accuracy of raster maps

[Jan Hartmann]

Hi,

Has anyone seen this before? I am georeferencing old cadastral maps and
have two layers in my map: the original scan (a raster layer with a
world file), and the reference points in a PostGIS database. Both are
the same local coordinate systems (0/0 - 4000/4000). On the first image
you see the green reference point lying over the black raster reference
lines. When zooming in further, (image2.gif), you see that the reference
point is somewhat to the left of the line. However, when panning the
image (image3.gif), the reference point has moved *over* the reference
line. Every further pan gives a slightly different position of the point
versus the raster. Differences are within the size of one pixel, but
they are irriting in a georeferencing application.

I can't see what I am doing wrong: both layers are in the same MapFile,
so when I change the map extent for zooming or panning, they should not
move relative to each other. My impression is that the point is rendered
exactly, but that the raster gets moved (rounding off error?) Any idea
what's going on here?

Jan
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