mis-joined tiles.

[Dave Weaver]

I'm experiencing a problem with horizontal lines on some of my maps, like this:

http://img363.imageshack.us/my.php?image=whitelinewm2.jpg

The lines only appear at some zoom levels, and at some tile sizes. For
example this request:

http://localhost:8080/cgi-bin/mapserv?map=map/Media_Base_Brown.map&layers=lod5&styles=&service=WMS&width=256&format=image/jpeg&request=GetMap&height=256&srs=EPSG:54004&version=1.1.1&bbox=-20037508.34,0.0,0.0,20037508.34

gets the tile with the line on, as in the above image. However if I simply
change the height= parameter to 255 or 257 then the line goes away.

After some investigation I can see that this gap is between 2 rows of my
source tiles, so I'm wondering if it's related to the remapping from
EPSG:4326 (the source tiles) to EPSG:54004 (the rendered map).

The source geotiffs have what looks like correct geographical info in them.
Here's the info from two tiles from the above image, one immediately above
and one immediately below the white line:

Driver: GTiff/GeoTIFF
Size is 2048, 2048
Coordinate System is:
GEOGCS["WGS 84",
    DATUM["WGS_1984",
        SPHEROID["WGS 84",6378137,298.2572235629972,
            AUTHORITY["EPSG","7030"]],
        AUTHORITY["EPSG","6326"]],
    PRIMEM["Greenwich",0],
    UNIT["degree",0.0174532925199433],
    AUTHORITY["EPSG","4326"]]
Origin = (-135.000000000000000,45.000000000000000)
Pixel Size = (0.021972656250000,-0.021972656250000)
Metadata:
  AREA_OR_POINT=Area
Corner Coordinates:
Upper Left  (-135.0000000,  45.0000000) (135d 0'0.00"W, 45d 0'0.00"N)
Lower Left  (-135.0000000,   0.0000000) (135d 0'0.00"W,  0d 0'0.01"N)
Upper Right ( -90.0000000,  45.0000000) ( 90d 0'0.00"W, 45d 0'0.00"N)
Lower Right ( -90.0000000,   0.0000000) ( 90d 0'0.00"W,  0d 0'0.01"N)
Center      (-112.5000000,  22.5000000) (112d30'0.00"W, 22d30'0.00"N)
Band 1 Block=2048x4 Type=Byte, ColorInterp=Red
Band 2 Block=2048x4 Type=Byte, ColorInterp=Green
Band 3 Block=2048x4 Type=Byte, ColorInterp=Blue
Band 4 Block=2048x4 Type=Byte, ColorInterp=Alpha

Driver: GTiff/GeoTIFF
Size is 2048, 2048
Coordinate System is:
GEOGCS["WGS 84",
    DATUM["WGS_1984",
        SPHEROID["WGS 84",6378137,298.2572235629972,
            AUTHORITY["EPSG","7030"]],
        AUTHORITY["EPSG","6326"]],
    PRIMEM["Greenwich",0],
    UNIT["degree",0.0174532925199433],
    AUTHORITY["EPSG","4326"]]
Origin = (-135.000000000000000,0.000000000000000)
Pixel Size = (0.021972656250000,-0.021972656250000)
Metadata:
  AREA_OR_POINT=Area
Corner Coordinates:
Upper Left  (-135.0000000,   0.0000000) (135d 0'0.00"W,  0d 0'0.01"N)
Lower Left  (-135.0000000, -45.0000000) (135d 0'0.00"W, 45d 0'0.00"S)
Upper Right ( -90.0000000,   0.0000000) ( 90d 0'0.00"W,  0d 0'0.01"N)
Lower Right ( -90.0000000, -45.0000000) ( 90d 0'0.00"W, 45d 0'0.00"S)
Center      (-112.5000000, -22.5000000) (112d30'0.00"W, 22d30'0.00"S)
Band 1 Block=2048x4 Type=Byte, ColorInterp=Red
Band 2 Block=2048x4 Type=Byte, ColorInterp=Green
Band 3 Block=2048x4 Type=Byte, ColorInterp=Blue
Band 4 Block=2048x4 Type=Byte, ColorInterp=Alpha

Can anyone give me a clue as to how I can fix this?

Many thanks,
Dave.