A grid system allows the location of a point on a map (or on the surface of the earth) to be described in a way that is meaningful and universally understood. Projecting the earth’s surface (or a portion of it) in one of the ways outlined in the Map Projections page, allows for a representation of an area on a flat piece of paper. Once this is accomplished, it is necessary to set up a coordinate system on the map that will allow a point to be described in X-Y space.
However, in order to describe this location in a universally understandable manner a grid system is necessary. A simple grid is shown with the location of a point of interest that we want to describe.
In order for a point designation on a grid to be meaningful, there must be an origin to the grid which can be used to reference the point to. Once an origin is assigned then there is only one correct designation for the point, and anyone looking at the grid will assign it the same value and be able to interpret what someone else means when they describe a point located at 3,3.
A few examples of possible origins for the grid are shown. In the first example, the designation for the point would be 3,3. In the second the designation would be 3,1, and in the third it would be 1,1. All of these designations describe the same point and the only thing that has changed is the origin of the grid. In order for any type of grid to be useful it is necessary for it to have an origin and a uniform grid spacing (i.e. the distance between grid lines should remain constant).
There are several types of grids (or coordinate systems), used to divide the earth’s surface.
Geographic . . . Uses degrees of latitude and longitude. One of the most common coordinate systems in use.
UTM . . . Preserves shape, and allows for precise measurements in meters.
State Plane . . . Developed for local surveying, with minimal distortion.
Public Land Survey . . . This one was used in Colonial America for surveying. Not as accurate as others.