Simon Haddon wrote:
Hi,
I wasn't quiet sure what subject heading to give this problem. I hope it
is ok.
I have a map file which creates maps in decimal degrees. The data is a
mix raster and vector. It is the raster data which I am having trouble
with.
If I have any raster data with a large grid cell size (ie: 25 km) the
generated map always has complete pixels (cells) created. There are never
any truncated pixels. As a result pixels don't even maintain their
correct relative position and can be seen differently, compared to the
overlayed point and polygon data, depending on how fair you move or resize
the map.
If I move the map a whole pixel cell size the pixels redraw in their
correct position. If a move the map 1/2 a pixel cell size then the pixels
move to fit the image. This can also extend to stretching the pixel cells
to always ensure that a complete set of cells are displayed on the map
Has anyone had any experience with this problem and does anyone have any
suggestions about how to fix it.
Simon,
This is due to an optimization in MapServer to use the GDAL RasterIO() call
for up/down sampling. The net result is that requests are always made
on "pixel boundaries" and you get the result you see. If you add the line:
PROCESSING "RESAMPLE=NEAREST"
then MapServer will use a more general resampling code chunk which will give
better results, but at some performance cost.
Best regards,
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