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Hi,<br>
I just want to clear up my mindset as to how a VRT is implemented in
QGIS.<br>
<br>
I'd like to understand when QGIS opens a file, when it reads the
contents, and when it writes (if need be) and closes a file.<br>
In this context, I am thinking about SHP files - especially the NGI
dataset which comes out "cut up" into degree squares.<br>
Let's just deal with 1 feature type: Rivers lines. My VRT looks
like:<br>
<font color="#006600"><tt><OGRVRTDataSource></tt><tt><br>
</tt><tt> <OGRVRTUnionLayer name="Rivers"></tt><tt><br>
</tt><tt> <OGRVRTLayer name="2730_RIVER_LINE_2006_06"></tt><tt><br>
</tt><tt> <SrcDataSource
relativeToVRT="1">2730/2730_RIVER_LINE_2006_06.shp</SrcDataSource></tt><tt><br>
</tt><tt> </OGRVRTLayer></tt><tt><br>
</tt><tt> <OGRVRTLayer name="2731_RIVER_LINE_2006_04"></tt><tt><br>
</tt><tt> <SrcDataSource
relativeToVRT="1">2731/2731_RIVER_LINE_2006_04.shp</SrcDataSource></tt><tt><br>
</tt><tt> </OGRVRTLayer></tt><tt><br>
</tt><tt> <OGRVRTLayer name="2732_RIVER_LINE_2006_04"></tt><tt><br>
</tt><tt> <SrcDataSource
relativeToVRT="1">2732/2732_RIVER_LINE_2006_04.shp</SrcDataSource></tt><tt><br>
</tt><tt> </OGRVRTLayer></tt><tt><br>
</tt><tt> </OGRVRTUnionLayer></tt><tt><br>
</tt><tt></OGRVRTDataSource></tt><tt><br>
</tt></font><br>
<ul>
<li>When I open the VRT in QGIS, does QGIS open ALL the VRT files
and look for the extent of each of the files?</li>
<ul>
<li>If my VRT had the extents included for each of the files,
would this stop QGIS from (at this stage) opening the files
and reading the extents?<br>
<br>
</li>
</ul>
<li>Before rendering the VRT, does QGIS look at the extents of my
viewport and only physically open my files and render it's
contents?<br>
In other words, if I first zoom into a known area, then open my
VRT, will QGIS at this stage still open all the subfiles,
instead of waiting until a specific subfile needs opening)<br>
<br>
</li>
<li>As I pan around my map, does QGIS open and close the VRT
subfiles that are out of my current viewing region?<br>
<br>
</li>
<li>Presumably if any of my VRT subfiles touch or overlap my
current viewport, they would be "processed" depending on what I
am doing?<br>
<br>
</li>
<li>Is there a way to structure a VRT file so that you can have
access to the underlying files that make up the VRT? (Even edit
access?)<br>
</li>
</ul>
<p><br>
Or, is the VRT just any easy way to bunch a whole lot of maps
under one name, and there is no processing benefit depending on
the area you are viewing or working in?<br>
</p>
<p>TIA,<br>
Zoltan<br>
</p>
<br>
<br>
<br>
<br>
<br>
<pre class="moz-signature" cols="72">--
===========================================
Zoltan Szecsei PrGISc [PGP0031]
Geograph (Pty) Ltd.
GIS and Photogrammetric Services
P.O. Box 7, Muizenberg 7950, South Africa.
Mobile: +27-83-6004028
Fax: +27-86-6115323 <a class="moz-txt-link-abbreviated" href="http://www.geograph.co.za">www.geograph.co.za</a>
===========================================</pre>
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