[mapserver-users] TileIndex - accuracy and truncation ?

[Lars I. Nielsen]

Hi,

For performance reasons, I've (used FME to) split a large PNG into 450 small tiles, serving them with a Mapserver tileindex as a WMS service to a web solution.

Howeever, white lines appear between some of the tile rows (not all) at some scales (not all).

I've looked deeper into the WLD files for the individual tiles, and found that the calculations are accurate to 1/10 of a nanometer. At least if one uses all 10 decimals.

Tiles are 2254 pixels high, and (as per the WLDs below) 6138899.3169212583 - 0.0204375000 * 2254 = 6138853.2507962583 (almost equal to 6138853.2507962584)

Which leads me to suspect that some sort of truncation goes on, yielding some sort of rounding error.

Do anyone know whether the tileindex calculations truncates any of the values in the WLD files ? And if so, by how much ?

Cheers.

--

A typical map picture with a white line can be seen via this link:

http://hvmgo01.hvenegaard.dk/mswms/mapserv.exe?map=odensekkgd_wms.map&LAYERS=ASSISTENS&TRANSPARENT=true&SERVICE=WMS&VERSION=1.1.1&REQUEST=GetMap&STYLES=&FORMAT=image/png&SRS=epsg:25832&BBOX=586609.80952408,6138781.1015875,586686.0222225,6138857.3142859&WIDTH=256&HEIGHT=256
Here's the WLD of the two tiles displayed in the picture:

Upper tile WLD:
0.0204375000
0.0000000000
0.0000000000
-0.0204375000
586620.6934394070
6138899.3169212583

Lower tile WLD:
0.0204375000
0.0000000000
0.0000000000
-0.0204375000
586620.6934394070
6138853.2507962584

--

Ps! As I wrote previously, I'm using FME to create the tiles. So I have little control over the actual coordinate and pixel values, and only specify the number of tiles I need. So manual truncation is not really a viable option.


Kind regards / Med venlig hilsen
Lars I. Nielsen
----------------------------------------------------------------
Landinspektør, Senior GIS Programmør og Konsulent
Hvenegaard Landinspektører A/S
Rugaardsvej 55, DK-5000 Odense C
Denmark
Tel. +45 6313 5050
http://www.hvenegaard.dk<http://www.hvenegaard.dk/>

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