[Qgis-user] trimming unwanted areas

[Alex Mandel]

On 02/03/2012 06:51 AM, Chuck Young wrote:
> I have  MfrSID raster layer and two Tiger file vector layers. The image
> file is for the north half of the county and the vector files (roads and
> water) cover the entire county.  I have imported them all  into the
> SpatiaLite database.  What I need to do is trim off  the data that is
> outside the area I need to work with.  Is there a method to select the area
> I want to keep or the area I want to delete, and remove the unwanted
> portions.  I know I can go to each entity and break it and delete the
> portion that is not needed but there are thousands of entities and this
> would take a lot of time.
> 
> Thanks!
> 
> Chuck
> 

Chuck,

Did you want to cut just vectors or also rasters? The method is slightly
different for each of those. The basic approach is to create a vector
layer that serves as your mask; this can be a simple box (the is a
plugin to make a perfect box if you don't want to draw by hand). Step 2
is to clip your other layers using the mask layer.

Vector->Geometry Management->Clip
Raster->Extraction->Clipper (This is GDAL Translate underneath)

If you have multiple layers to do and want to automate that might take a
little python or some command line with GDAL/OGR.

An alternate method is to use the selection tool (Rectangle), once you
have the data selected you want to keep, do a "save selection as" to
create a new file of just the selection.

I don't think there's a plugin to just do all this for a stack of layers
(sounds like a good idea), we could all it cookie cutter...

Thanks,
Alex