Percentage of area overlaped / covered by feature of another layer

[celati Laurent]

Dear all,

I'm working with QGIS and, if necessary, PostGIS.
I have two vector layers :

 - A first polygonal layer resulting from vectorization work using
photointerpretation of the aerial imagery (in blue in the attached
screenshot below).
 - A second polygonal layer in the same area resulting from segmentation of
the same aerial imagery (in yellow in the attached screenshot below).

[image: SS_seg_PI_identifiants.png]


Each feature in the two layers has a unique identifier (representing thanks
to labels in the screenshot).

I'd like to be able to produce a small performance indicator of the
segmentation performs in faithfully matching the photointerpretation. I was
thinking there's probably something that could be done with geoprocessing
(union, intersection?) and a area calculation ? An indicator of
segmentation performance could be: for each photo-interpreted
feature/polygon, calculate the percentage of the surface area covered by
the feature in the segmentation layer with the biggest overlap ?
To be more specific, in the example screenshot, this could be:
"For feature 11 in photo-interpretation layer (blue), I would like to know
the percentage of the surface area covered by the feature in the
segmentation layer that has the highest overlap with this feature 11 (in
this specific case, it's probably feature 42 in yellow)."

This could be: area of feature 42 overlapping feature 11 / total area of
feature 11?

I don't know if I'm on the right path. How could we translate this needs
toward a QGIS and/or PostGIS tool?

Thanks so much
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