[mapserver-users] Creating lower aeriel image cache from the best resolution tiles

[Rahkonen Jukka]

Hi,

I took some numbers from the real system. We use a custom tile schema with 15 resolution levels
between 0.5 and 8192 meters. Six of the resolutions levels 0.5-16 m) come from the same original aerial images so it would be possible to build lower resolution level from previously generated tiles. Nothing special here, basically it is the same process than I have done for all the originals by running "gdalddo 2 4 8 16 32"  so I believe that the image quality would be comparable.

There are roughly 24 million tiles on the disk for the 0.5 m resolution and with 5 by 5 sized metatiles it has
taken nearly 1 million WMS requests to seed the cache. There is no alternative for seeding this resolution level, it just has to be seeded from WMS and we want to have a completely seeded cache. The interesting question concerns seeding the 1 m resolution level. In our existing situaltion we have sent about 250000 WMS requests and split the metatiles into 8 million final tiles. It would be nice to know how long it would take to build the tiles by doing 8 million times "take four tiles, combine and resample". Because there are only 86400 seconds in a day even small differences count. Even if combining tiles would be somewhat slower it might be a good alternative because the WMS server would take 250000 requests less and it is a production server which has other things to do as well.

In our case the source data of WMS suits very well for seeding because the native resolution is the same than the final one and images have overviews which also fit perfectly, and the native imagery and cache are using the same projection. Obviously systems where WMS server must to reprojecting and adjust pixel size would get more advantage from aggregating higher resolution tiles.

-Jukka Rahkonen-


________________________________
Lähettäjä: Puneet Kishor [punk.kish at gmail.com]
Lähetetty: 9. kesäkuuta 2013 20:22
Vastaanottaja: thomas bonfort
Cc: Rahkonen Jukka; Mapserver-Users (mapserver-users at lists.osgeo.org)
Aihe: Re: [mapserver-users] Creating lower aeriel image cache from the best resolution tiles

Yesterday at SOTMUS here in SF, Dane Springmeyer (Maobox) demonstrated something they call "overzooming" that might be worth investigating. It seems to solve the kind of problem Thomas is articulating.



--
Puneet Kishor
Policy Coordinator for Science and Data
Creative Commons

On Jun 9, 2013, at 9:14 AM, thomas bonfort <thomas.bonfort at gmail.com<mailto:thomas.bonfort at gmail.com>> wrote:

Jukka,
No this is not considered, and you'll have to be really persuasive to make this happen as I do not see any advantage of adding this :) The reason is that in order to create e.g. the tile for level 0 for the first time, you'll have to downsample (and maybe previously seed) the full z=21 level (and all the intermediate ones).

regards,
thomas


On 9 June 2013 18:02, Rahkonen Jukka <jukka.rahkonen at mmmtike.fi<mailto:jukka.rahkonen at mmmtike.fi>> wrote:
Hi,

Have you been considering to seed the cache for aerial or satellite image layers in opposite order compered to the normal Z0, Z1, Z2... so that only the best resolution Z(max) tiles (or perhaps also Z(max-1) were requested from the WMS? The lower resolution tiles could me created by combining and resamplint tiles from the tiles of the higher zoom level in groups of 4. This should be much faster than asking lower level images from the WMS, especially if the WMS is a remote one and connection is slow. If resampling from Z+1 tiles yields to artifacts I feel that quality by resampling from Z+2 would normally be good and speed migh be still fantastic.

-Jukka Rahkonen-

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