Projections

The problem with maps


We create maps on a globe or on a 2-dimensional plane (computer screen or piece of paper).

But the earth is neither a perfect sphere or flat.

The goals of map projectons

  • define where the data is located on the earth’s surface.
  • define how to draw those locations on a flat surface.

Define location on the earth’s surface

The earth is not a sphere

Define location on the earth’s surface

Geographic Coordinate Systems (GSC) approximate the shape of the Earth using a reference ellipsoid.

  • WGS84: World Geodetic System of 1984 is the most common - designed as a one-size fits all GCS.
  • NAD83: North American Datum of 1983 - the ellipsoid that best approximates North America.

Draw those locations on a flat surface.

Flat is really hard too

Projected Coordinate Systems (PSC)


A Projected Coordinate Systems (PSC) is an equation to draw on a flat surface, like on a paper map or a computer screen using a specific GCS.

Cylinders, Cones, and Planes

(Cylindrical, Conic, or Azimuthal / Planar)

source

Distortion

All projections distort.

They generally focus on preserving one (or two) of:

direction, area, or shape.

source

Mercator

EPSG:3395 (check out the mercator puzzle)

Conformal cylindrical - preserves direction (any straight line drawn on a map represents an actual compass bearing) – only good for marine navigation.

Albers Equal-Area Conic Projection

EPSG:102008

Equal area conic projection. Shapes, directions, angles, and distances are generally distorted. Good for North America – used by the U.S. Census Bureau. source

USA


———– Web Mercator (3857) —————– Albers (102008) ———

New York


—— Web Mercator (3857) ————- NY State Plane, LI (2263)—-

Projections tips


  • Know the units that your projection uses (feet, meters, degrees, etc)
  • Select your projection based on your needs:
    • webmap/static map/geographic calculations
    • use the projection that is commonly used in the area you’re mapping
    • use epsg.io to search for the projections for any area
    • deep-dive
  • Projections are defined using their EPSG code (EPSG lookup)

Projections tips - webmaps


  • Leaflet and Google expect you to use WGS84 (EPSG:4326)
    • transform your data to EPSG::4326 before you add them as layers to a webmap

Projections tips - static maps


  • United States maps:
    • Use Albers (EPSG:102008)
  • New York City maps:
    • Use NY State Plane Long Island Zone
      (EPSG:2263)

  • State or school district maps:
    • Use the projection commonly used in a state
    • Transform your data to that projection for your static maps