[Qgis-user] intersection question

[BS]

HI Matthew,

this seems to be less of a QGIS specific question than a general GIS 
question IMHO. Anyways, to address the first question I would store the 
polygons' area in the intersection layer's attribute table. After 
intersecting you can compare the polygon's current area to the one 
stored in the attributes and thus identify those polygons that were 
intersected (the areas differ).
The second question is more tricky. AFAIK there is no tool to 
automagically fix the topology of two layers if they were digitized 
lousily. Maybe there is some tool that defines two vertices as being 
identical if they are only a certain distance apart but that won't help 
if one layer contains the vertex and the other does not. So the answer 
might depend on the reason why the two boundaries do not align. However 
you can intersect the two and try to identify the sliver polygons 
afterwards. Normally their area to circumference ratio is pretty "small" 
(as in tiny area and long circumference) compared to the "real" 
polygons. Use the "eliminate selected polygons" algorithm to produce a 
final result.

Hope that helps
Bernhard

Am 11.11.23 um 08:45 schrieb Matthew Carroll via QGIS-User:
> Hi there,
>
> I'm fairly new to qgis, and hoping someone can point me in the right
> direction for a problem I'm trying to solve.
>
> I need to intersect two polygon layers, and identify which polygons from
> the original layer are, essentially, the ones that were "split" by the
> intersection process.
>
> In this example I'd want to identify polygons 1,2,4,5 since 3,6 are the
> same as in the original layer.
>
> [image: intersection_example.png]
>
> Ideally those polygons would be identified somehow in the attribute table
> for the intersection layer.
>
> In addition, is there any way to build some tolerance into the process to
> account for spots where the polygon boundaries on layer 2 don't align 100%
> perfectly with layer one.
>
> Thanks so much for the help,
> Matthew
>
>
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