GEOG 862
GPS and GNSS for Geospatial Professionals

Lesson 6 Overview



In the opening sentence of my book Basic GIS Coordinates, “Coordinates are slippery devils. A stake driven into the ground holds a clear position, but it is awfully hard for its coordinates to be so certain, even if the figures are precise.” I try to give the sense that coordinate systems must and do evolve as measurement technology improves. That has certainly been the case with the advent of GPS/GNSS. That is reflected in the commonly used systems of State Plane Coordinates and also in heights. It may come as a bit of surprise, but the height, or elevation, that is most easily derived from satellite positioning is almost certainly not the sort of height you are after. Also, did you know that the string of a plumb bob is actually very slightly curved? We will discuss these issues and the concepts at their foundation.


At the successful completion of this lesson, students should be able to:

  • explain the basics of State Plane Coordinates;
  • describe NAD83 positions and plane coordinates;
  • identify map projections;
  • define map distortion;
  • differentiate between SPCS27 and SPCS83;
  • describe scale and distance in State Plane Coordinates;
  • explain the basics of heights;
  • identify ellipsoidal heights;
  • recognize orthometric heights;
  • discuss the evolution of the vertical datum in North America; and
  • recognize the geoid.


If you have any questions now or at any point during this week, please feel free to post them to the Lesson 6 Discussion Forum. (To access the forum, return to Canvas and navigate to the Lesson 6 Discussion Forum in the Lesson 6 module.) While you are there, feel free to post your own responses if you, too, are able to help out a classmate.