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I have a dataset, in shapefile format, for agricultural land grades in England. It is divided into 7 categories in its attribute table. As the file is a large file and I only want a small section I want to select a rectangle out of it. This was done for all other layers with the select rectangle tool. However it seems to select the entire data when i try to select the small area.

Is there a way to cut the data down to this small area which is undefined in the attribute table???

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you probably have multipolygon there..which software are you using? ArcGIS,QGIS..? –  vinayan Jul 4 '12 at 9:11
    
sorry i am using qgis for this project –  Gerald Jul 4 '12 at 10:06
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2 Answers

up vote 2 down vote accepted

You probabley have Multipart Features. From Vector Menu Select Geometry Tools > Multipart To Single Part and Run that tool. This will create a new shapefile where parts are separated in attribute table. From there you can make your rectangle selection.

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oh man, I am slow. I used this method and then set about further narrowing it down. I used intersect in geoprocessing and then seen that one layers CRS was wrong. Created new guideline boundary and then stumbled into CLIP....deadly. Knew i was looking for it...just didnt know where. Thanks all the same! –  Gerald Jul 4 '12 at 11:24
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I'm not familiar with the Select by rectangle tool, as I think it's new in ArcGIS 10 (I'm still at version 9.3), but as @vinayan says, your land grade polygons have probably been aggregated together so that you can't just select the ones in your area of interest.

The Clip tool in the Analysis toolbox should solve the problem. You'll need to create a new polygon shapefile containing a rectangle which defines your area of interest. Open the Clip (analysis) tool and set input features to your land grade polygons and clip features to your area of interest rectangle. The tool uses the clip features a bit like a pastry cutter to extract just the area of interest.

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Sorry I am using QGIS for this project –  Gerald Jul 4 '12 at 10:06
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