NODATA values and TRANSPARENCY Question

[Stephen Woodbridge]

Hi and Season Greetings to all,

I'm playing around with GDAL and raster images for the first time in 
about 4 years and I have some questions about how NODATA values are handled.

I generated some hillshade GeoTiff images and added overviews to them 
ans generated a tile-index and the mapfile is basically working, EXCEPT 
the NODATA areas don't make any sense to me. They seem to be generating 
some type of image where I would expect them to be totally transparent 
BECAUSE they is NODATA.

Also any ideas what the large lakes, the Great Lakes and the Oceans are 
handled differently? The hillshade was create from srtm dems.

-Steve

The mapserver image:

http://imaptools.com/cgi-bin/mapserv-4.10?mode=map&layers=all&scale=39000001&map=/u/data/dem/hillshade.map&mapxy=-122+42

notice the dark blue areas in the Oceans adjacent to the land masses.
The mapfile:

MAP
     NAME "Hillshade"
     EXTENT -180 -90 180 90
     SIZE 600 300
     IMAGETYPE png
#    TRANSPARENT on
     IMAGECOLOR 0 0 255
     SHAPEPATH "/u/data/dem/"
     UNITS DD

     LAYER
         NAME "Countries"
         TYPE POLYGON
         DATA "/u/data/data/mdata/world/world_countries_shp"
         STATUS DEFAULT
         CLASS
             COLOR 255 255 255
             OUTLINECOLOR 255 255 0
         END
     END

     LAYER
         NAME "Hillshade"
         TYPE RASTER
         TILEINDEX "/u/data/dem/hillshade-index"
         TRANSPARENCY 35
         STATUS DEFAULT
         CLASS
             COLOR 0 0 0
             MAXSCALE 40000000
             MINSCALE    80000
         END
     END

     LAYER
         NAME "Hillshade"
         TYPE RASTER
         TILEINDEX "/u/data/dem/hillshade-index"
         TRANSPARENCY 10
         STATUS DEFAULT
         CLASS
             COLOR 0 0 0
             MAXSCALE    80000
             MINSCALE    40000
         END
     END

END


The gdalinfo:

woodbri at carto:/u/data/dem$ gdalinfo z_10_1t.tif
Driver: GTiff/GeoTIFF
Size is 6000, 6000
Coordinate System is:
GEOGCS["WGS 84",
     DATUM["WGS_1984",
         SPHEROID["WGS 84",6378137,298.2572235630016,
             AUTHORITY["EPSG","7030"]],
         AUTHORITY["EPSG","6326"]],
     PRIMEM["Greenwich",0],
     UNIT["degree",0.0174532925199433],
     AUTHORITY["EPSG","4326"]]
Origin = (-135.000000,60.000000)
Pixel Size = (0.00083333,-0.00083333)
Corner Coordinates:
Upper Left  (-135.0000000,  60.0000000) (135d 0'0.00"W, 60d 0'0.00"N)
Lower Left  (-135.0000000,  55.0000000) (135d 0'0.00"W, 55d 0'0.00"N)
Upper Right (-130.0000000,  60.0000000) (130d 0'0.00"W, 60d 0'0.00"N)
Lower Right (-130.0000000,  55.0000000) (130d 0'0.00"W, 55d 0'0.00"N)
Center      (-132.5000000,  57.5000000) (132d30'0.00"W, 57d30'0.00"N)
Band 1 Block=256x256 Type=Byte, ColorInterp=Gray
   NoData Value=-32768
   Overviews: 3000x3000, 1500x1500, 750x750, 375x375, 188x188, 94x94, 47x47