artifacts at tiled image borders

[Dylan Beaudette]

Greetings,

I have been doing some research on better ways to store my aerial imagery, so 
that I can keep a higher resolution version of my source files. Until now I 
have been downsampling USGS DOQQ images from 1 meter resolution to 10 meters, 
saving the result as a geoTiff. With overviews, these file are about 1.5 mb 
per quarter quad. A shapefile tile index is used to render the images in 
Mapserver.

Last night I re-created a section of the tiled dataset in JPEG format at 3 
meter resolution. I am much happier with the resulting increase in detail, 
however I am now noticing some strange artifacts showing up at the "seams" of 
the tile index.

here is an example of what I am talking about:
http://169.237.35.250/~dylan/temp/mapserver/bad_edges.jpg

the source data do not appear to have these type of "rough edges":
http://169.237.35.250/~dylan/temp/mapserver/original_clean_edges.jpg

I am using the OFFSITE operator in my map file to ignore pixels with an RGB 
value of '0 0 0'.

Does anyone have any idea what could be causing these rough looking seams at 
the tileindex boundaries?

thanks in advance!


-- 
Dylan Beaudette
Soils and Biogeochemistry Graduate Group
University of California at Davis
530.754.7341