Hi,
I just want to clear up my mindset as to how a VRT is implemented in QGIS.
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.
In this context, I am thinking about SHP files - especially the NGI
dataset which comes out "cut up" into degree squares.
Let's just deal with 1 feature type: Rivers lines. My VRT looks like:
<OGRVRTDataSource>
<OGRVRTUnionLayer name="Rivers">
<OGRVRTLayer name="2730_RIVER_LINE_2006_06">
<SrcDataSource
relativeToVRT="1">2730/2730_RIVER_LINE_2006_06.shp</SrcDataSource>
</OGRVRTLayer>
<OGRVRTLayer name="2731_RIVER_LINE_2006_04">
<SrcDataSource
relativeToVRT="1">2731/2731_RIVER_LINE_2006_04.shp</SrcDataSource>
</OGRVRTLayer>
<OGRVRTLayer name="2732_RIVER_LINE_2006_04">
<SrcDataSource
relativeToVRT="1">2732/2732_RIVER_LINE_2006_04.shp</SrcDataSource>
</OGRVRTLayer>
</OGRVRTUnionLayer>
</OGRVRTDataSource>
* When I open the VRT in QGIS, does QGIS open ALL the VRT files and
look for the extent of each of the files?
o 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?
* Before rendering the VRT, does QGIS look at the extents of my
viewport and only physically open my files and render it's contents?
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)
* As I pan around my map, does QGIS open and close the VRT subfiles
that are out of my current viewing region?
* Presumably if any of my VRT subfiles touch or overlap my current
viewport, they would be "processed" depending on what I am doing?
* 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?)
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?
TIA,
Zoltan
--
===========================================
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 www.geograph.co.za
===========================================
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