For latitudes like in India a city, which has span of 40 Km. or a distance of 25 miles, can make a difference of 25' in longitude and latitude. This is particularly so in case of Delhi and Mumbai, where the city stretches to over 40 km. diagonally. In Delhi, where the accepted coordinates 28°39' N & 77°13' E are for New Delhi Railway Station, easily makes a difference of over 25' in longitude and latitude from one end to the other.
For example Nangal in South-West of Delhi has coordinates 28°33'N and 77°06'E, whereas Vikas Kunj in North-East coordinates of 28°45'N and 77°18'E, thus reflecting a difference of 12' in longitude and 12' in latitude, with a total difference of 24'. Similarly in Bombay Dahisar in North has coordinates 19°16'N and 72°51'E, whereas Colaba in South has coordinates 18°54'N and 72°49'E, making a difference of 22' in latitude and 2' in longitude, again making a total difference of 24'.
However small cities are normally only half or even less than half the size of Delhi or Bombay. Towns are only a few kms. in length or breadth, thus making a total difference of few minutes in all. If a centre point is chosen then the difference does not exceed more than 1' or 2' in latitude and longitude combined. For this reason for most of the towns and places, 1' accuracy in longitude or latitude is just sufficient, whereas in metropolitan cities a further breakup into small area is advisable.
To understand the total difference caused by longitude and latitude, let us convert the maximum combined difference in Delhi or Bombay (~25') into time. We find it is equivalent to 100s of time. And from a centre point it is only ±50s. i.e. less than a minute! Thus when time of birth is accurate only to a minute level, taking the centre point of even the metros for longitude or latitude is not going to add much to the inaccuracy in results.
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