$polygon geometry definition$

If you don’t order the points correctly you’ll just have overlapping polygons. Effectively this makes an ESRI Shapefile Polygon a Simple Feature MultiPolygon Topologically the island in a lake would be just another clockwise outer ring. Since multiple outer boundaries are allowed island-in-a-lake-on-an-island configurations are possible with this polygon definition. The order of rings in the points array is not significant. Vertices for a single, ringed polygon are, therefore, always in clockwise order. Vertices of rings defining holes in polygons are in a counterclockwise direction. The neighborhood to the right of an observer walking along the ring in vertex order is the neighborhood inside the polygon.

The order of vertices or orientation for a ring indicates which side of the ring is the interior of the polygon. A polygon may contain multiple outer rings. A ring is a connected sequence of four or more points that form a closed, non-self-intersecting loop. have to be represented as MultiPolygons ( see figure 2.10 (4)), since there can be only one exterior border and deeper nestings than interior rings are not defined.Ī polygon consists of one or more rings. Oracle Spatialĭeeper nestings, aka island-in-a-lake-on-an-island. In most implementations the order of rings in a POLYGON is important (as opposed to shapefiles)įor a polygon with holes, its first subelement is its exterior ring, its second subelement is its first interior ring, its third subelement is its second interior ring, and so on. The interior LinearRings will have the opposite orientation, and appear as clockwise when viewed from the “top”. The exterior boundary LinearRing defines the “top” of the surface which is the side of the surface from which the exterior boundary appears to traverse the boundary in a counter clockwise direction. Each interior boundary defines a hole in the Polygon. interior rings (holes): clockwise direction.Ī Polygon is a planar Surface defined by 1 exterior boundary and 0 or more interior boundaries.Here is an overview on outer ring directions of polygons in various formats by their specifications:Īlso used in WKT/GML/KML and various SQL implementations: Determining which of those two areas is the bigger one is possible, but still leaves the ambiguity. Ring (boundary) directions are needed to prevent ambiguities for geographic coordinate systems that cover a finite surface, since the boundary would define two areas, one left and one right of the boundary along it's direction. One solution is computing the area of each polygon and, if I remember correctly, if it is positive the order is clockwise and if negative the order is counterclockwise.Īrea computation is not an intensive task, so it will not slow my plugin so much.īut in the special case of QGIS, anybody knows if it stores the polygons clockwise or counterclockwise, regardless the order in the original source?īy now I'm working with ESRI shapefiles and the coordinates in layer.getFeatures().geometry().asPolygon() are stored in clockwise for the outer border and counterclockwise for the holes, i. As I need to check the angles between consecutive sides of polygons using their azimuths I need to known if the order is clockwise. My purpose is to write a plugin for QGIS, but the source of data can be ESRI shapefiles or other formats. Is the clockwise order only for ESRI shapefiles or for general GIS formats?Īnd what about the internal representation for a GIS software?įor example, if I use QGIS and I read a *.shp containing polygons I suppose the internal representation of the outer bound is clockwise as in the original shapefile, but what about for all the file formats supported by QGIS?Īnd in the case there exists a file format with polygons stored in counterclockwise, if these files are loaded in QGIS, ArcGIS, etc., is the orientation changed internally, so if I read the data using PyQGIS, for example, the polygons are clockwise ordered? That question was answered ( Are polygons stored clockwise or counterclockwise in a shapefile?) and it was also answered in an old post ( Polygon creation (Clockwise rotation or not))īut now my question is more general, and I don't know if it has an unique answer. Two days ago I asked a question about the internal storing order for the vertices of a polygon in ESRI shapefiles.

Polygon geometry definition