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/* ---------------------------------------------------------------- */
/* Transforms a GeoTiff file as produced by ASTER GDEM, see:        */
/*                                                                  */
/* http://gdem.ersdac.jspacesystems.or.jp                           */
/*                                                                  */
/* to 'hgt' format as used by SRTM, see:                            */
/*                                                                  */
/* https://lpdaac.usgs.gov/products/measures_products_table/srtmgl3 */
/*                                                                  */
/* Foa a nice explanation of the (geo)tiff format see:              */
/*                                                                  */
/* http://www.awaresystems.be/imaging/tiff.html                     */
/*                                                                  */
/* ---------------------------------------------------------------- */
/*                                                                  */
/* Originally by Ruurd J. Idenburg                                  */                                                            
/*                                                                  */
/* No copyright, no licence, no guarantees or warrantees, be it     */
/* explicit, implicit or whatever. Usage is totally and completely  */
/* at the users own risk, the author shall not be liable for any    */
/* damages whatsoever, for any reason whatsoever.                   */
/*                                                                  */
/* Please keep this comment block intact when modifying this code   */
/* and add a note with date and a description.                      */
/*                                                                  */
/* ---------------------------------------------------------------- */
/*  Parameter(s):                                                   */
/*                                                                  */
/*    file - the (geo)tiff path and filename to be converted        */
/*                                                                  */
/*                                                                  */
/*  Result:                                                         */
/*                                                                  */
/*    A file on the same path and filename as the input tiff file   */
/*    but with file extension 'hgt' and in the SRTM format          */
/*                                                                  */
/* ---------------------------------------------------------------- */
/* 2014/02/01 - Initial version                                     */
/* ---------------------------------------------------------------- */

use arg geoTiffFile
geoTiff = .stream~new(geoTiffFile)
geoTiff~open("read")
/* ---------------------------------------------------------------- */
/* The Image File Directory(IFD) is always at offset 4 in the file. */
/* ---------------------------------------------------------------- */
ifdOffset = (geoTiff~charin(5,4)~reverse~c2d)+1   -- cater for base 1
say "Found the IFD at offset" ifdOffset "('"ifdOffset~d2x(8)"'X)."
/* ---------------------------------------------------------------- */
/* Get the number of tags present in this IFD.                      */
/* ---------------------------------------------------------------- */
tagCount = geoTiff~charin(ifdOffset,2)~reverse~c2d
say "IFD contains" tagCount "tags."
/* ---------------------------------------------------------------- */
/* The tags follow directly after the count and each 12 bytes long. */
/* ---------------------------------------------------------------- */
tags = geoTiff~charin(ifdOffset+2,tagCount*12)
nextIfdOffset = geoTiff~charin(ifdOffset+2+tagCount*12,4)~reverse~c2d
-- say nextIfdOffset  -- for now assume no following IFD's
tagDir = .directory~new
/* ---------------------------------------------------------------- */
/* Each tag consists of Key, Datatype, Valuecount and Value(s).     */
/* If the values don't fit in 4 bytes, the value is an offset to    */
/* where the values can be found.                                   */
/* ---------------------------------------------------------------- */
do t=0 to tagCount-1  --cater for base 1
  aTag = tags~substr(1+12*t,12)
  tagKey = aTag~substr(1,2)~reverse~c2x
  dataType = aTag~substr(3,2)~reverse~c2x
  valueCount = aTag~substr(5,4)~reverse~c2d
  valuePointer = aTag~substr(9,4)~reverse~c2d -- can be a value only
  tag = .directory~new
  tagDir~put(dataType valueCount valuePointer,tagKey)
end
columns = tagDir~0100~word(3)
say "The elevation information is" columns "columns wide."
rows = tagDir~0101~word(3)
say "The elevation information is" rows "rows deep."
--rowsPerStrip = tagDir~0116~word(3) -- not needed for further processing
--say rowsPerStrip
stripByteCountValues = tagDir~0117~word(2)
say "There are" stripByteCountValues "byte counts for the row strips to read."
stripByteCountsOffset = tagDir~0117~word(3)+1 -- cater for base 1
--say stripByteCountsOffset
stripByteCounts = geoTiff~charin(stripByteCountsOffset,stripByteCountValues*4)
--say stripByteCounts~c2x
stripCount = tagDir~0111~word(2)
say "The strip count" stripCount "should be equal to previous value."
stripOffsetsPointer = tagDir~0111~word(3)+1 -- cater for base 1
--say stripOffsetsPointer
stripPointers = geoTiff~charin(stripOffsetsPointer,stripCount*4)
heightFile = geoTiffFile~substr(1,geoTiffFile~lastPos(".")-1)".hgt"
srtm = .stream~new(heightFile)
srtm~open("write" "replace")
do a=0 to stripCount-1  -- cater for base 1
  charinLength = stripByteCounts~substr(1+4*a,4)~reverse~c2d
  stripOffset = stripPointers~substr(1+4*a,4)~reverse~c2d+1
  strip = geoTiff~charin(stripOffset,charinLength)
  -- make strip contents big-endian from little-endian
  test = makeBigEndian(strip)
  --say test~length
  srtm~charout(test)
end
geoTiff~close
srtm~close
exit
   
::routine makeBigEndian
  use arg string
  big = .mutablebuffer~new(,string~length)
  do i=1 by 2 to string~length
    big~append(string~substr(i,2)~reverse)
  end
  return big
exit

 
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All content © Ruurd Idenburg, 2007–2018, except where marked otherwise. All rights reserved. This page is primarily for non-commercial use only. The Idenburg website records no personal information and sets no ‘cookies’. This site is hosted on a VPS(Virtual Private System) rented from CloudVPS, a Dutch company, falling under Dutch (privacy) laws.

This page updated on Thu, 28 Apr 2016 14:59:56 +0200 by Ruurd Idenburg.