When creating convex hulls based on input field in qgis 1.8, there seems to be always one less convex hull created for the unique values in the input field.

For example;

I have a point vector file, and it has a field in its attribute table called 'type'. This field has two unique values, 1 and 2. When trying to create convex hulls for these types, only one convex hull is created, encompassing the 'type' value 2.

The point vector file also has a field called 'year'. This field has three unique values, 2007, 2008 and 2009. When creating convex hulls for these types, only two convex hulls are created, encompassing 2008 and 2009 values only. I've tested this further with other vector files and it always seems to be the case that the lowest integer value is ignored when the convex hull process is run.

I have also tested this with text based fields, it also creates one less convex hull than the number of unique variables in the text field.

This issue was not present in previous verisons of qGIS. Can any of the developers or anyone else shed some light on this?

As a workaround, a dummy feature has been included in the attribute table with an additional variable where its value is lower than that of the other unique variables.

  • 1
    Update; The SEXTANTE plugin includes a 'Convex Hull' function among its QGIS geoalgorithms -> geometry tools. This produces Convex Hull(s) as expected without the issue described above. I will now use this plugin to produce Convex Hulls instead. It also appears to have many other useful fucntions in its toolbox. – Walter May 20 '13 at 12:04

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