Projections

GIS contains spatial data (unlike other information systems).  Spatial data includes coordinates that define geographic objects.  Geodesy was mentioned in the last post. Geodesy is “the science of measuring the shape of the Earth (Bolstad).”  Because it’s hard to perceive the curvature of the Earth on a human scale, we’ve been using flat maps for the majority of our lifetime.  Flat maps distort geometry due to not being able to take into consideration the curvature of the Earth.

A flat map complicates defining coordinates because measurements are affected by the distortion created by flat maps.  The irregular shape of the Earth and imperfect measurements also complicate the defining of coordinates.  Earth is not a perfect sphere shape and is actually very deformed because of natural forces.  Our measurements contain error but with improvements over time and with the sophistication of models, our measurements of positions on Earth improve.

All these differences in defining coordinates cause us to create “several sets of coordinates that define the same location on Earth (Bolstad).”  The points created at a location on Earths’ surface are coordinates that depend on how the points from a flat map are translated onto a curved map surface, what type of reference is used in creating the coordinates and what shape of the Earth is taken into consideration when formulating the measurements for the points.

 

 

Reference:

 

Bolstad, Paul. (2012).  GIS Fundamentals:  A First Text on Geographic Information Systems.  White Bear Lake, MN:  Eider Press.