[mapserver-users] VRT overlaps and transparency

[TC Haddad]

Hi all,

I'm using mapserver to display a single VRT that brings together multiple
linear TIF mosaics of historic aerial images.

LAYER
    NAME "Photos1967"
    DATA "Photos1967.vrt"
    STATUS ON
    METADATA
        "wms_title"         "ODOT 1967 Aerial Photos"
        "wms_group_title"     "historic_photos"
        IMAGEFORMAT         "PNG"
        "wms_srs"             "EPSG:3857"
    END
    PROJECTION
       "init=epsg:3857"
    END
    TYPE RASTER
    OFFSITE 255 255 255
END


I'm having trouble in that the settings I am using seem to cause the
individual TIFs  to "cancel out" the overlap areas between the files, which
contains gaps in a sequence that should look seamless.

See http://bit.ly/1N5iOZM for an example of the blank spaces created
between TIF files.

An example snippet of the VRT looks like this:

<VRTDataset rasterXSize="137160" rasterYSize="940380">
  <GeoTransform> 2.2045589280046773e+005, 1.6409343787230763e+000,
0.0000000000000000e+000, 1.6572602506651357e+006,
0.0000000000000000e+000,-1.6409343787192308e+000</GeoTransform>
  <VRTRasterBand dataType="Byte" band="1">
    <NoDataValue>2.55000000000000E+002</NoDataValue>
    <HideNoDataValue>1</HideNoDataValue>
    <ColorInterp>Gray</ColorInterp>
    <SimpleSource>
      <SourceFilename
relativeToVRT="1">flightlines1967_1.tif</SourceFilename>
      <SourceBand>1</SourceBand>
      <SourceProperties RasterXSize="8142" RasterYSize="43231"
DataType="Byte" BlockXSize="8142" BlockYSize="1" />
      <SrcRect xOff="0" yOff="0" xSize="8142" ySize="43231" />
      <DstRect xOff="124188" yOff="119485" xSize="8143" ySize="43234" />
    </SimpleSource>
    <SimpleSource>
      <SourceFilename relativeToVRT="1">flightlines1967
_2.tif</SourceFilename>
      <SourceBand>1</SourceBand>
      <SourceProperties RasterXSize="14367" RasterYSize="43419"
DataType="Byte" BlockXSize="14367" BlockYSize="1" />
      <SrcRect xOff="0" yOff="0" xSize="14367" ySize="43419" />
      <DstRect xOff="115051" yOff="160932" xSize="14367" ySize="43418" />
    </SimpleSource>
  </VRTRasterBand>
</VRTDataset>

Any suggestions as to how I can resolve the issue of the overlaps?

My Mapserver version is 6.2.1 (gdal 1.10)...

Tanya
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