Tuesday, March 8, 2016

Reshaping Polygons in QGIS

We have shapefile giving the glacier outlines on Svalbard, now after a few years we want to update these outlines since glaciers have changed in extent:


For reshaping this polygon, activate the advanced digitizing menue in QGIS:


Then select the shapefile layer, choose "toggle editing" to make the shapefile layer editable and choose the "Reshape Feature" Button:

 Now start digitizing, first click outside the polygon, crossing the old glacier front, the digitize the new glacier front, and at the end again cross the old front line as indicated here. Below you see this for e retreating glacier, similarly an advancing glacier you cross the old front line before digitizing the new front. THe principle is nicely explained at this blog post. You can zoom into the image use the arrow keys to move the image while digitzing:

Start Digitizing:

End of Digitizing: 

For an advancing glacier you start and end by crossing the polygon like this, again look at this blog post to understand the principle: 



When you now right click, the polygon is adjusted to the new front line:

Now select the polygon you just edited, right click the layer and choose "open attribute table. In the table you choose to show the selected polygon, where you can update the relevant columns (year, date, analyst, source2 and sensor): 






 

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