PRO read_calipso_altitudes

;  This routine demonstrates reading the altitude data stored
;  in a CALIPSO Lidar Level 1B, Lidar Level 2 Aerosol Profile,
;  or Lidar Level 2 Cloud Profile data file.  The altitudes are
;  stored in an HDF vdata (table) structure named "metadata".
;  One field (column) in the table is named "Lidar_Data_Altitudes".
;  The table contains only one row, and the value in that row under
;  the "Lidar_Data_Altitudes" column is an array of the altitude
;  values.

;  There are no input or output parameters.  The CALIPSO file is
;  specified through a file selection dialog box, and any errors
;  are reported in pop-up message dialog boxes.

cfile = DIALOG_PICKFILE(TITLE="Select CALIPSO file")

fid = HDF_OPEN(cfile,/READ)
IF (fid eq -1) THEN BEGIN
  msg_text = "Problem opening file " + filename
  GOTO, error
ENDIF

vdref = HDF_VD_FIND(fid,"metadata")
IF (vdref eq 0) THEN BEGIN
  msg_text = "Can't find vdata metadata in file " + filename
  GOTO, error
ENDIF

vdid = HDF_VD_ATTACH(fid,vdref,/READ)
IF (vdref eq 0) THEN BEGIN
  msg_text = "Problem attaching vdata " + vdname
  GOTO, error
ENDIF

status = HDF_VD_READ(vdid,altitudes,FIELDS="Lidar_Data_Altitudes")

IF (status NE 1) THEN BEGIN
  msg_text = "Problem reading altitude field in vdata"
  GOTO, error
ENDIF

PRINT,altitudes

HDF_VD_DETACH,vdid
HDF_CLOSE,fid
RETURN

;  error handler
error:
  status = DIALOG_MESSAGE(msg_text,/INFORMATION,TITLE='read_vdata message')
  vdata_struc = 0

END