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</o:shapelayout></xml><![endif]--></head><body lang=EN-US link=blue vlink=purple><div class=WordSection1><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D'>I do get 2.23606 on PostGIS so we are in agreement there.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D'>As to what the right answer is I have no clue and reading math equations gives me a headache.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D'>I’ve added geos-develop to mailing list for comment.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D'>Thanks,<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D'>Regina<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D'><o:p> </o:p></span></p><div style='border:none;border-left:solid blue 1.5pt;padding:0in 0in 0in 4.0pt'><div><div style='border:none;border-top:solid #E1E1E1 1.0pt;padding:3.0pt 0in 0in 0in'><p class=MsoNormal><b><span style='font-size:11.0pt;font-family:"Calibri",sans-serif'>From:</span></b><span style='font-size:11.0pt;font-family:"Calibri",sans-serif'> Mobilitydb-dev [mailto:mobilitydb-dev-bounces@lists.osgeo.org] <b>On Behalf Of </b>Esteban Zimanyi<br><b>Sent:</b> Saturday, September 11, 2021 4:35 AM<br><b>To:</b> mobilitydb-dev@lists.osgeo.org<br><b>Subject:</b> [Mobilitydb-dev] Problem with ST_FrechetDistance in PostGIS/GEOS<o:p></o:p></span></p></div></div><p class=MsoNormal><o:p> </o:p></p><div><div><div><p class=MsoNormal>Dear Regina<o:p></o:p></p><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>We started the implementation of the discrete Frechet distance in MobilityDB and found out that we obtain a different result than PostGIS/GEOS.<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><div><p class=MsoNormal>test=# select frechetDistance(tgeompoint '[Point(1 1)@2000-01-01, Point(2 2)@2000-01-02, Point(3 1)@2000-01-03]',<br>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]');<br> frechetdistance<br>-----------------<br> 3<br>(1 row)<br><br>test=# select ST_FrechetDistance(geometry 'Linestring(1 1,2 2,3 1)',<br>test(# geometry 'Linestring(1 4,2 3,3 4,4 3)');<br> st_frechetdistance<br>--------------------<br> 2.23606797749979<br>(1 row)<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div></div><div><p class=MsoNormal>We used the simple algorithm referenced in the PostGIS manual<o:p></o:p></p></div><div><p class=MsoNormal><a href="https://postgis.net/docs/ST_FrechetDistance.html" target="_blank">https://postgis.net/docs/ST_FrechetDistance.html</a><o:p></o:p></p></div><div><p class=MsoNormal>and according to our understanding the correct result is 3.<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>Indeed the matrix of Euclidean distances between a vertex of the first linestring and a vertex of the second linestring is as follows<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>3.6 2.23 2.23<br>3.6 2.23 3<br>2.23 1 2.23<br>3 2.23 3.6 <br><br>And the matrix of the computation of the Frechet distance (ca in the algorithm) is as follows<br><br>3.6 3 3<br>3.6 3 3<br>3 3 3<br>3 3 3.6<br><br>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.<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>Many thanks<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>Esteban<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div></div></div></div></div></div></body></html>