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I suspect I'm trying to do the same thing as in Convert image in KMZ to polygon. Given a kml displaying an image within a given lat/long boundary, what are the steps I follow to use Quantum GIS to convert this KML to a vectorized polygon? The image is simply a solid color map -- I.E. the associated PNG simply has each pixel as a set color or as a transparency, and I'm intrerested in being able to query if an arbitrary lat/long is within the bounds of the vectorized polygon.

Edit: It doesnt look like I can open the kml directly. Its raster image is a PNG. How do I set the lat/long bounds of the image if I simply open this PNG as a raster layer?

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up vote 0 down vote accepted

It sounds like you need to first georeference your png file before converting it to a polygon.

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Yep, that's the keyword I needed. Georeference the image, enable the gdaltools plugin, Raster->Conversion->Polygonize, save the resulting vector in whatever format. Thanks. – Oesor Feb 14 '12 at 17:39

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