Curated OER
X Marks the Spot
Fourth graders use games and puzzles to explore co-ordinate systems and their uses. Students are given the opportunity to invent their own systems by locating counters on a sheet of paper and by finding the dead center of a set of objects.
Wolfram Research
Wolfram Math World: Z Axis
Contains a graphical representation of the z-axis in a 3D cartesian coordinate plane and expresses it's normal orientation. Site has related links.