pro read_sds_one,filename,tagname,f
;   read one variable from the HDF file 
;   usage: idl> read_sds_one,'filename','parameter_1',variable_name
;                 chuntao liu
;                           6/28/05
f=-1
tags=filename
m=0
sdsfileid = hdf_sd_start(filename,/read)
hdf_sd_fileinfo,sdsfileid,numsds,ngatt
for i = 0, numsds - 1 do begin
   sds_id = hdf_sd_select(sdsfileid, i )
   hdf_sd_getinfo, sds_id, name = na, ndim = nd,type= typ, label=label
   if keyword_set(tag_label) and strlen(label) gt 0 then na=label
   tics=strsplit(na,' ',/extract)
   if(n_elements(tics) gt 1) then na=tics[0]+'_'+stringit(long(tics[1]))
   iftags=where(strupcase(na) eq tags)
   if(iftags[0] ge 0) then na=na+'_'+stringit(i)
   if(strupcase(na) eq strupcase(tagname)) then begin
        hdf_sd_getdata, sds_id, data
        f=data
        m=1 
   endif
   tags=[tags,na]
   hdf_sd_endaccess,sds_id
endfor
hdf_sd_end,sdsfileid
if(m eq 0) then print,tagname,' not found in the file ',filename
end