ZHANG, Ruda October 23, 2017:  digitize variational principles [bad00f3]
Ruda Zhang January 08, 2017:  Add new page to Category. [b590935]
Ruda Zhang October 18, 2016:  Definition of Mercator [c2b5e0b]
Ruda Zhang October 17, 2016:  terrestrial reference system vs map projection [6e72fd3]
Ruda Zhang June 26, 2016:  add category tag to wiki pages [a9b356c]
Ruda Zhang June 26, 2016:  use canonical filename and no internal H1 [eeca8ff]