<html xmlns:v="urn:schemas-microsoft-com:vml" xmlns:o="urn:schemas-microsoft-com:office:office" xmlns:w="urn:schemas-microsoft-com:office:word" xmlns:m="http://schemas.microsoft.com/office/2004/12/omml" xmlns="http://www.w3.org/TR/REC-html40"><head><meta http-equiv=Content-Type content="text/html; charset=utf-8"><meta name=Generator content="Microsoft Word 15 (filtered medium)"><style><!--
/* Font Definitions */
@font-face
{font-family:"Cambria Math";
panose-1:2 4 5 3 5 4 6 3 2 4;}
@font-face
{font-family:Calibri;
panose-1:2 15 5 2 2 2 4 3 2 4;}
/* Style Definitions */
p.MsoNormal, li.MsoNormal, div.MsoNormal
{margin:0in;
margin-bottom:.0001pt;
font-size:12.0pt;
font-family:"Times New Roman",serif;}
a:link, span.MsoHyperlink
{mso-style-priority:99;
color:blue;
text-decoration:underline;}
a:visited, span.MsoHyperlinkFollowed
{mso-style-priority:99;
color:purple;
text-decoration:underline;}
span.EmailStyle17
{mso-style-type:personal-reply;
font-family:"Calibri",sans-serif;
color:#1F497D;}
.MsoChpDefault
{mso-style-type:export-only;
font-family:"Calibri",sans-serif;}
@page WordSection1
{size:8.5in 11.0in;
margin:1.0in 1.0in 1.0in 1.0in;}
div.WordSection1
{page:WordSection1;}
--></style><!--[if gte mso 9]><xml>
<o:shapedefaults v:ext="edit" spidmax="1026" />
</xml><![endif]--><!--[if gte mso 9]><xml>
<o:shapelayout v:ext="edit">
<o:idmap v:ext="edit" data="1" />
</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'>Doesn’t look like Martin’s response made it to mobilitydev list probably because he’s not on the list so forwarding.<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><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'> geos-devel [mailto:geos-devel-bounces@lists.osgeo.org] <b>On Behalf Of </b>Martin Davis<br><b>Sent:</b> Friday, September 17, 2021 4:09 PM<br><b>To:</b> GEOS Development List <geos-devel@lists.osgeo.org><br><b>Subject:</b> Re: [geos-devel] [Mobilitydb-dev] Problem with ST_FrechetDistance in PostGIS/GEOS<o:p></o:p></span></p><p class=MsoNormal><o:p> </o:p></p><div><p class=MsoNormal>Well, it looks like there is a possible bug in the GEOS DiscreteFrechetDistance class. This line:<o:p></o:p></p><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal><a href="https://git.osgeo.org/gitea/geos/geos/src/branch/main/src/algorithm/distance/DiscreteFrechetDistance.cpp#L112">https://git.osgeo.org/gitea/geos/geos/src/branch/main/src/algorithm/distance/DiscreteFrechetDistance.cpp#L112</a><o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>should probably be<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal> ca[i][j] = p_ptDist;<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>And indeed that gives the expected answer of 3.<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>Working on a fix now, and will file a GEOS issue.<o:p></o:p></p></div></div><p class=MsoNormal><o:p> </o:p></p><div><div><p class=MsoNormal>On Fri, Sep 17, 2021 at 10:43 AM Martin Davis <<a href="mailto:mtnclimb@gmail.com">mtnclimb@gmail.com</a>> wrote:<o:p></o:p></p></div><blockquote style='border:none;border-left:solid #CCCCCC 1.0pt;padding:0in 0in 0in 6.0pt;margin-left:4.8pt;margin-right:0in'><div><p class=MsoNormal>I agree with the analysis that there might be a problem with the Frechet Distance algorithm in GEOS. Although, there is a recent PR against JTS for Frechet Distance with a different codebase, and it produces the same result as the GEOS code. So this is puzzling.<o:p></o:p></p><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>The GEOS code is here: <a href="https://git.osgeo.org/gitea/geos/geos/src/branch/main/src/algorithm/distance/DiscreteFrechetDistance.cpp" target="_blank">https://git.osgeo.org/gitea/geos/geos/src/branch/main/src/algorithm/distance/DiscreteFrechetDistance.cpp</a><o:p></o:p></p></div><div><p class=MsoNormal>The JTS code is here: <a href="https://github.com/locationtech/jts/blob/ff6476cd8fe4e4ee85304ebc049d05a7cafc3c00/modules/core/src/main/java/org/locationtech/jts/algorithm/distance/DiscreteFrechetDistance.java" target="_blank">https://github.com/locationtech/jts/blob/ff6476cd8fe4e4ee85304ebc049d05a7cafc3c00/modules/core/src/main/java/org/locationtech/jts/algorithm/distance/DiscreteFrechetDistance.java</a><o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>MobilityDB can compare with their algorithm and see if there is a bug. Or else provide their source code to allow us to compare?<o:p></o:p></p></div></div><p class=MsoNormal><o:p> </o:p></p><div><div><p class=MsoNormal>On Thu, Sep 16, 2021 at 2:09 PM Regina Obe <<a href="mailto:lr@pcorp.us" target="_blank">lr@pcorp.us</a>> wrote:<o:p></o:p></p></div><blockquote style='border:none;border-left:solid #CCCCCC 1.0pt;padding:0in 0in 0in 6.0pt;margin-left:4.8pt;margin-right:0in'><div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><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.</span><o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D'> </span><o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><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.</span><o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D'> </span><o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D'>I’ve added geos-develop to mailing list for comment.</span><o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D'> </span><o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D'>Thanks,</span><o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D'>Regina</span><o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D'> </span><o:p></o:p></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 style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><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:<a href="mailto:mobilitydb-dev-bounces@lists.osgeo.org" target="_blank">mobilitydb-dev-bounces@lists.osgeo.org</a>] <b>On Behalf Of </b>Esteban Zimanyi<br><b>Sent:</b> Saturday, September 11, 2021 4:35 AM<br><b>To:</b> <a href="mailto:mobilitydb-dev@lists.osgeo.org" target="_blank">mobilitydb-dev@lists.osgeo.org</a><br><b>Subject:</b> [Mobilitydb-dev] Problem with ST_FrechetDistance in PostGIS/GEOS</span><o:p></o:p></p></div></div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'> <o:p></o:p></p><div><div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>Dear Regina<o:p></o:p></p><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'> <o:p></o:p></p></div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>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 style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'> <o:p></o:p></p></div><div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>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 style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'> <o:p></o:p></p></div></div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>We used the simple algorithm referenced in the PostGIS manual<o:p></o:p></p></div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><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 style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>and according to our understanding the correct result is 3.<o:p></o:p></p></div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'> <o:p></o:p></p></div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>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 style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'> <o:p></o:p></p></div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>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 style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'> <o:p></o:p></p></div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>Many thanks<o:p></o:p></p></div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'> <o:p></o:p></p></div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>Esteban<o:p></o:p></p></div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'> <o:p></o:p></p></div></div></div></div></div></div></div><p class=MsoNormal>_______________________________________________<br>geos-devel mailing list<br><a href="mailto:geos-devel@lists.osgeo.org" target="_blank">geos-devel@lists.osgeo.org</a><br><a href="https://lists.osgeo.org/mailman/listinfo/geos-devel" target="_blank">https://lists.osgeo.org/mailman/listinfo/geos-devel</a><o:p></o:p></p></blockquote></div></blockquote></div></div></body></html>