In aerial photogrammetry the location of a point on a map by spherical co-ordinates i.e., latitudes and longitudes is a difficult process whereas it is simple and convenient to use the rectangular co-ordinate. For this, best reference system is a set of squares or rectangles ribled on map with reference to which the position of points or described by ordinary east or north or xyz co-ordinates. In aerial photogrammetry a grid is a network of perfect squares drawn on a series of maps of a zone on a selected projection. It is named after a projection of a map on which is supposed to be drawn Lambert grid is a set of equal and perfect squares when drawn on a series of maps having Lambert conical orthomorphic projections.
Origin and axes of grid co-ordinates in Aerial Photogrammetry
In aerial photogrammetry for each grid zone, origin of grid co-ordinates is the origin of projection on which grids are constructed. The directions of axes of co-ordinate are those of the central meridian and 1r to it through the origin. Grid co-ordinates are expressed in easting and northing. In aerial photogrammetry the co-ordinates of the origin are assumed to be large positive constant. So that nowhere in the grid zone any of the co-ordinates are in negative term. In the Indian grids, the constant assumed for the co-ordinates of origin are 3000,000 and 1000,000-grid yards/ meter for easting and northing respectively. The point whose grid co-ordinates are co, o is called the false origin. Its exact position is a matter of no consequence.