" (=relation??) between lat & long constantly changes as
you move away from the equator.

Navigation is not done "on" a chart (it's extremely difficult to be
accurate with pencils, instruments etc on a chart, even more so on a
small boat:-)) but traditionally it's done via math calculations, then
the results are "plotted" to a chart, hence an electronic chart plotter
is programmed to calculate the answers very quickly & display the
plotted track.

1 minute of latitude is 1 nautical mile in length on the face of the
earth; anywhere on earth, but

1 minute of longitude is ONLY 1 nautical mile in length at the equator.

Going north or south of the equator, the length of a minute of
longitude gets shorter the further you move away from the equator, till
at the poles it's length is nil.

Basically everything can be reduced to right angled triangles (they're
spherical triangles). After that there are several simple formulae (many
others do the same thing) which given some "knowns" you can work out the
"unknowns"

So plotting a track onto a chart from various knowns is just a matter
of applying the required formula from what you do know.

Some of the most commonly used formulae a-)

Difference in latitude
Distance = ------------------------
COS of the course

or the same thing is

Distance = difference in latitude X SECANT of the course.

Difference in longitude X COS of the mean latitude
Tan course = ---------------------------------------------------
difference in latitude.

Difference in lat. = distance X COS of the course.

Difference in long.= distance X SIN course X SECANT mean lat.

or

distance X SIN course
Difference in long = ------------------------
COS mean lat.


So once you have a chip programmed with the right formulae, then as the
GPS delivers the actual positions, the plotter can calculate & place the
track on the screen, or if you want to manually draw it to a chart then
you can calculate the lat & long with your "knowns" & just join the dots.

K

& the Krause lie of the day???

Not only does this delusional liar see himself as a "real" boat owner;
he also makes up lies about every facet of boating including always
buying below the market & yes you guessed it, our lying Krause always
claims he sells (just before anyone actually checks) well above it,
indeed he says he operates in his socialist union fantasy where
"markets" don't even exist:-)


The Hatteras is sold. The SeaPro is loosely for sale against a guaranteed
trade-in value good through 30 June towards a new boat. I'm not
concerned with
the "next" issue and I never have been. I've bought and sold upwards of
25-30
boats over the years without ever once "consulting" any of the guides.
Time and
again, I've had surveyors tell me, "Wow...this boat shows way off the
top of the
bluebook allowances..."


No but seriously:-) if he let go of it he'd just fly away; yes????