[Qgis-user] Mosaicking aerial images

[morgan-hesperus]

When I need a blended mosaic of overlapping images, I've had good results
using gdalwarp's -cblend switch, as follows.

1. Open the overlapping images in QGIS.
2. Create a new polygon layer. This is so you can create a masking polygon
for one of the images.
3. Draw a masking polygon whose edge cuts through the overlap area where
you want blending. (The rest of the polygon can be outside the image.)
Save.
4. Go Raster>Extraction>Clipper.
5. Choose Mask layer and select your polygon.
6. If your image has only three bands, check Create alpha band. Don't
check this if your image already has 4 bands.
7. Specify an output file. This cannot already exist.
8. Edit the command line at the bottom to replace the "-clip-to-cutline"
switch with "-cblend 40". The 40 is just a suggestion. It means the new
image will fade to completely transparent across the 40 pixels around the
polygon edge. 
9. Hit OK. The image produced will have an alpha channel and fade from
fully opaque to fully transparent along the clipping polygon edge.

- Morgan


> Message: 3
> Date: Wed, 10 Dec 2014 12:21:14 -0600
> From: Michael Treglia <mtreglia at gmail.com>
> To: Andreas Neumann <a.neumann at carto.net>,
> 	"qgis-user at lists.osgeo.org" <qgis-user at lists.osgeo.org>
> Subject: Re: [Qgis-user] Mosaicking aerial images
> Message-ID:
> 	<CAPKp32vuf8OmVenc=fh54xnnQSTnWp0J31=aR0dvBppXBA7TVQ at mail.gmail.com>
> Content-Type: text/plain; charset="utf-8"
> 
> Hi Andreas,
> 
> I'm Bringing this back to the list, as others here probably have some
> better thoughts than I do.  Googling around, I found this tool which
might
> be useful (?), though I can't tell how it handles overlaps [haven't even
> downloaded it...]: https://trac.osgeo.org/gdal/wiki/Correlator
> 
> Not sure about enblend (can't access the site right now)
> 
> Depending on how different the images are, you could just try mosaicing
and
> see what it looks like (gdal_merge takes the values of last image added
for
> overlaps).  If there's a way to standardize color values based on
> brightness, or by RGB bands, that might help, either before or after
> mosaicing. (might depend on if you're dealing with multi-band image, or
> single band? - it would be fairly easy on multi-band like Landsat)
> 
> Hope that helps,
> mike
>