[mapserver-users] raster query strategy

[Jan Hartmann]

On 08/18/10 00:46, Richard Greenwood wrote:
> On Tue, Aug 17, 2010 at 2:57 PM, Frank Warmerdam<warmerdam at pobox.com>  wrote:
>> Jan Hartmann wrote:
>>>   Is this a bug? Shouldn't MapServer use the gdaladdo overviews at the
>>> appropriate scales?
>>>
>> Rich / Jan,
>>
>> No, generally I treat raster query as an "exact" operation done
>> against the full resolution data.  The overviews are only used
>> to satisfy downsampled RasterIO requests such as drawing a reduced
>> resolution map.
> I wouldn't classify it as a bug, but it is an interesting question.
> Displaying a map at 1:1,000,000 the user clicks and gets 30,000 cells
> from my 10m grid. Which one is right, or "exact"? Right now I'm
> averaging them, which is pretty much what gdaladdo has already done.
If you click on a raster map and want to know the value of the area 
under that click, shouldn't that be the value of the pixel in the 
original resolution, just as Frank implemented it in GDAL? What you are 
doing now is essentially querying a square, with an area varying 
according to scale,  and there is no other way to do that than by 
generalizing. Nothing wrong with that, of course, but is that really 
what you intend? And if MapServer's query function forces you to use 
that option, isn't *that* a bug?

Jan

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