Projections create distortion

ArcGIS Help on projections says

All map projections distort shape, area, distance or direction to some extent. The impact of this distortion on your work depends on what you will be using your map for, and its scale:

……and David adds, the extent of area of interest.  Find a good overview of these distortions at this web site

The map has different names, depending on what you hold constant:

  1. equal area – homolographic — if you put a dime on a map, it’ll always cover the same area, but shapes and angles are distorted
  2. shape – conformal — at a point, relative local angles are preserved, meridians intersect parallels at right angles, areas enlarged or reduced
  3. scale – equidistant — equidistant between two points and rest of map only, or along meridians
  4. directional – azimuthal — all rhumblines (lines of constant direction) are straight

Open the Virginia Counties map from your GIS\demo\introPro project and change the coordinate system of the map to

  • Projected/State Plane/VA state plane south zone (polyconic)
  • Projected/UTM/NAD 1983/grid zone 17N and then try zone 5N (transverse Mercator)
  • Projected/world/Mollweide (equal area?)
  • Geographic/North America/NAD 1983 (simply longitude as x and latitude as y, or GCS- “geographic coordinate system”)

Compare size and shape. (Could you make a layout with 4 different maps using different projections? Have you learned layouts yet?)

The maker of the cartoon XKCD claims to have the “best” map considering all four of these potential distortions. The “Waterman” projection.