, y 
  
  
  
  
p 
A m ee T7 
Ll X Grid plane 
utt \ 
\ 
Jm \ by 
Ki . 
C 
Wy 
| => Db, 
? b 
zy 
h 
+t 
X 
Lo 
Z 
P d 
Am n em -—— P 
Projection plane 
y 
Fig. 1 
Definition of the coordinate system etc.