c this subroutine performs bivariate interpolation when the pro- c jections of the data points in the x-y plane are irregularly c distributed in the plane. c the input parameters are c md = mode of computation (must be 1, 2, or 3), c = 1 for new ncp and/or new xd-yd, c = 2 for old ncp, old xd-yd, new xi-yi, c = 3 for old ncp, old xd-yd, old xi-yi, c ncp = number of additional data points used for esti- c mating partial derivatives at each data point c (must be 2 or greater, but smaller than ndp), c ndp = number of data points (must be 4 or greater), c xd = array of dimension ndp containing the x c coordinates of the data points, c yd = array of dimension ndp containing the y c coordinates of the data points, c zd = array of dimension ndp containing the z c coordinates of the data points, c nip = number of output points at which interpolation c is to be performed (must be 1 or greater), c xi = array of dimension nip containing the x c coordinates of the output points, c yi = array of dimension nip containing the y c coordinates of the output points. c the output parameter is c zi = array of dimension nip where interpolated z c values are to be stored. c istat = error message. c status= Starlink error status c the other parameters are c iwk = integer array of dimension c max0(31,27+ncp)*ndp+nip c used internally as a work area, c wk = array of dimension 8*ndp used internally as a c work area. c the very first call to this subroutine and the call with a new c ncp value, a new ndp value, and/or new contents of the xd and c yd arrays must be made with md=1. the call with md=2 must be c preceded by another call with the same ncp and ndp values and c with the same contents of the xd and yd arrays. the call with c md=3 must be preceded by another call with the same ncp, ndp, c and nip values and with the same contents of the xd, yd, xi, c and yi arrays. between the call with md=2 or md=3 and its c preceding call, the iwk and wk arrays must not be disturbed. c use of a value between 3 and 5 (inclusive) for ncp is recom- c mended unless there are evidences that dictate otherwise. c this subroutine calls the idcldp, idlctn, idpdrv, idptip, and c idtang subroutines.