[Mapserver-dev] Raster Queries / Query Caches

Chris G. Nicholas cgn at globexplorer.com
Tue Mar 9 15:49:18 EST 2004

or perhaps, in the spirit of "GetFeatureInfo" - where one simply changes out "GetMap" - one might just make the qualification that returned results of raster queries have been resampled using the same manner they would have been to present them on an image....?


-----Original Message-----
From:	Daniel Morissette [mailto:dmorissette at dmsolutions.ca]
Sent:	Tue 3/9/2004 11:16 AM
To:	Frank Warmerdam
Cc:	mapserver-dev at lists.gis.umn.edu
Subject:	Re: [Mapserver-dev] Raster Queries / Query Caches
Frank Warmerdam wrote:
> My big question is how should I fit this into the query model?

Not an easy one I agree!

> 2) What should be done about queries that return very large result sets?
> How do folks avoid accidentally very large query results in the vector
> realm?  Would it be sensible to have a "max hits" value in the query API?
> Perhaps the default could be unbounded, but a max hits provided by some
> mechanism optionally?

With very hi-res raster images viewed at very small scale, even a point 
query with a tolerance of 2-3 pixels could generate thousands or even 
millions of results.

Setting a "max hits" may be a good way to handle this, but that would 
return only a subset of the results, which may or may not be 
representative of the complete queried area.

Would it be possible to offer the option to return a kind of histogram 
of the selected pixels?  In this case the result would be a set of 
raw_value/red/green/blue/count where count is the number of pixels that 
have that discrete value.

I'm thinking out loud here... maybe the entries in the histogram could 
be driven by the set of classes defined on the layer?  i.e. you get one 
count value per class.

> 3) Are there presentation methods that should be implemented for raster
> query results other than the normal template mechanisms used for vectors?

We may have to think about how we represent those results with a WMS 
GetFeatureInfo in GML format... um... probably just a collection of GML 
points.  In the case of text/html GetFeatureInfo, the results would be 
driven by the same query template mechanism as the mapserv CGI.

  Daniel Morissette               dmorissette at dmsolutions.ca
  DM Solutions Group              http://www.dmsolutions.ca/

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