[Qgis-user] QGIS: Reduction of resolution via pyramids only for defined scale range

Thu Jun 12 08:00:15 PDT 2014

Dear Jonathan,

thanks for answering.

Ive forgotten to tell you an maybe important- detail: Im working with a
virtual raster catalogue which i built out of 875 .jpgs (4000x4000).

Zoom to best scale (100%) ends in scale 1:668 (for one of the 875 tiles as
well as for the virtual raster catalogue of the 875 tiles).

So if I understand you right I have to calculate the 106x106 pyramide and
smaller ones respectively level 37,5 and higher ones, right?

Kind regards,

Julian Schall

Julian Schall

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Von: Jonathan Moules [mailto:jonathanmoules at warwickshire.gov.uk] 
Gesendet: Donnerstag, 12. Juni 2014 11:34
An: Julian Schall
Cc: qgis-user at lists.osgeo.org
Betreff: Re: [Qgis-user] QGIS: Reduction of resolution via pyramids only for
defined scale range

Hi Julian,

  You can specify at what levels pyramids are created. That's what the
"resolutions" box on the right does. The problem is, knowing which levels
you want.

So if your data is a 4000x4000 tiff then it'll offer to create pyramids at:

2000*2000
1000*1000
500*500
250*250
125*125
63*63

I'm going to guess at the answer to your question - I believe what you need
to do is the following:

- Find out what the scale is for "Zoom to best scale (100%)" for the raster.
For my raster it's 1:9,449 (yours will be obviously be different).

- So the 2000x2000 pyramid will have a scale of 1:18898 (i.e. double 9449).
So you wouldn't want that level. In this scenario you may want 1000*1000 -
which would be for a scale of 1: 37796 (double 1:18898).

So with some simple maths you can figure out which pyramid levels you want.
I suppose it would be a good feature for QGIS to show scales rather than
tile sizes in the "resolutions" box. I'll go open a feature suggestion for
that one.

Cheers,

Jonathan

On 12 June 2014 10:00, Julian Schall <j.schall at ff-forst.de> wrote:

Hello members of the mailing list, 

I would appreciate help on the following problem: 

I would like to improve the performance of a raster layer. Of course I can
build pyramids for that purpose. But in the scale ranges from 1:1 to
1:25.000 I need maximum resolution and detail. In this scale range, the
performance is sufficient even without pyramids. 

Ideal would be that the pyramids respectively the reduction of the
resolution with improvement of performance is only applied to scales smaller
than 1:25,000. Is this possible in QGIS? 

Thanks for helpful answers! 

Kind regards 

Julian Schall

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