Virtual dataset - how to?

[jlawheadus]

Thanks for the info!

-- Joel

--- In gdal-dev at y..., Frank Warmerdam <warmerdam at p...> wrote:
> Joel Lawhead wrote:
> > I've been having fun with GDAL 1.1.7 and Python 2.2.1 on Windows
2000. I 
> > was wondering if anyone could give me an example or pointer to
create a 
> > virtual dataset for use with CreateCopy(). I realize Frank is
working on 
> > rewriting 'gdal_translate' to use virtual datasets and was
wondering if 
> > there were any examples out there in C or preferably Python. I'm 
> > experimenting with creating jpegs from other gdal formats.
> > 
> > I haven't located any direct examples in the list archive, gdal
example 
> > sources or OpenEV sources. I'm aware of the two-step method to
create a 
> > JPEG by doing the resampling in another format but was just curious 
> > about the virtual dataset concept. At first glance it seems like that 
> > method might be faster. I haven't checked the UMN MapServer source
code 
> > to see whether it uses the two-step process or virtual datasets to 
> > convert data to jpegs. If I've overlooked anything please let me
know.
> 
> Joel,
> 
> As you note, once the rewrite of gdal_translate is done to utilize the
> virtual datasets the two step process will not longer be needed but for
> now it is. Currently there is no programatic way of creating virtual
> datasets ... they have to be initialized from the XML .vrt files. The
> closest you could get would be to write out a temporary .vrt file with
> the description you want.
> 
> I had intended to do more today, but I have been distracted by a high
> priority task, but what I have got done is add a CreateCopy() method
> for VRTDataset. That means "gdal_translate -of VRT utm.tif utm.vrt"
> will create a .vrt file that is an image (and still referring to) the
> utm.tif file. This .vrt file can then be hand edited to modify some
> information.
> 
> If you were working with very large files to translate to JPEG with
> an altered configuration you could conveivably do:
> 
> gdal_translate -of VRT big.tif big.vrt
> hand edit big.vrt to take the subset, band combinatoin etc desired.
> gdal_translate -of JPG big.vrt big.jpg
> 
> But that isn't my intended final solution.
> 
> > I've been working with Perl for several years and Python for a
couple of 
> > years and in the last year have been introduced to the GIS/remote 
> > sensing world. I started out trying to understand image manipulation 
> > using the Python Imaging Library which lead me to GDAL. I've found
GDAL 
> > a great tool for learning about the different geo-spatial data
formats. 
> > 
> > What would be really nice would be a GDAL Cookbook' on the Yahoo
Groups 
> > site. More experienced programmers could post their successful code 
> > snippets (maybe in the 'Files' section?) which would make it easy to 
> > find examples to give us intermediate programmers a little boost. I 
> > found the API tutorial on the GDAL site extremely helpful and
would love 
> > to see more of those types of examples. Once I get comfortable with 
> > GDAL in Python I'll try to write a comprehensive tutorial about what 
> > I've learned.
> 
> I like the idea, but I am not sure I want to depend on Yahoo groups
to hold
> it all. I would like to even transition the mailing list off yahoo
eventually
> since their ads are getting increasingly annoying.
> 
> I have been impressed with the MapServer Wiki, and I am wondering if we
> should establish something similar for GDAL on remotesensing.org. In
> the meantime we could likely colonize a corner of the MapServer Wiki
with
> GDAL related information. The MapServer Wiki is at:
> 
> http://mapserver.gis.umn.edu/cgi-bin/wiki.pl
> 
> I also agree with the need for more and better tutorial material of
various kinds.
> I would be happy to incorporate submissions from GDAL users.
> 
> Best regards,
> 
> -- 
>
---------------------------------------+--------------------------------------
> I set the clouds in motion - turn up | Frank Warmerdam, warmerdam at p...
> light and sound - activate the windows | http://pobox.com/~warmerdam
> and watch the world go round - Rush | Geospatial Programmer for Rent