ORIGINAL Q:
I have a MrSID image that appears to have the correct UTM coordinates in the
.sdw world file (about 477,000-x and 1,555,000-y). In ArcMap, I can draw it
just fine on top of other layers. In ArcView 3.2, it plots way off in space
so that when you get the "two dots" when you zoom to all themes. Zooming to
the MrSID image theme, the cursor coordinates show up as the regular coords
times 100 (about 47,700,000-x and 155,500,000-y).

WHAT I FOUND OUT:
1) The MrSID image contains georeferencing information, in addition to the
.sdw world file. Deleting the world file has no effect in AV3.2, so it looks
like AV3.2 ignores the .sdw file altogether and gets its coordinates from
the image.
2) ArcMap can figure out the projection, ArcView 3.2 cannot. Looks like
ArcMap uses the world file.
3) Defining the projection/units in the ArcView view doesn't seem to make
any difference.
4) I downloaded the (free) MrSID GeoViewer from LizardTech. It looks like
the Viewer will default to the coordinates in the .sdw world file, if it
exists. If not, it will use the coordinates in the .sid image. The
coordinates in the GeoViewer were correct when it had a world file, and were
incorrect when I deleted the world file. So, it looks like the problem is
with the image's coordinates (times 100 of what they should be).
5) I got the images from someone else, so I have no idea how the image's
coordinates got messed up.  Since I got emails from others with the same
problem, my guess is that it's caused by a software bug or some user error
that's easy to do.
6) Aside from this problem of the coordinates being off by a factor of 100,
the images were misregistered. I figured out the approximate vector of the
shift, added it to the world files' coordinates, opened with the GeoViewer,
and saved as GeoTIFF. Success: the orthophotos overlay my data in AV3.2
correctly! Since the images will be used only as a background for viewing,
and not digitizing, this was close enough for my purposes.
7) Making many small GeoTIFF tiles (less than 50MB each) allows me to
convert my GeoTIFFs to MrSID image tiles in ArcCatalog. If you buy the
MrSID license, you can make bigger tiles and/or mosaic them.

Hope that helps, thanks to the many who provided input.

David Askov
mapgeek@hotmail.com


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