[postgis-tickets] [PostGIS] #3728: Bug in geography ST_Segmentize

[PostGIS]

#3728: Bug in geography ST_Segmentize
----------------------+---------------------------
  Reporter:  petermj  |      Owner:  pramsey
      Type:  defect   |     Status:  new
  Priority:  medium   |  Milestone:  PostGIS 2.3.3
 Component:  postgis  |    Version:  2.3.x
Resolution:           |   Keywords:
----------------------+---------------------------
Changes (by robe):

 * cc: hmercier (added)


Comment:

 Okay I see your point.  I reduced the query to one offending example:

 IN POSTGIS="2.3.2 r15302" GEOS="3.6.1-CAPI-1.10.1 r4317" PROJ="Rel. 4.9.1,
 04 March 2015" GDAL="GDAL 2.1.3, released 2017/20/01" LIBXML="2.7.8"
 LIBJSON="0.12" RASTER

 {{{
 With f AS  (SELECT ST_Segmentize(ST_GeogFromText('SRID=4326;
 LINESTRING(-4.0 49.8, -4.0 59.0)'),1000) AS geog_s,
 ST_GeogFromText('SRID=4326; LINESTRING(-4.0 49.8, -4.0 59.0)') AS geog),
     f2 AS (SELECT ST_NPoints(geog_s::geometry) AS geog_s_npoints, geog,
 geog_s FROM F)
 SELECT  geog_s_npoints, ST_Length(geog_s) AS seg_length, ST_Length(geog)
 AS orig_length, ST_Length(geog)/1000 As exp_np,
         ST_Length(ST_MakeLine(ST_PointN(geog_s::geometry, 1),
 ST_PointN(geog_s::geometry, 2))::geography) As first_seg_length,
         ST_Length(ST_MakeLine(ST_PointN(geog_s::geometry, geog_s_npoints -
 1), ST_PointN(geog_s::geometry, geog_s_npoints))::geography) As
 last_seg_length
 FROM f2;
 }}}

 Yields:


 {{{
  geog_s_npoints |    seg_length    |   orig_length   |     exp_np      |
 first_seg_length | last_seg_length
 ----------------+------------------+-----------------+-----------------+------------------+------------------
             894 | 1024067.47066481 | 1024067.4706648 | 1024.0674706648 |
 1000.26657219012 | 131222.895347768
 (1 row)
 }}}

 Answer in
 POSTGIS="2.2.2 r14797" GEOS="3.6.1-CAPI-1.10.1 r4317" PROJ="Rel. 4.9.1, 04
 March 2015" GDAL="GDAL 2.1.3, released 2017/20/01" LIBXML="2.7.8"
 LIBJSON="0.12" RASTER


 {{{
  geog_s_npoints |   seg_length    |   orig_length   |     exp_np      |
 first_seg_length | last_seg_length
 ----------------+-----------------+-----------------+-----------------+------------------+------------------
            1024 | 1024067.4706648 | 1024067.4706648 | 1024.0674706648 |
 995.986340600015 | 997.506446092082
 (1 row)
 }}}

 Much more pleasant indeed.

 SO yes this looks like another regression resulting from the 2.3
 segmentize change.

--
Ticket URL: <https://trac.osgeo.org/postgis/ticket/3728#comment:2>
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