[postgis-users] postgis 1.4 and curves
mario at geosar.ch
mario at geosar.ch
Wed Jul 29 05:56:30 PDT 2009
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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