if y > 0 then
     if x > 0 then tc1 = arctan(y/x)
     if x < 0 then tc1 = 180 - arctan(-y/x)
     if x = 0 then tc1 = 90
   if y < 0 then
     if x > 0 then tc1 = -arctan(-y/x)
     if x < 0 then tc1 = arctan(y/x)-180
     if x = 0 then tc1 = 270
   if y = 0 then
     if x > 0 then tc1 = 0
     if x < 0 then tc1 = 180
     if x = 0 then [the 2 points are the same]