逐像元法（麻烦，占空间）:
inData = read_tiff(inputfile,GEOTIFF=Geovar,INTERLEAVE=2)
iSize = size(inData,/DIMENSIONS)
iNs = iSize[0]
iNl = iSize[1]
outData = fltarr(iNs,iNl)
for ii = 0,iNs-1 do begin
for ij = 0,iNl-1 do begin
if outData[ii,ij] le 0 then begin
outData[ii,ij] = 1
endif else begin
outData[ii,ij] = 2
endelse
endfor
endfor
索引法：
bgIdx=WHERE(inData le 0,bgCnt)
IF bgCnt GT 0 THEN BEGIN
inData[bgIdx]=!VALUES.F_NAN
ENDIF
