Why Are There Lines on the Map That Are Not on the Ground? Somebody Drew a Grid

Lines of latitude run east to west parallel to the equator, and lines of longitude run north to south from pole to pole, and together they form a grid covering the whole globe. The system is imaginary in the sense that nothing on the ground marks it, and it is not arbitrary, since the equator and the poles are fixed by the Earth's rotation and latitude is measured from the equator naturally. Longitude has no such natural origin, and the meridian counted as zero was chosen by agreement, which is why that choice took an international conference to settle in 1884.

The two families of lines behave quite differently:

• •Latitude lines are circles that shrink towards the poles
• •Only the equator among them is a great circle
• •Longitude lines are all great circles of the same length
• •They converge and meet at the poles
• •A degree of latitude is nearly constant in distance
• •A degree of longitude shrinks to nothing towards the poles

On a globe the network is regular, and on any flat map it is distorted, which is an unavoidable consequence of flattening a curved surface. Different projections distort it differently and the pattern of the lines is the quickest way to identify which projection is being used. A rectangular grid of straight lines meeting at right angles with widening spacing towards the poles indicates one family. Curved converging lines indicate a conic projection. Lines radiating from a point indicate a polar one. Reading the network therefore tells a map user what has been preserved and what has been sacrificed before they read anything else.

Most detailed national mapping overlays a second grid that is entirely different in character and easier to use for measurement. That grid is rectangular, with lines a fixed distance apart in metres rather than a fixed angle apart in degrees, which means a square on the map is a square on the ground and distances can be measured with a ruler. It is built on a projection covering a limited area, so the distortion within that area is small enough to ignore. The trade is that such a grid works only within its own zone and cannot cover the world, which is why positions stated in it must name the zone as well.

The word also names something quite different that shares the idea of an overlaid reference. In optical instruments it is the pattern of lines or marks etched onto a glass plate inside the eyepiece, which appears superimposed on whatever is being viewed and provides a scale or an aiming point. Microscopes use one to measure specimens, surveying instruments to read angles, and telescopic sights to aim. The crucial requirement is that it sits exactly in the plane where the image forms, so that both are in focus together and the mark does not shift relative to the image when the observer moves their eye.

Latitude and longitude lines form a reference grid that corresponds to nothing physical, with the equator and poles fixed by rotation and the zero meridian settled by agreement in 1884. Latitude circles shrink towards the poles while longitude lines are all great circles, so a degree of longitude covers less distance the further north or south it is measured.