| Zp (peak)
|\
| \
|  \
| a \
|    \
|-----\ (Xa,Ya)
|      \
|  b    \
|        \
|---------\ (Xb,Yb)
|          \    
|           \   
|    c       \  
|             \ 
|--------------\ (Xc,Yc)
|               \    
|                \   
|      d          \  
|                  \ 
|-------------------\ (Xd,Yd)
(Xp,Yp)

##</code><code>##

Ha   Hb   Hc   Hd
-- = -- = -- = --
Ba   Bb   Bc   Bd

##</code><code>##

Ha   Hb
-- = --
Ba   Bb

Ha   Hc
-- = --
Ba   Bc

Hb   Hc
-- = --
Bb   Bc