Get weird result when calculating distance between two points, using latitude longitude?

Question

I use algorithm (see: calculating distance between two points) to calculate distance between two points. but it get wrong result, see Java code:

/**
 * group1：set latitude to 89
 */
// set longitude1 = 0, longitude2 = 1
System.out.println(distance(89, 89, 0, 1, 0 , 0)); // 1941.0            result1

// set longitude1 = 0, longitude2 = 2
System.out.println(distance(89, 89, 0, 2, 0 , 0)); // 3881.0            result2

// set longitude1 = 0, longitude2 = 179
System.out.println(distance(89, 89, 0, 179, 0 , 0)); // 222381.0        result3

// set longitude1 = 0, longitude2 = 180
System.out.println(distance(89, 89, 0, 180, 0 , 0)); // 222390.0        result4

/**
 * group2：set latitude to 0
 */
// set longitude1 = 0, longitude2 = 1
System.out.println(distance(0, 0, 0, 1, 0 , 0)); // 111195.0            result5

// set longitude1 = 0, longitude2 = 2
System.out.println(distance(0, 0, 0, 2, 0 , 0)); // 222390.0            result6

// set longitude1 = 0, longitude2 = 179
System.out.println(distance(0, 0, 0, 179, 0 , 0)); // 19903892          result7

// set longitude1 = 0, longitude2 = 180
System.out.println(distance(0, 0, 0, 180, 0 , 0)); // 20015087          result8

In group1:

a = result2 - result1 = 1940 

b = result4 - result3 = 9

In group2:

c = result6 - result5 = 111195

d = result8 - result7 = 111195

It's very strange. c = d, group2 is right, but a ≠ b, group1 is wrong.

How do I solve this issue?

Is there other higher accuracy algorithm for calculate distance between two coordinate points?

Answer 1

No issue here - the formula calculates shortest path across the globe which in case #3 and #4 goes across the pole, not along a line of constant latitude which you are assuming.

Cross reference results using this tool here.