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In QGIS (1.8.0-Lisboa) on Windows (OSGeo4W Install), how would you resize a feature proportionally?

The only way I've found to do this is by hand using either the Freehand Editing (0.2.6) plugin or the Reshape Features button. Neither of these provides a means to uniformly scale (as in resize) the selected feature up or down.

For example, I want to increase the features size by 130% so that you retain the shape of the feature but expand the area of the shape itself proportionally in all directions.

PolyGeo
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paperclip
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1 Answers1

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In QGIS you can use the Affine functions for scaling vector features. In the Vector menu:

enter image description here

And the dialog looks like this:

enter image description here

If you enter 1.3 in both the "Scale X" and "Scale Y" boxes, then the layer (or just the selected feature(s) will be scaled up by 130%. This operation will scale the features proportionally, but be aware that depending on the coordinate reference system (CRS) you are using the results may look warped. For example, if you are using a Mercator projection and scale a large polygon near to the north or south pole, then it will appear to stretch more the closer it is to the pole. For smaller polygons near the center of the given CRS there will be no noticeable distortion.

To see what I mean about distortions, try the Mercator Puzzle. It's fun!

Update (January 2019) for QGIS 3.4: Affine transformations can be are available through the Processing Toolbox with the GRASS algorithm v.transform.

enter image description here

PolyGeo
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katahdin
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    Good answer. Took me a second to understand that Affine functions needed to be installed as a plugin.

    I imagine with the constraints of the CRS (in my case I see: EPSG:4326 - WGS 84) this is not as straightforward to arbitrarily increasing the size of the vector shape itself without affecting the coordinates of the feature.

    The Mercator Puzzle was a nice addition as an example.

     – paperclip Jun 10 '13 at 08:37
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    Tip: To Use v.transfrom, selected Layer should be in editable mode. Else the python error has accured. – Can Bayat May 17 '19 at 13:18