[mapserver-users] performance problem with ortho images

[Rahkonen Jukka]

Hi,
 
Is it necessary to show 2 m data at all in the areas where 20-50 cm imagery is available?  If not you can first make tileindex shapefiles from 2 m images and 20-50 cm images.  Open both shapefiles with for example OpenJUMP and select and remove those polygons from 2 m shapefiles which are totally covered by you 20-50 cm tiles.  Combine the two layer, save and use the result as a tileindex file for Mapserver.
 
There may be problems in the areas with overlapping images. To get the desired image to show on top it may be necessary to rewrite the shapefile by so that it will be ordered so that the small pixel size images will be written last in the shapefile.  Then they should be drawn on top of 2 m imagery.
 
If you want the show 2 m images for the whole area when zoomed far away, prepare still the combined shapefile with covered by -images removed.  Then make two scale dependent layers under the same group.  Read 2 m data when scale is between 200k and 100k and switch then to the combined layer.
 
There may be better ways, this is the best I have invented this far for our own use.
 
Take care of creating enough many low-resolution layers with gdaladdo for far away zooms.  Even then if Mapserver must open more than 20-40 image files for filling the screen I would consider making a separate overview image by mosaicing the whole area together with something like 20-50-100 meter pixel size and using that image in the same image group.
 
-Jukka Rahkonen-


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	Lähettäjä: mapserver-users-bounces at lists.osgeo.org [mailto:mapserver-users-bounces at lists.osgeo.org] Puolesta Liang Jian
	Lähetetty: 16. marraskuuta 2009 13:02
	Vastaanottaja: mapserver-users at lists.osgeo.org
	Aihe: [mapserver-users] performance problem with ortho images
	
	
	
	Hi all,
	 
	I have a problem with mapserver handling ortho images. The situation is we have 2m data cover the whole country and 20-50cm data cover most cities. We setup mapfiles as 2m data shows between scale 200k to 500 and 20-50cm data show between scale 100k to 500. Problem is when requirst data in the city ranges, mapserver first read data from 20-50cm dataset and then read from 2m dataset which is unnecessary as the 20-50cm data is enough and the best solution is when there is no 20-50cm data, 2m data should be used instead. 
	 
	Do you have any idea how to avoid the unnecessary reading? 
	 
	Any suggestions will be welcome!
	 
	Thanks
	 
	/Liang

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