1.3.5 Scale Distortion

Referring back to example in 1.3.3, it is clear that a projection will distort the distance between two points. Clearly the true (Earth surface measured) distance between anchorage and Washington DC has not changed, but the length on the map can change depending on the projection. Most projections we use have a sub unity ‘central scale factor’. In the example above the Mississippi West state plane projection has a central scale factor of 0.99975. This means that on the central meridian a 1,000 meter line measured on the ground will be represented by a line 999.75 meters at the scale of the map. If the map has a scale of 1:10,000, then the line on the map will be represented by a line 9.9975 cms long. If you plotted the same line on the central meridian of a UTM projection with a central scale factor of 0.9996, then the line on the map would be represented by a line 9.996 cms long.