function read_grid_attr,filename,gridname,attrname,attrdata
  fid = EOS_GD_OPEN(filename,/READ)
  if (fid lt 0) then begin
    msg_text = "Problem opening HDF-EOS file " + filename
    goto, error
  endif
  gid = EOS_GD_ATTACH(fid,gridname)
  if (gid lt 0) then begin
    msg_text = "Problem attaching grid " + gridname
    goto, error
  endif
  status = EOS_GD_ATTRINFO(gid, attrname, attrtype, attrcount)
  if (status lt 0) then begin
    msg_text = "Problem getting attribute info for " + attrname
    goto, error
  endif
  status = EOS_GD_READATTR(gid,attrname,attrdata)
  if (status lt 0) then begin
    msg_text = "Problem reading attribute " + attrname
    goto, error
  endif
  ; account for bug in HDF-EOS attributes that makes data array length = number of bytes
  ; rather than number of data items
  case attrtype of
    3 : ; unsigned character
    4 : ; charcter (string)
    5 : attrdata = attrdata[0:(attrcount/4)-1]; float 32
    6 : attrdata = attrdata[0:(attrcount/8)-1]; float 64 (double)
    20 : ; int8
    21 : ; uint8
    22 : attrdata = attrdata[0:(attrcount/2)-1]; int16
    23 : attrdata = attrdata[0:(attrcount/2)-1]; uint16
    24 : attrdata = attrdata[0:(attrcount/4)-1]; int32
    25 : attrdata = attrdata[0:(attrcount/4)-1]; uint32
    else : ;unknown so treat as 1 byte length
  endcase

  status = EOS_GD_DETACH(gid)
  if (status lt 0) then begin
    msg_text = "Problem detaching grid " + gridname
    goto, error
  endif
  status = EOS_GD_CLOSE(fid)
  if (status lt 0) then begin
    msg_text = "Problem closing HDF-EOS file " + filename
    goto, error
  endif
  return,0
error:
  status = DIALOG_MESSAGE(msg_text,/INFORMATION,TITLE='read_grid_attr message')
  return,-1
end