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C Copyright 1981-2007 ECMWF
C
C Licensed under the GNU Lesser General Public License which
C incorporates the terms and conditions of version 3 of the GNU
C General Public License.
C See LICENSE and gpl-3.0.txt for details.
C
REAL FUNCTION KNFROM4(LVEGGY,RLAT,RLON,OLAT,OLON,PT,OLDFLD)
C
C---->
C**** KNFROM4
C
C Purpose
C -------
C
C Generates a new field point from its four nearest neighbours.
C
C
C Interface
C ---------
C
C VALUE = KNFROM4(RLAT,RLON,OLAT,OLON,PT,OLDFLD)
C
C Input
C -----
C
C LVEGGY - 'True' if 'nearest neighbour' processing required
C RLAT - Latitude of the new point
C RLON - Longitude of the new point
C OLAT - Latitudes of the four neighbouring points
C OLON - Longitudes of the four neighbouring points
C PT - Positions in old field array of the four neighbours
C OLDFLD - Array of old field values
C
C
C Output
C ------
C
C Function returns a value for the new field point.
C
C
C Method
C ------
C
C Uses neighbours to calculate a value by bi-linear interpolation.
C
C
C Externals
C ---------
C
C None.
C
C
C Author
C ------
C
C J.D.Chambers ECMWF November 2000
C
C----<
C
IMPLICIT NONE
C
C Function arguments
C
LOGICAL LVEGGY
REAL RLAT,RLON,OLAT(2),OLON(4),OLDFLD(*)
INTEGER PT(4)
C
#include "intisl.h"
#include "parim.h"
#include "nifld.common"
C
C Local variables
C
INTEGER COUNT, LOOP, NEAREST
REAL WEIGHT(4), NWEIGHT, SWEIGHT, NORMAL
C
C Externals
C
C
C Statement function
C
REAL A, B
LOGICAL NOTEQ
NOTEQ(A,B) = (ABS((A)-(B)).GT.(ABS(A)*1E-3))
C
C -----------------------------------------------------------------|
C* Section 1. Calculate weighting values for each neighbour
C -----------------------------------------------------------------|
C
100 CONTINUE
C
C Calculate the north/south weighting values allowing for
C southern latitude to be same as northern latitude
C
NWEIGHT = OLAT(NORTH) - RLAT
SWEIGHT = RLAT - OLAT(SOUTH)
IF( SWEIGHT.EQ.0.0 ) NWEIGHT = 1.0
C
C Calculate the weighting values for each matching neighbour
C
WEIGHT(NWEST) = ABS((OLON(SEAST) - RLON) * SWEIGHT)
WEIGHT(NEAST) = ABS((RLON - OLON(SWEST)) * SWEIGHT)
WEIGHT(SWEST) = ABS((OLON(NEAST) - RLON) * NWEIGHT)
WEIGHT(SEAST) = ABS((RLON - OLON(NWEST)) * NWEIGHT)
C
C Normalise the weights
C
NORMAL = ( WEIGHT(NWEST) + WEIGHT(NEAST) +
X WEIGHT(SWEST) + WEIGHT(SEAST) )
C
WEIGHT(NWEST) = WEIGHT(NWEST)/NORMAL
WEIGHT(NEAST) = WEIGHT(NEAST)/NORMAL
WEIGHT(SWEST) = WEIGHT(SWEST)/NORMAL
WEIGHT(SEAST) = WEIGHT(SEAST)/NORMAL
C
C -----------------------------------------------------------------|
C* Section 3. Interpolate
C -----------------------------------------------------------------|
C
300 CONTINUE
C
C Use nearest neighbour for vegetation field
C
IF( LVEGGY ) THEN
NEAREST = NWEST
IF( WEIGHT(NEAST).GT.WEIGHT(NEAREST) ) NEAREST = NEAST
IF( WEIGHT(SWEST).GT.WEIGHT(NEAREST) ) NEAREST = SWEST
IF( WEIGHT(SEAST).GT.WEIGHT(NEAREST) ) NEAREST = SEAST
KNFROM4 = OLDFLD(PT(NEAREST))
C
ELSE
C
C Count non-missing data values
C
COUNT = 0
IF( NOTEQ(OLDFLD(PT(NWEST)),RMISSGV) ) COUNT = COUNT + 1
IF( NOTEQ(OLDFLD(PT(NEAST)),RMISSGV) ) COUNT = COUNT + 1
IF( NOTEQ(OLDFLD(PT(SWEST)),RMISSGV) ) COUNT = COUNT + 1
IF( NOTEQ(OLDFLD(PT(SEAST)),RMISSGV) ) COUNT = COUNT + 1
C
C Interpolate using four neighbours if none are missing
C
IF( COUNT.EQ.4 ) THEN
KNFROM4 = ( (OLDFLD(PT(NWEST)) * WEIGHT(NWEST)) +
X (OLDFLD(PT(NEAST)) * WEIGHT(NEAST)) +
X (OLDFLD(PT(SWEST)) * WEIGHT(SWEST)) +
X (OLDFLD(PT(SEAST)) * WEIGHT(SEAST)) )
C
C Set missing if all neighbours are missing
C
ELSE IF( COUNT.EQ.0 ) THEN
KNFROM4 = RMISSGV
C
C Otherwise, use the nearest neighbour
C
ELSE
NEAREST = NWEST
IF( WEIGHT(NEAST).GT.WEIGHT(NEAREST) ) NEAREST = NEAST
IF( WEIGHT(SWEST).GT.WEIGHT(NEAREST) ) NEAREST = SWEST
IF( WEIGHT(SEAST).GT.WEIGHT(NEAREST) ) NEAREST = SEAST
KNFROM4 = OLDFLD(PT(NEAREST))
ENDIF
ENDIF
C
C -----------------------------------------------------------------|
C* Section 9. Return
C -----------------------------------------------------------------|
C
900 CONTINUE
C
RETURN
END
|
