Span of 1' of Longitude or Latitude :

Let us determine the distance represented by 1' of the longitude or latitude; that is, how much distance changes the coordinates by 1'. 
Earth's mean radius "a" = 6371 km.
Considering the earth as sphere 1degree of longitude at latitude f= p. acosf km.180.   ....{ i }.
At Delhi (f = 28° 39' N) 1° of longitude = 97.6 Km. Thus 1' of longitude at Delhi =1.626Km. and 1° of latitude = ª 1 mile p x a Km.180......{ ii }.
For all longitudes 1° of latitude = 111.2 Km. or 1' of latitude= 1.853 Km. ª 1.16 mile. For all practical purposes, in India we can consider 1 mile, making a difference of 1' in longitude or latitude or combined difference of both.
 From formula 1 and 2 it is obvious that the distance in North or South makes a variation in latitude, which is constant for all places on earth. However the longitude at least changes as much as latitude and the variation becomes more and more prominent as latitude increases. This is obvious because of the fact that equator the circumference of earth is maximum, where as it reduces as latitude increases and rate of change of circumference also increases with the increase in latitude. A table can be drawn for distance covered by 1' of longitude at different latitudes.