[Tilecache] Empty pixel rows across bottom of certain tiles

[Roger André]

Just in case someone else wants to see this for themselves, here are the
steps needed to replicate the condition I'm seeing.

Step 1.  Create a large rectangular image of Size 14400 x 7200.

Step 2.  Assign georeferencing to the image:
gdal_translate -a_ullr -180 90 180 -90 -a_srs "EPSG:4326" big_black.tif
wgs84_big_black.tif

Step 3. Load this image into some sort of WMS rendering system and assign a
background color value of red to it.

Step 4. Make sure you have the spherical mercator projection defined in your
epsg file.
<900913> +proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0
+y_0=0 +k=1.0 +units=m +nadgrids=@null +no_defs <>

Step 5. Make a WMS request for a tile that replicates one that TileCache
makes:
http://localhost/cgi-bin/mapserv?map=/var/www/mapfiles/test/test.map
&layers=imagery
&srs=EPSG%3A900913
&version=1.1.1
&bbox=-20037508.34%2C-20037508.34%2C-10018754.1704%2C-10018754.1704
&service=WMS
&width=256
&styles=
&format=image%2Fpng
&request=GetMap
&height=256

Step 6. Admire the red line at the bottom of the image which indicates no
data.
--


On Mon, Oct 20, 2008 at 10:38 AM, Roger André <randre at gmail.com> wrote:

> Ok, I think I've narrowed this down to being a reprojection problem.  My
> source data is actually unprojected lat/lon raster data, with pixels that
> are .025 x ,025 degrees in size.  I've set this to be EPSG:4326 in my
> MapServer LAYER, and am specifying a spherical mercator reprojection to
> EPSG:900913 via TileCache.  I figure I have either a problem in my source
> projection, or in my output projection, with a slight suspicion that I might
> be wrong in assigning EPSG:4326 to the source data.
>
> Does this sound reasonable, and if so, can I define lat/lon geographic
> without specifying an ellipsoid?
> --
>
>
> On 10/18/08, Roger André <randre at gmail.com> wrote:
>>
>> Hi Everyone,
>>
>> I've encountered something new that is perplexing me.  I'm seeding my
>> cache and finding that certain tiles have a row of pixels missing at the
>> bottom of the tile which is being filled with the background color I've set
>> in my MAPFILE.  It looks suspiciously like TileCache is having to "split" my
>> seed image in order to fit it into the Google Maps frame, which would
>> indicate something wrong with my georeferencing - or extents setup, but I
>> can't figure out what.  Is this a symptom that rings a bell with anyone?
>>
>> Thanks,
>>
>> Roger
>> --
>>
>
>
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