Map Scale

The map scale is a fraction with a numerator of 1. The denominator shows how much length on the ground a unit of length on the map represents. On a 1:24,000 map, one cm on the map represents 24,000 cm on the ground. Since the scale is dimensionless, one of any unit represents 24,000 of the same unit on the ground.

Three standard paper map scales from USGS, showing the Naval Academy in Annapolis.  Printed on paper, each map is approximately the same physical size, within about a factor of two.  This size reflects an easy size for printing presses to handle, and for the user to manipulate. 

They are referred to as quadrangle maps, because they are quasi-rectangles bounded by parallels and meridians. 

  • 1:24,000 map
  • covers 7.5' by 7.5'
  • on this screen each pixel is 4 m
  • Largest scale: small area, large detail
  • About 50,000 maps at this scale cover the continental United States (CONUS)
  • 1:100,000 map
  • covers 30' x 60'
  • on this screen each pixel is 16 m
  • About 3200 maps at this scale cover CONUS
  • This example shows a digital merge of two separate maps
  • 1:250,000 map
  • covers 1°x2°
  • on this screen each pixel is 64 m
  • Smallest scale: large area, low detail
  • About 800 maps at this scale cover CONUS
  • This example shows a digital merge of two separate maps

As a fraction, 1:24,000 is bigger than 1:250,000, so the 1:24,000 map is a larger scale map than the 1:250,000 map. As a corollary, the large scale map covers less area on the ground (unless it is physically much larger).  The large scale map should have more detail.

The boundary between large scale and small scale changes with the context. 

With digital data, the computer can change the scale for display, but the data was still collected at a specific scale which limits how much it can be blown up or reduced. Map scale and effective resolution.

With vector data, scale determines the number of points used to characterize a line segment and the spacing between the points. Large scale maps need many closely spaced points to accurately depict where a curved line goes. Small scale maps must be more generalized, and need fewer, more widely spaced points.

With raster data, scale determines the size of each pixel used to represent the data.  Large scale data has many more smaller pixels.

Approximate conversions, degrees to metric.

Last revision 6/17/2020