GIS Lecture: Projections

Outline Map Projections Coordinate Systems

Map Projections

Map Projections and Distortion Mathematical transformation that behaves as if it were projecting features of the world onto one of three surfaces: a plane, cone, or cylinder

When Projection Is Important Measurements used to make important decisions Comparing shapes, areas, distances, or directions of map features Feature and image themes are aligned New York New York Los Angeles Los Angeles Projection: Mercator Distance: 3,124.67 miles Projection: Albers Equal Area Distance: 2,455.03 miles Actual distance: 2,451 miles

When Projection Is Not Important Business applications Not of critical importance Concerned with the relative location of different features On large scale maps - street maps Distortion may be negligible Map covers only a small part of the Earth's surface.

Coordinate Systems

Projection vs. Coordinate System the method used for flattening the paper on the spherical earth Coordinate System = the method for referencing specific locations - on a sphere (Geographic Coordinates) - on that flattened paper (Rectangular Coord)

Geographic Coordinates Geographic Coordinate System (GCS) Spherical coordinates “Un-Projected” Angles of rotation of a radius anchored at earth’s center Latitude and Longitude

Latitude and Longitude Longitude (Meridians) Latitude (Parallels)

Latitude and Longitude Longitude (prime meridian) Latitude (equator)

Latitude and Longitude Coordinates Pittsburgh, PA USA 40°26’2”N latitude -80°0’58”W longitude 40 -80

Rectangular Coordinates Used for locating an intersection on a flat sheet of paper Cartesian Coordinates (X,Y) X>0 Y>0 X<0 Y<0 (0,0) Y X

State Plane Coordinates Used by local US Governments All positive coordinates in feet

State Plane Coordinates Zones 125 zones, following state and county boundaries At least one for each state

City of Pittsburgh as Geographic Coordinates

City of Pittsburgh as State Plane Coordinates

Universal Transverse Mercator (UTM) Coordinate system used by U.S. Military Covers entire world Metric coordinates

How do I know what my data is?

Summary Map Projections Coordinate Systems