; this code is used to find the contour of footprint that encircles a
; specific percent of total footprint area

FUNCTION findcontour,ft,gridsize,ratio,source_area=area

; find the max ft, firstly

; ft--  arary, footpint, ttotal(ft)*grid*grid=1
; gridszie-  as the name
; ratio  --  percent of the ft area, e.g., 0.9, 0.95
; area  -- the area the contour encircles

; normalized
 ft=ft/(gridsize*gridsize*total(1d*ft))

peak=1.0d*max(ft)

n=1000.
dp=abs(peak)/n

value=-999.

FOR i=0,n-1 DO BEGIN

    value=peak-dp*i
    r=where(ft ge value,count)
  IF count LE 0 THEN BEGIN
   value=-999.
   r=0.0
   GOTO,ccc
  ENDIF

    rrr=1.0d*gridsize*gridsize*total(1.0d*ft(r))
    area=gridsize*gridsize*float(len(r))
   IF rrr GE ratio THEN begin
     value=value-dp
     GOTO, ccc     
   endif

ENDFOR


ccc:

return,value
END