I do get 2.23606 on PostGIS so we are in agreement there.
As to what the right answer is I have no clue and reading math equations gives me a headache. I’ve added geos-develop to mailing list for comment. Thanks, Regina From: Mobilitydb-dev [mailto:mobilitydb-dev-boun...@lists.osgeo.org] On Behalf Of Esteban Zimanyi Sent: Saturday, September 11, 2021 4:35 AM To: mobilitydb-...@lists.osgeo.org Subject: [Mobilitydb-dev] Problem with ST_FrechetDistance in PostGIS/GEOS Dear Regina We started the implementation of the discrete Frechet distance in MobilityDB and found out that we obtain a different result than PostGIS/GEOS. test=# select frechetDistance(tgeompoint '[Point(1 1)@2000-01-01, Point(2 2)@2000-01-02, Point(3 1)@2000-01-03]', tgeompoint '[Point(1 4)@2000-01-01, Point(2 3)@2000-01-02, Point(3 4)@2000-01-03, Point(4 3)@2000-01-04]'); frechetdistance ----------------- 3 (1 row) test=# select ST_FrechetDistance(geometry 'Linestring(1 1,2 2,3 1)', test(# geometry 'Linestring(1 4,2 3,3 4,4 3)'); st_frechetdistance -------------------- 2.23606797749979 (1 row) We used the simple algorithm referenced in the PostGIS manual https://postgis.net/docs/ST_FrechetDistance.html and according to our understanding the correct result is 3. Indeed the matrix of Euclidean distances between a vertex of the first linestring and a vertex of the second linestring is as follows 3.6 2.23 2.23 3.6 2.23 3 2.23 1 2.23 3 2.23 3.6 And the matrix of the computation of the Frechet distance (ca in the algorithm) is as follows 3.6 3 3 3.6 3 3 3 3 3 3 3 3.6 Could you please have a look ? If you confirm that there is a problem I will post a ticket in PostGIS and/or GEOS mailing lists. Many thanks Esteban
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