I'm new at this, a novice at best. I'm using QGIS 1.7.3. I simply want to select a large number of points on a single polygon and delete them. Is this possible?
All the answers given will work, depending on how you define 'select' and 'make it smaller'. However, my guess from your latest comment is that you want to just chop a portion off your polygon rather than simplify the number of vertices while maintaining the general shape of the whole polygon. Spilth's answer ought to work, but for me it only deletes one node from the selection and not ALL the selected vertices (I'm using version 1.9.9 on Windows). If you have this same issue, an alternative method for cutting out a section of your polygon is this:
- Enable editing
- From the advanced editing tools choose Reshape feature
- Draw a line (point and click) across your polygon where you want to cut it (start and finish your cut-line outside the polygon and right-click when you have finished).
This will chop off the smaller part of the polygon. You can also use this tool to add to a polygon if you start and finish drawing from inside the polygon.
- Start an edit session on the layer that you want to delete features
- Run a Spatial Query to perform the selection
- Delete features
As you added as a comment: "Well, I have a big shapefile that is a river. I want to select half of the shapefile and delete it, to make the file smaller. I don't think performing a spatial query would work for that. "
I suggest to use: Vector -> Geometry Tools -> Simpliyfy Geometries.
if you want only one line-feature of many line-features processed, you can first select this line on the canavas and then tick the box "use only selected features" in the dialog-windows of Simplify Geometries. You may have to try different tolerance values to get the desired resultat...
Hope this helps