Index: gdal_merge.py
===================================================================
--- gdal_merge.py	(revision 21833)
+++ gdal_merge.py	(working copy)
@@ -48,13 +48,13 @@
 # =============================================================================
 def raster_copy( s_fh, s_xoff, s_yoff, s_xsize, s_ysize, s_band_n,
                  t_fh, t_xoff, t_yoff, t_xsize, t_ysize, t_band_n,
-                 nodata=None ):
+                 nodata=None, a_nodata=None ):
 
     if nodata is not None:
         return raster_copy_with_nodata(
             s_fh, s_xoff, s_yoff, s_xsize, s_ysize, s_band_n,
             t_fh, t_xoff, t_yoff, t_xsize, t_ysize, t_band_n,
-            nodata )
+            nodata, a_nodata )
 
     if verbose != 0:
         print('Copy %d,%d,%d,%d to %d,%d,%d,%d.' \
@@ -66,6 +66,10 @@
 
     data = s_band.ReadRaster( s_xoff, s_yoff, s_xsize, s_ysize,
                               t_xsize, t_ysize, t_band.DataType )
+
+    if a_nodata is not None:
+        t_band.SetNoDataValue(a_nodata)
+        
     t_band.WriteRaster( t_xoff, t_yoff, t_xsize, t_ysize,
                         data, t_xsize, t_ysize, t_band.DataType )
         
@@ -75,7 +79,7 @@
 # =============================================================================
 def raster_copy_with_nodata( s_fh, s_xoff, s_yoff, s_xsize, s_ysize, s_band_n,
                              t_fh, t_xoff, t_yoff, t_xsize, t_ysize, t_band_n,
-                             nodata ):
+                             nodata, a_nodata ):
     try:
         import numpy as Numeric
     except ImportError:
@@ -95,6 +99,9 @@
 
     nodata_test = Numeric.equal(data_src,nodata)
     to_write = Numeric.choose( nodata_test, (data_src, data_dst) )
+
+    if a_nodata is not None:
+        t_band.SetNoDataValue(a_nodata)
                                
     t_band.WriteArray( to_write, t_xoff, t_yoff )
 
@@ -164,7 +171,7 @@
         print('UL:(%f,%f)   LR:(%f,%f)' \
               % (self.ulx,self.uly,self.lrx,self.lry))
 
-    def copy_into( self, t_fh, s_band = 1, t_band = 1, nodata_arg=None ):
+    def copy_into( self, t_fh, s_band = 1, t_band = 1, nodata_arg=None, a_nodata_arg=None ):
         """
         Copy this files image into target file.
 
@@ -234,14 +241,15 @@
         return \
             raster_copy( s_fh, sw_xoff, sw_yoff, sw_xsize, sw_ysize, s_band,
                          t_fh, tw_xoff, tw_yoff, tw_xsize, tw_ysize, t_band,
-                         nodata_arg )
+                         nodata_arg, a_nodata_arg )
 
 
 # =============================================================================
 def Usage():
     print('Usage: gdal_merge.py [-o out_filename] [-of out_format] [-co NAME=VALUE]*')
     print('                     [-ps pixelsize_x pixelsize_y] [-tap] [-separate] [-q] [-v] [-pct]')
-    print('                     [-ul_lr ulx uly lrx lry] [-n nodata_value] [-init "value [value...]"]')
+    print('                     [-ul_lr ulx uly lrx lry] [-init "value [value...]"]')
+    print('                     [-n nodata_value] [-a_nodata output_nodata_value]')
     print('                     [-ot datatype] [-createonly] input_files')
     print('                     [--help-general]')
     print('')
@@ -265,6 +273,7 @@
     separate = 0
     copy_pct = 0
     nodata = None
+    a_nodata = None
     create_options = []
     pre_init = []
     band_type = None
@@ -322,6 +331,10 @@
             i = i + 1
             nodata = float(argv[i])
 
+        elif arg == '-a_nodata':
+            i = i + 1
+            a_nodata = float(argv[i])
+
         elif arg == '-f':
             # for backward compatibility.
             i = i + 1
@@ -473,9 +486,9 @@
 
         if separate == 0 :
             for band in range(1, bands+1):
-                fi.copy_into( t_fh, band, band, nodata )
+                fi.copy_into( t_fh, band, band, nodata, a_nodata )
         else:
-            fi.copy_into( t_fh, 1, t_band, nodata )
+            fi.copy_into( t_fh, 1, t_band, nodata, a_nodata )
             t_band = t_band+1
             
         fi_processed = fi_processed+1