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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