I'm measuring distance of 2 points on Google Map and then in my program converting them into ECEF using this formula. Then using Pythagorean theorem to calculate distance between those 2 points. Difference is about 100-200m, which is fairly big for what I am doing. In my program I use double for precision.
Big question will be what method Google is using in order to calculate distance between 2 points?
Update, example:
Lon/Lat : 44.50310895983847,40.16009701404741 and 44.50151117520584,40.15672510042466. Google Earth says it's 400m.
After converting to ECEF I get 3482269.0815067333, 2938590.6893090927 and 3482536.9833229352, 2938466.2491839984. I measure distance of 276.99011m