[postgis-users] postgis 1.4 and curves

Wed Jul 29 06:37:30 PDT 2009

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Bo Coughlin
bo at rekindl.com
704.414.0805

On Wed, Jul 29, 2009 at 8:56 AM, <mario at geosar.ch> wrote:

> Thanks for you answer Mark,
> but st_linetocurve in my test cases always add a segment at the end or at
> the start of the original curve.
> St_linetocurve behaviour in this way is not good for me because I need the
> same curves with the same original points ( millimeter differences can be
> accepted ).
>
> Postgis functions curve geometry support is planed for development?
>
> Anyway thanks
>
> mario
>
> -----Original Message-----
> From: postgis-users-bounces at postgis.refractions.net
> [mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of Mark
> Cave-Ayland
> Sent: mercoledì, 29. luglio 2009 14:33
> To: PostGIS Users Discussion
> Subject: Re: [postgis-users] postgis 1.4 and curves
>
> mario at geosar.ch wrote:
>
> > Hi,
> > the geometry is :
> > GeomFromText('COMPOUNDCURVE((723702.522 121248.15,723892.918
> > 121297.093),(723892.918 121297.093,724230.191
> > 121220.96),CIRCULARSTRING(724230.191 121220.96,724268.27
> > 121144.826,724251.951 121025.188))')
> >
> > Then try:
> > select st_astext( st_linetocurve(
> > st_curvetoline(GeomFromText('COMPOUNDCURVE((723702.522
> 121248.15,723892.918
> > 121297.093),(723892.918 121297.093,724230.191
> > 121220.96),CIRCULARSTRING(724230.191 121220.96,724268.27
> > 121144.826,724251.951 121025.188))')) ) )
> >
> > I obtain:
> > COMPOUNDCURVE((723702.522 121248.15,723892.918 121297.093,724230.191
> > 121220.96),CIRCULARSTRING(724230.191 121220.96,724271.107624879
> > 121128.220456049,724253.605225062 121028.378284577),(724253.605225062
> > 121028.378284577,724251.951 121025.188))
> >
> > After the arc another line is added and the points of the arc are
> different.
> >
> > Thanks
> > Mario
>
> Hi Mario,
>
> I don't think that this is actually a PostGIS bug. The reason for this
> is that the transformation from curve to line is lossy in that any
> CIRCULARSTRINGs are converted from smooth curves into a series of short
> linestring segments, and so when converting back from line to curve, the
> curve fitting algorithm has no knowledge of the makeup of the original
> geometry. So as it traverses the input line segments, it will generate
> CIRCULARSTRINGs for lines that appear to fit around a given radius,
> regardless of whether they were originally mapped as LINESTRINGs or
> CIRCULARSTRINGs.
>
> Note that you can test yourself to see that your original input geometry
> and the output geometry are equivalent by comparing the results of the
> conversion from curves back to lines:
>
>
> postgis14=# select
> st_astext(st_curvetoline(geomfromtext('COMPOUNDCURVE((723702.522
> 121248.15,723892.918 121297.093,724230.191
> 121220.96),CIRCULARSTRING(724230.191 121220.96,724271.107624879
> 121128.220456049,724253.605225062 121028.378284577),(724253.605225062
> 121028.378284577,724251.951 121025.188))')));
>
>
>
>
>
>
>                        st_astext
>
>
>
>
>
>
> ----------------------------------------------------------------------------
>
> ----------------------------------------------------------------------------
>
> ----------------------------------------------------------------------------
>
> ----------------------------------------------------------------------------
>
> ----------------------------------------------------------------------------
>
> ----------------------------------------------------------------------------
>
> ----------------------------------------------------------------------------
>
> ----------------------------------------------------------------------------
>
> ----------------------------------------------------------------------------
>
> ----------------------------------------------------------------------------
>
> ----------------------------------------------------------------------------
> -----------------------------------------------------------------------
>  LINESTRING(723702.522 121248.15,723892.918 121297.093,724230.191
> 121220.96,724235.615995416 121214.326233812,724240.708952709
> 121207.434266378,724245.457602499 121200.300701051,724249.850504871
> 121192.942723214,724253.87707694 121185.378058881,724257.527618341
> 121177.62493199,724260.793334599 121169.702020504,724263.666358319
> 121161.62841141,724266.139768134 121153.423554739,724268.207605384
> 121145.10721671,724269.864888467 121136.69943211,724271.107624843
> 121128.220456028,724271.932820652 121119.69071506,724272.338487925
> 121111.1307581,724272.323649373 121102.561206832,724271.888340744
> 121094.002706058,724271.033610735 121085.475873955,724269.761518465
> 121077.001252409,724268.075128515 121068.599257524,724265.978503548
> 121060.290130444,724263.476694515 121052.093888581,724260.575728495
> 121044.030277402,724257.282594168 121036.118722851,724253.605225062
> 121028.378284577,724251.951 121025.188)
> (1 row)
>
>
> postgis14=# select
> st_astext(st_curvetoline(GeomFromText('COMPOUNDCURVE((723702.522
> 121248.15,723892.918
> 121297.093),(723892.918 121297.093,724230.191
> 121220.96),CIRCULARSTRING(724230.191 121220.96,724268.27
> 121144.826,724251.951 121025.188))')));
>
>
>
>
>
>
>                        st_astext
>
>
>
>
>
>
> ----------------------------------------------------------------------------
>
> ----------------------------------------------------------------------------
>
> ----------------------------------------------------------------------------
>
> ----------------------------------------------------------------------------
>
> ----------------------------------------------------------------------------
>
> ----------------------------------------------------------------------------
>
> ----------------------------------------------------------------------------
>
> ----------------------------------------------------------------------------
>
> ----------------------------------------------------------------------------
>
> ----------------------------------------------------------------------------
>
> ----------------------------------------------------------------------------
> -----------------------------------------------------------------------
>  LINESTRING(723702.522 121248.15,723892.918 121297.093,724230.191
> 121220.96,724235.61599542 121214.326233776,724240.708952716
> 121207.434266306,724245.457602506 121200.300700942,724249.850504877
> 121192.942723069,724253.877076943 121185.378058699,724257.527618339
> 121177.624931772,724260.793334591 121169.70202025,724263.666358302
> 121161.62841112,724266.139768107 121153.423554414,724268.207605344
> 121145.107216351,724269.864888414 121136.699431717,724271.107624775
> 121128.220455602,724271.932820567 121119.690714601,724272.338487822
> 121111.130757609,724272.32364925 121102.561206312,724271.8883406
> 121094.002705508,724271.033610568 121085.475873377,724269.761518274
> 121077.001251803,724268.075128299 121068.599256892,724265.978503305
> 121060.290129786,724263.476694244 121052.0938879,724260.575728195
> 121044.030276699,724257.282593838 121036.118722128,724253.605224622
> 121028.378283815,724251.951 121025.188)
> (1 row)
>
>
> (Note that there are some small rounding errors since the CIRCULARSTRING
> conversions occur in different places, but again remember that these
> conversions are lossy)
>
>
> HTH,
>
> Mark.
>
> --
> Mark Cave-Ayland - Senior Technical Architect
> PostgreSQL - PostGIS
> Sirius Corporation plc - control through freedom
> http://www.siriusit.co.uk
> t: +44 870 608 0063
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