Function NormalForm,X,Y
;+
;n=NormalForm(X,Y) returns a vector n=[rho,theta] that
;represents the normal form of a line that is devined
;by the points represented by X and Y. The vector X
;contains the x-coordinates and Y contains the y-coordinates
;of the points.
;
;HISTORY
;H. Rhody, November 16, 2001--version 1.0
;-
;Check to see that at least two points are supplied
n=N_ELEMENTS(X)<N_ELEMENTS(Y)
IF n LT 2 THEN Message,'At least two points are needed.'

;Choose the first and last point from the input data
x1=X[0] & x2=X[n-1]
y1=Y[0] & y2=Y[n-1]

;Compute the direction cosines of the normal vector
r=sqrt((x2-x1)^2+(y2-y1)^2)
ct=(y2-y1)/r
st=-(x2-x1)/r
theta=ATAN(st,ct)
rho=x1*ct+y1*st
RETURN,[rho,theta]
END