Topographical Map is a large scale map

Maps are divided on the basis of scale and on the basis of content

A topographic map is a detailed and accurate three-dimensional representation of natural and human-made features on the Earth's surface.

Post office's position is 2D

Library is in E1

As the topographic map is a map of a large area, many lines of latitudes and longitudes pass through it.

In order to have a system for locating features/places on the map, the system of Eastings and Northings are used.

Black vertical and Horizontal lines on the topographical sheets are called eastings and northings respectively.

The four figure grid system is used to locate places/features on a topographical map.

In eastings values increase toward the North

In northings values increase towards the east.