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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?

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In a comment below you mention you want to 'make the file smaller'. Is that the case? You may be looking for a 'simplify' operation which will run an algorithm to reduce the points - try the plugin DPSimplify, or the Vector|Geometry Tools|Simplify menu option. –  Simbamangu Mar 13 '12 at 5:13

4 Answers 4

  1. Start an edit session on the layer that you want to delete features
  2. Run a Spatial Query to perform the selection
  3. Delete features
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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. –  Mike Guillen Mar 13 '12 at 3:58
    
@Mike Guillen, Please update your question to better represent your comment. My answer above was in-line with your question as stated. –  artwork21 Mar 13 '12 at 11:25
    
@artwork21 - won't a spatial query select a set of features, whereas OP wants to edit components of an individual feature? –  Simbamangu Mar 13 '12 at 14:07
1  
Seems like a legitimate answer to the users question. Select points by poly, delete points. spot on. –  RyanDalton Mar 13 '12 at 15:00
  1. Select your layer
  2. Enabled Editing
  3. Choose the Node Tool
  4. Click a point on your polygon
  5. Use Command-click & drag to select multiple points with the marquee tool. (might be shift-click on Windows)
  6. Use Delete key to delete the selected points (Fn+Delete on MacBook Pro)
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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

Kurt

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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:

  1. Enable editing
  2. From the advanced editing tools choose Reshape feature
  3. 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.

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This worked best for me! Thanks so much! –  Mike Guillen Mar 13 '12 at 16:32
    
@Sylvester: Are you using Shift or Alt-click & drag to make sure nodes get added to your selection as you select more and more? –  Brian Kelly Mar 13 '12 at 17:39

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