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I'm completely new to GIS and this is a very basic question about editing existing shapefiles.

I am working off an existing shapefile which was digitized (by a colleague) so that polygons represent building cover. The shapefile contains information about the land use type and area. I am using QGIS to edit the shapefile.

Now, I wish to add/remove/edit polygons to improve accuracy as the extent of building area has changed a lot recently.

However, when I add new polygons into the shapefile (by digitizing it using an updated basemap), they are not added into the same polygon class as the earlier shapefiles, even though I input the right LandUse type (i.e. Buildings) in the attribute form. Each new polygon seems to create a new LandUse category, even though they're all namd "Buildings".

How do I ensure that any new polygons I add are added into the same LandUse class (all previously created polygons are in this "Buildings" class)?

Thanks in advance for any help on this.

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Can you post a image of your data? –  Nathan W Jan 21 '13 at 9:06
    
Hi, I can't upload an image just yet as I'm new to the site and don't have enough points. Will try to do that as soon as possible! –  lhmv Jan 21 '13 at 9:54
    
If you upload it somewhere and link to it I will update your post –  Nathan W Jan 21 '13 at 9:57
    
Oh that's right. Thanks for that! Here's the attribute table -- flickr.com/photos/92457754@N06/8401865312 –  lhmv Jan 21 '13 at 10:09
    
And here's an image of the map. The light pink polygons are the new ones I've added. And the dark pink ones with the red outlines are from the old dataset. --- flickr.com/photos/92457754@N06/8400780429/in/photostream –  lhmv Jan 21 '13 at 10:11
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1 Answer 1

up vote 4 down vote accepted

What you are doing is fine and good practice. You are not creating a new class but a new feature each time. A new class in your case would be if you added a new LandUse code, say "Big Building" or something like that.

What has happened in the original dataset is your colleague has draw in each building and then combined them into a single object called a multipolygon. What you have been creating is called a polygon. A multipolygon is a single object made up of many polygon objects.

I would continue to do what you are doing as having many single objects makes it easier to work with later. If you wanted to change the LandUse of one object to something else later you can just select that one object and change the value. You can always combine them later into one big single object if need be.

Tip:

If you don't want to type the word "Buildings" each type, because who really wants too and it's error prone, you can set the field in the layer to use a Unique Values Edit Widget. The Unique Values Edit Widget will give you a drop down of values already used in that field e.g Buildings

enter image description here

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Thanks! That was really helpful! –  lhmv Jan 21 '13 at 11:07
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