NavList:

Three members of navlist have contacted with me off-line asking information.

As I think this information might be of interest, here it is.

The Meridional Parts can be calculated like this:

Fi = latitude

a = main axis

Flattening: f=1-b/a

Eccentricity: eoe = sqrt(2.0*f-SQ(f));

Put the values on this formula, and you obtain the values.

M1 = log(10)*log10(tan( 45.0+fi/2.0) );

M2 = SQ( eoe )*sin( fi );

M3 = pow( eoe, 4 )/3.0*pow( sin( fi ), 3);

M4 = pow( eoe, 6 )/5.0*pow( sin( fi ), 5);

M = a*(M1-M2-M3-M4)

Values for "a" in nautical miles, and "f" for various Earth models:

·         BOWDITCH   21600.0/(2.0*PI), 1.0/298.26

·         WGS84      6378137.0/1852.0, 1.0/298.257223563

·         WGS72      6378135.0/1852.0, 1.0/298.26

·         ESFERA     21600.0/(2.0*PI), 0

·         NAD83      6378137.0/1852.0, 1/298.257222100880

·         GRS80       NAD83

·         EUROPE1950 6378388.0/1852.0, 1/297.0

If a spherical-Earth model is used, then the to axis are equal: a=b, and f=0, eoe = 0

More information at my web site: “The Sailings” paper.

You can see that BOWDITCH use "a" for "f" spherical model and f for the WGS72. This is the error of concept.

Andrés Ruiz

Navigational Algorithms

http://www.geocities.com/andresruizgonzalez