artifacts at tiled image borders

[Ed McNierney]

Dylan -

In addition to Frank's accurate description of the problem, I'll also
note that JPEG is a very poorly-performing format for geospatial
applications.  Yes, you do lose something if you want to save space!

How many DOQQs do you need to deal with?  They're big, but they're not
THAT big.

	- Ed

Ed McNierney
President and Chief Mapmaker
TopoZone.com / Maps a la carte, Inc.
73 Princeton Street, Suite 305
North Chelmsford, MA  01863
Phone: +1 (978) 251-4242
Fax: +1 (978) 251-1396
ed at topozone.com 

-----Original Message-----
From: UMN MapServer Users List [mailto:MAPSERVER-USERS at LISTS.UMN.EDU] On
Behalf Of Dylan Beaudette
Sent: Wednesday, July 27, 2005 4:56 PM
To: MAPSERVER-USERS at LISTS.UMN.EDU
Subject: [UMN_MAPSERVER-USERS] artifacts at tiled image borders

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