Name          :   vertical_check()

 Purpose:      :   determine good (non -1) endpoint locations in a data
                       vector of 512 elements (-1 indicates missing data)

 Inputs        :   crit = Max number of -1 data points acceptable for 
                            return value to be 1
                   d = data vector

 Outputs       :   y1 = first non -1 data value location in d
               :   y2 = last non -1 data value location in d

 Return value  : 
                   1 = Usable y1 and y2 values are returned.
                   2 = The number of -1 data values exceeded crit.
                   3 = The data vector had no -1 data values.

 Contact       : sgreer@sec.noaa.gov
 CALLED BY
	sxi_interp