Geographic Information Systems Stack Exchange is a question and answer site for cartographers, geographers and GIS professionals. It only takes a minute to sign up.
Sign up to join this community
Adding a new temporary scratch layer in QGIS, selecting the Geometry type, there are different options, between else MultiSurface.
In the documentation, I could not find any information about this Geometry type. What is the difference from MultiPolygon? As I understand it, a MultiPolygon is used to create features with several non contigent parts - like countries consisting of the mainland + islands (say Italy: the "boot"-shaped mainland + Sicily, Sardinia etc.). So what are MultiSurfaces for?
The OGC standard about Simple Features describes LineString and LinearRing
A LineString is a Curve with linear interpolation between Points. Each consecutive pair of Points defines a Line segment.
A LinearRing is a LineString that is both closed and simple
Polygons are made of LinearRings so it means that in a polygon all the segments are straight lines.
Curvepolygon is a geometry that allows also curved segments (arcs). Geometry that is combined from several Curvepolygons is called MultiSurface.
An example about the difference using the PostGIS function https://postgis.net/docs/ST_CurveToLine.html
Source geometry is a curvepolygon that could be an element in a multisurface:
'CURVEPOLYGON(CIRCULARSTRING(1 3, 3 5, 4 7, 7 3, 1 3))';
Apply the PosGIS function ST_CurveToLine:
select ST_AsText(
ST_CurveToLine(ST_GeomFromText(
'CURVEPOLYGON(CIRCULARSTRING(1 3, 3 5, 4 7, 7 3, 1 3))')));
Resulting output:
"POLYGON((1 3,1.313519436051493 3.228634069139761,1.615442682167029 3.472376408113742,1.905042378801364 3.730639820278051,2.18162085491937 4.00280212645158,2.444511808746663 4.288207663800152,2.693081912948945 4.586168865385723,2.926732340373023 4.895967916575354,3.144900206673873 5.21685848431954,3.347059926352328 5.548067515133887,3.532724478936582 5.888797097452681,3.701446582257166 6.238226383867729,3.852819769988868 6.595513568621666,3.986479370863718 6.959797915591758,4 7,4.153336482273182 6.996235800641164,4.306303563529878 6.984952270850615,4.458532732673006 6.966176593639941,4.609657256300401 6.939954001260095,4.7593130621977 6.90634766623295,4.907139616420195 6.865438549163153,5.052780791850688 6.81732520369694,5.195885726140906 6.762123539097772,5.336109666969632 6.69996654101076,5.473114802581243 6.631003951088609,5.606571075603817 6.55540190625085,5.736156978186257 6.473342538445454,5.861560326538854 6.385023535877015,5.982479013011392 6.290657666758554,6.098621733896932 6.190472266734247,6.209708691207961 6.084708691207961,6.315472266734247 5.973621733896932,6.415657666758554 5.857479013011392,6.510023535877016 5.736560326538854,6.598342538445454 5.611156978186257,6.68040190625085 5.481571075603817,6.756003951088609 5.348114802581243,6.82496654101076 5.211109666969632,6.887123539097772 5.070885726140905,6.94232520369694 4.927780791850688,6.990438549163153 4.782139616420195,7.03134766623295 4.6343130621977,7.064954001260095 4.484657256300401,7.091176593639941 4.333532732673005,7.109952270850616 4.181303563529878,7.121235800641164 4.028336482273182,7.125 3.875,7.121235800641164 3.721663517726819,7.109952270850616 3.568696436470123,7.091176593639941 3.416467267326995,7.064954001260095 3.265342743699599,7.03134766623295 3.1156869378023,6.990438549163153 2.967860383579805,6.94232520369694 2.822219208149312,6.887123539097772 2.679114273859095,6.824966541010761 2.538890333030369,6.756003951088609 2.401885197418757,6.68040190625085 2.268428924396182,6.598342538445454 2.138843021813743,6.510023535877016 2.013439673461146,6.415657666758554 1.892520986988608,6.315472266734247 1.776378266103067,6.209708691207961 1.665291308792039,6.098621733896933 1.559527733265753,5.982479013011392 1.459342333241447,5.861560326538854 1.364976464122985,5.736156978186257 1.276657461554546,5.606571075603817 1.19459809374915,5.473114802581243 1.118996048911391,5.336109666969631 1.05003345898924,5.195885726140906 0.987876460902229,5.052780791850688 0.93267479630306,4.907139616420195 0.884561450836847,4.7593130621977 0.84365233376705,4.609657256300402 0.810045998739905,4.458532732673006 0.783823406360059,4.306303563529878 0.765047729149385,4.153336482273181 0.753764199358836,4 0.75,3.84666351772682 0.753764199358836,3.693696436470123 0.765047729149384,3.541467267326995 0.783823406360059,3.390342743699599 0.810045998739905,3.2406869378023 0.84365233376705,3.092860383579806 0.884561450836847,2.947219208149312 0.93267479630306,2.804114273859095 0.987876460902229,2.663890333030369 1.050033458989239,2.526885197418757 1.118996048911391,2.393428924396182 1.19459809374915,2.263843021813744 1.276657461554545,2.138439673461146 1.364976464122984,2.017520986988608 1.459342333241446,1.901378266103067 1.559527733265753,1.790291308792039 1.665291308792039,1.684527733265754 1.776378266103067,1.584342333241448 1.892520986988607,1.489976464122984 2.013439673461146,1.401657461554546 2.138843021813743,1.31959809374915 2.268428924396182,1.243996048911391 2.401885197418757,1.17503345898924 2.538890333030367,1.11287646090223 2.679114273859093,1.05767479630306 2.822219208149313,1.009561450836848 2.967860383579805,1 3))"
MultiSurface is related in the same way to Curvepolygons as Multipolgons to Polygons or MultiLines to Lines? So then the terminology is somewhat confusing - so MultiSurface is in fact a MultiCurvepolygon?
MultiPolygoncan be found here : gis.stackexchange.com/questions/225368/…. Information aboutMultiSurfacecan be found for instance here : datypic.com/sc/niem21/e-geo_MultiSurface.html. Briefly saying: Italy is yourMultiPolygon, but as soon as you adda Surfacee.g. a TIN to it using theST_MultiSurface(), it becomes aMultiSurface. So, basicallyMultiSurfaceextendsMultiPolygon,