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The usual approach I've seen in evaluating a remote sensing
classification is the confusion matrix [1]<br>
<br>
I'd either rasterize the polygons at whatever resolution seems
appropriate and compute the confusion matrix from the rasters, or
split the one polygon layer with the other and take the areas of
each resulting polygon and assign them based on if they are in one,
the other, both, or neither.<br>
<br>
[1] <a class="moz-txt-link-freetext" href="https://en.wikipedia.org/wiki/Confusion_matrix">https://en.wikipedia.org/wiki/Confusion_matrix</a><br>
<br>
<div class="moz-cite-prefix">On 11/4/25 06:19, celati Laurent wrote:<br>
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<blockquote type="cite"
cite="mid:CAHByMH1uxXSfyB7v_F4NkX43_339g9vOBrLu5yg2jRW+c2+nrA@mail.gmail.com">
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<div>Dear all, </div>
<div><br>
</div>
<div>A few weeks ago, I posted an initial message:</div>
<a
href="https://lists.osgeo.org/pipermail/postgis-users/2025-October/046930.html"
moz-do-not-send="true"><br>
https://lists.osgeo.org/pipermail/postgis-users/2025-October/046930.html</a><br>
<br>
But after further thought, I'm creating a new one to
provide additional inputs/clarification.<br>
As a reminder, my goal is to find a method/tool to compare
the similarity/dissimilarity between two polygonal vector
layers consisting of multiple features. In my use case,
this comparison concerns a polygonal layer resulting from
photo-interpretation work (modeling of large physiognomic
units/habitats). I reattached a screenshot; this layer
appears in blue.<br>
The other polygonal vector layer is a polygonal vector
layer resulting from a segmentation process (using an OTB
tool available via QGIS). (It is yellow in the attached
screenshot):<br>
<br>
<img src="cid:part1.Rl0f9IM8.0vvy0cIc@gmail.com"
alt="SS_seg_PI_identifiants.png" width="578"
height="278" class=""><br>
<br>
<br>
In my previous post, I mentioned a possible way for one
similarity indicator: "for each photo-interpreted
feature/polygon, calculate the percentage (%) of area
covered by the feature from the segmentation layer that
has the highest overlap."<br>
To be more specific, in the example in the screenshot, it
could be:<br>
"for feature number 11 (blue) from the
photo-interpretation layer, I would like to know the
proportion of the area covered by the feature from the
segmentation layer that has the highest overlap with this
feature 11 (in this specific case, it is probably feature
42 in yellow)."<br>
This could be: area of feature 42 overlapping with feature
11 / total area of feature 11.<br>
<br>
But after further thought, I think this is insufficient.
That i need to go a little further.<br>
<br>
Regarding these methods for quantifying the accuracy of a
segmentation/fidelity to photo-interpretation. Regarding
the level/degree of similarity, I think i should integrate
a "bi-similarity" aspect/logic: using an approach based <b>on
pairs</b> of the most overlapping polygons:<br>
<br>
<b>step 1</b><br>
- For each polygon feature in the input layer, retrieve
the ID and geometry of the polygon from the segmentation
layer that has the highest overlap. Calculate the overlap
rate.<br>
(e.g., surface area of feature 42 overlapping with feature
11 / <b>total surface area of feature 11</b>).<br>
<b><br>
Step 2:</b><br>
- Once this first indicator is calculated, perform the
calculation for the same pair/sequence of polygons
calculated in step 1: perform the same calculation but
this time on the total surface area of feature 42 (polygon
from the segmentation).<br>
(e.g., surface area of feature 42 overlapping with feature
11 /<b> total surface area of feature 42</b>).<br>
<br>
<b>Step 1/2 variant:</b><br>
I was thinking that a one-step variant/synthesis/summary
of steps 1 and 2 could be:<br>
- (e.g., surface area of feature 42 overlapping with
feature 11 / <b>total surface area constituted by feature
11 AND feature 42).</b>?<br>
<br>
<b> Step 3:</b><br>
The idea would be, based on steps 1/2, to have a kind of
overall score at the layer level. This could be a kind of
average/median for all pairs of polygons? Or another
metric?<br>
<br>
I imagine that this tool does not exist/is not available
in QGIS. Although I discovered this plugin:<br>
<a
href="https://plugins.qgis.org/plugins/similarity_plugin/#plugin-about"
target="_blank" moz-do-not-send="true"
class="moz-txt-link-freetext">https://plugins.qgis.org/plugins/similarity_plugin/#plugin-about</a><br>
<a href="https://github.com/panickspa/SimilarityPlugin"
target="_blank" moz-do-not-send="true"
class="moz-txt-link-freetext">https://github.com/panickspa/SimilarityPlugin</a><br>
<br>
Interesting plugin. But in my humble opinion, it only
covers part of my need.<br>
To be complete, I did some research on the internet. I
also found a potentially interesting indicator:<br>
<a
href="https://postgis.net/docs/ST_HausdorffDistance.html"
target="_blank" moz-do-not-send="true"
class="moz-txt-link-freetext">https://postgis.net/docs/ST_HausdorffDistance.html</a><br>
<a href="https://en.wikipedia.org/wiki/Hausdorff_distance"
target="_blank" moz-do-not-send="true"
class="moz-txt-link-freetext">https://en.wikipedia.org/wiki/Hausdorff_distance</a><br>
<br>
Is there a possible way, either through QGIS or PostGIS?<br>
</div>
Thanks so much.</div>
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