SUBROUTINE ZLEDGE(N,YDEF,TOT,YPOS,STATUS)
C
C
C Module Number: 13.8.2
C
C Module Name: ZLEDGE
C
C Keyphrase:
C ----------
C  Locate yposition of upper or lower HRS photocathode mask edge
C
C Description:
C ------------
C Using an edge profile the location of the upper or lower photocathode
C mask edge is located at the point where the counts equals 1/2 the
C maximum + minimum counts.
C
C FORTRAN Name: zledge.for
C
C
C Keywords of Accessed Files :
C --------------------------
C NONE
C
C Modules Called:
C ---------------
C SDAS:
C       UMSPUT
C
C History:
C --------
C Version       Date      Author        Description
C  1.0       10/MAR/86   D. Lindler     Designed and coded
C  2.0       DEC 87      D. LINDLER     new sdas i/o and standards
C------------------------------------------------------------------------
C     INCLUDE FILE FOR THE IRAF77 FORTRAN INTERFACE TO THE IRAF VOS
C
C
C     FILE I/O ACCESS MODES
C
      INTEGER   RDONLY
      PARAMETER (RDONLY = 1)
      INTEGER   RDWRIT
      PARAMETER (RDWRIT = 2)
      INTEGER   WRONLY
      PARAMETER (WRONLY = 3)
      INTEGER   APPEND
      PARAMETER (APPEND = 4)
C
C     CODES FOR DATA TYPES
C
      INTEGER   TYBOOL
      PARAMETER (TYBOOL = 1)
      INTEGER   TYCHAR
      PARAMETER (TYCHAR = 2)
      INTEGER   TYINT
      PARAMETER (TYINT = 4)
      INTEGER   TYREAL
      PARAMETER (TYREAL = 6)
      INTEGER   TYDOUB
      PARAMETER (TYDOUB = 7)
C
C     UMSPUT DESTINATIONS -- CB, DAO, 4-SEP-87
C
      INTEGER STDOUT
      PARAMETER (STDOUT = 1)
      INTEGER STDERR
      PARAMETER (STDERR = 2)
C
C     UHDAS HEADER PARM TYPES -- CB, DAO, 5-SEP-87
C
      INTEGER GENHDR
      PARAMETER (GENHDR = 0)
      INTEGER IMSPEC
      PARAMETER (IMSPEC = 1)
C
C     THIS SECTION IS FOR PARAMETERS RELEVANT TO TABLE I/O.
C
C     THESE MAY BE SET BY UTPPTI AND/OR READ BY UTPGTI:
C
C                                       LENGTH OF ROW (UNIT = SIZE OF REAL)
      INTEGER   TBRLEN
      PARAMETER (TBRLEN = 1)
C                                       INCREASE ROW LENGTH
      INTEGER   TBIRLN
      PARAMETER (TBIRLN = 2)
C                                       NUMBER OF ROWS TO ALLOCATE
      INTEGER   TBALLR
      PARAMETER (TBALLR = 3)
C                                       INCREASE ALLOC NUM OF ROWS
      INTEGER   TBIALR
      PARAMETER (TBIALR = 4)
C                                       WHICH TYPE OF TABLE? (ROW OR COLUMN)
      INTEGER   TBWTYP
      PARAMETER (TBWTYP = 5)
C                                       MAXIMUM NUMBER OF USER PARAMETERS
      INTEGER   TBMXPR
      PARAMETER (TBMXPR = 6)
C                                       MAXIMUM NUMBER OF COLUMNS
      INTEGER   TBMXCL
      PARAMETER (TBMXCL = 7)
C                                       TYPE = ROW-ORDERED TABLE
      INTEGER   TBTYPR
      PARAMETER (TBTYPR = 11)
C                                       TYPE = COLUMN-ORDERED TABLE
      INTEGER   TBTYPC
      PARAMETER (TBTYPC = 12)
C
C     THESE MAY BE READ BY UTPGTI BUT MAY NOT BE SET:
C
C                                       NUMBER OF ROWS WRITTEN TO
      INTEGER   TBNROW
      PARAMETER (TBNROW = 21)
C
C     END IRAF77.INC
C
C INPUT:
C
        INTEGER         N
C                                    ---> NUMBER OF EDGE OBSERVATIONS
        DOUBLE PRECISION YDEF(1)
C                                    ---> Y-DEFLECTIONS OF THE OBS.
        DOUBLE PRECISION TOT(1)
C                                    ---> TOTAL COUNTS AT EACH Y-DEF
C
C OUTPUT:
C
        DOUBLE PRECISION YPOS
C                                    ---> Y-DEFLECTION OF EDGE
        INTEGER         STATUS
C                                    ---> STATUS
C
C LOCAL VARIABLES
C
        DOUBLE PRECISION MINTOT,MAXTOT
C                                    ---> MIN AND MAX OF TOT
        DOUBLE PRECISION HTOT
C                                    ---> 1/2 (MIN AND MAX OF TOT)
        INTEGER         I
C                                    ---> INDEX
        LOGICAL         UPPER
C                                    ---> UPPER EDGE?
        CHARACTER*130   CONTXT
C                                    ---> WHAT HAPPENED
        INTEGER         ISTAT
C
C-----------------------------------------------------------------------
C
C DETERMINE MIN AND MAX OF TOT
C
        MINTOT=1.0E10
        MAXTOT=0.0
        DO 1 I=1,N
                IF(TOT(I).GT.MAXTOT) MAXTOT=TOT(I)
                IF(TOT(I).LT.MINTOT) MINTOT=TOT(I)
1       CONTINUE
        HTOT = (MINTOT+MAXTOT)/2.0
        IF(MINTOT.EQ.MAXTOT) THEN
                CONTXT='MIN AND MAX OF EDGE PROFILE IDENTICLE'
                STATUS=1
                GO TO 999
        ENDIF
C
C DECIDE IF UPPER OR LOWER EDGE
C
        UPPER=.TRUE.
        IF(YDEF(1).GT.2048) UPPER=.FALSE.
C
C FIND TWO POINTS HTOT LIES BETWEEN
C
        DO  2 I=1,N-1
                IF(UPPER)THEN
                        IF(TOT(I+1).GT.HTOT) GO TO 10
                   ELSE
                        IF(TOT(I+1).LT.HTOT) GO TO 10
                ENDIF
2       CONTINUE
C
C INTERPOLATED TO COMPUTE POSITION
C
10      YPOS=YDEF(I) + (HTOT-TOT(I))*(YDEF(I+1)-YDEF(I))/
     *                                  (TOT(I+1)-TOT(I))
        STATUS=0
C
C DONE
C
        GO TO 1000
999     CALL UMSPUT(CONTXT,STDOUT+STDERR,0,ISTAT)
1000    RETURN
        END