How can I measure the distance between two cities in the map? Well i know that the distance between Moscow and London by km it's about 2,519 km and the distance between Moscow and London in my map by cm it's about 30.81 cm and the Scale for my map is 1 cm = 81.865 km but when i tried to measure the distance between other two cities for example between London and Berlin with my map scale the result was wrong so i think that's because the spherical of earth ???!! 
Now i want to know how can i measure the distance between tow cities in the map also how can i know the scale of a map ?

The calculation is somewhat complex. A simplification is to assume that the Earth is a sphere and finding the great-circle distance. A more complex calculation instead uses an oblate spheroid as a closer approximation.
