lakshmanok/asgbook

Java code accompanying "Automating the Analysis of Spatial Grids" by Valliappa Lakshmanan, Springer, 2012

asgbook's Issues

Question about effective earth radius in distanceInKms

At line

asgbook/src/edu/ou/asgbook/core/LatLon.java

Line 43 in d320b4d

double R=a * (1-esq)/Math.pow(sq(1-esq*(Math.sin(lat0))),1.5);

of the function distanceInKms, the effective earth radius is computed as:

R = a * (1-esq)/Math.pow(sq(1-esq*(Math.sin(lat0))),1.5);

where sq is the squared number (sq(x) = x*x), lat0 is the mean of the two input latitude, a is the semi major axis of the ellipsoid model WGS-84.

I am expecting the radius to be equal to a = 6378.137 when lat0 = 0, i.e. when both points are on the equator. Or I am expecting the denominator to be equal to (1-esq). With the sin dependency factor, this is not the case, and so when both points are on the equators, the effective radius is 6335.439

How is that possible?
Also where is the formula coming from? The resource cited is the Snyder 1987, though it would help to know the page where this formula is taken.

Thanks