Geometric Operations

In this section, we consider some of the common kinds of geometries that arises in applications. These include Point, LineString, Polygon, GeometryCollection, MultiPolygon, MultiPoint, and MultiLineString. For brevity in the examples, we will use the prefix const AG = ArchGDAL.

Geometry Creation

To create geometries of different types.

point = AG.createpoint(1.0, 2.0)
linestring = AG.createlinestring([(i,i+1) for i in 1.0:3.0])
linearring = AG.createlinearring([(0.,0.), (0.,1.), (1.,1.)])
simplepolygon = AG.createpolygon([(0.,0.), (0.,1.), (1.,1.)])
complexpolygon = AG.createpolygon([[(0.,0.), (0.,j), (j,j)] for j in 1.0:-0.1:0.9])
multipoint = AG.createlinearring([(0.,0.), (0.,1.), (1.,1.)])
multilinestring = AG.createmultilinestring([[(i,i+1) for i in j:j+3] for j in 1.0:5.0:6.0])
multipolygon = AG.createmultipolygon([[[(0.,0.), (0.,j), (j,j)]] for j in 1.0:-0.1:0.9])

Alternatively, they can be assembled from their components.

point = AG.createpoint()
    AG.addpoint!(point, 1.0, 2.0)
linestring = AG.createlinestring()
    for i in 1.0:3.0
        AG.addpoint!(linestring, i, i+1)
    end
linearring = AG.createlinearring()
    for i in 1.0:3.0
        AG.addpoint!(linearring, i, i+1)
    end
polygon = AG.createpolygon()
    for j in 1.0:-0.1:0.9
        ring = AG.createlinearring([(0.,0.), (0.,j), (j,j)])
        AG.addgeom!(polygon, ring)
    end
multipoint = AG.createmultipoint()
    for i in 1.0:3.0
        pt = AG.createpoint(i, i+1)
        AG.addgeom!(multipoint, pt)
    end
multilinestring = AG.createmultilinestring()
    for j in 1.0:5.0:6.0
        line = AG.createlinestring([(i,i+1) for i in j:j+3])
        AG.addgeom!(multilinestring, line)
    end
multipolygon = AG.createmultipolygon()
    for j in 1.0:-0.1:0.9
        poly = AG.createpolygon([(0.,0.), (0.,j), (j,j)])
        AG.addgeom!(multipolygon, poly)
    end

They can also be constructed from other data formats such as:

• Well-Known Binary (WKB): AG.fromWKB([0x01,0x01,...,0x27,0x41])

• Well-Known Text (WKT): AG.fromWKT("POINT (1 2)")

• JavaScript Object Notation (JSON): AG.fromJSON("""{"type":"Point","coordinates":[1,2]}""")

Geometry Modification

The following methods are commonly used for retrieving elements of a geometry.

• AG.getcoorddim(geom): dimension of the coordinates. Returns 0 for an empty point

• AG.getspatialref(geom)

• AG.getx(geom, i)

• AG.gety(geom, i)

• AG.getz(geom, i)

• AG.getpoint(geom, i)

• AG.getgeom(geom, i)

The following methods are commonly used for modifying or adding to a geometry.

• AG.setcoorddim!(geom, dim)

• AG.setspatialref!(geom, spatialref)

• AG.setpointcount!(geom, n)

• AG.setpoint!(geom, i, x, y)

• AG.setpoint!(geom, i, x, y, z)

• AG.addpoint!(geom, x, y)

• AG.addpoint!(geom, x, y, z)

• AG.addgeom!(geom1, geom2)

• AG.addgeomdirectly!(geom1, geom2)

• AG.removegeom!(geom, i)

• AG.removeallgeoms!(geom)

Unary Operations

The following is an non-exhaustive list of unary operations available for geometries.

Attributes

• AG.getdim(geom): 0 for points, 1 for lines and 2 for surfaces

• AG.getcoorddim(geom): dimension of the coordinates. Returns 0 for an empty point

• AG.getenvelope(geom): the bounding envelope for this geometry

• AG.getenvelope3d(geom): the bounding envelope for this geometry

• AG.wkbsize(geom): size (in bytes) of related binary representation

• AG.getgeomtype(geom): geometry type code (in OGRwkbGeometryType)

• AG.getgeomname(geom): WKT name for geometry type

• AG.getspatialref(geom): spatial reference system. May be NULL

• AG.geomlength(geom): the length of the geometry, or 0.0 for unsupported types

• AG.geomarea(geom): the area of the geometry, or 0.0 for unsupported types

Predicates

The following predicates return a Bool.

• AG.isempty(geom)

• AG.isvalid(geom)

• AG.issimple(geom)

• AG.isring(geom)

• AG.hascurvegeom(geom, nonlinear::Bool)

Immutable Operations

The following methods do not modify geom.

• AG.clone(geom): a copy of the geometry with the original spatial reference system.

• AG.forceto(geom, targettype): force the provided geometry to the specified geometry type.

• AG.simplify(geom, tol): Compute a simplified geometry.

• AG.simplifypreservetopology(geom, tol): Simplify the geometry while preserving topology.

• AG.delaunaytriangulation(geom, tol, onlyedges): a delaunay triangulation of the vertices of the geometry.

• AG.boundary(geom): the boundary of the geometry

• AG.convexhull(geom): the convex hull of the geometry.

• AG.buffer(geom, dist, quadsegs): a polygon containing the region within the buffer distance of the original geometry.

• AG.union(geom): the union of the geometry using cascading

• AG.pointonsurface(geom): Returns a point guaranteed to lie on the surface.

• AG.centroid(geom): Compute the geometry centroid. It is not necessarily within the geometry.

• AG.pointalongline(geom, distance): Fetch point at given distance along curve.

• AG.polygonize(geom): Polygonizes a set of sparse edges.

Mutable Operations

The following methods modifies the first argument geom.

• AG.setcoorddim!(geom, dim): sets the explicit coordinate dimension.

• AG.flattento2d!(geom): Convert geometry to strictly 2D.

• AG.closerings!(geom): Force rings to be closed by adding the start point to the end.

• AG.setspatialref!(geom, spatialref): Assign spatial reference to this object.

• AG.transform!(geom, coordtransform): Apply coordinate transformation to geometry.

• AG.transform!(geom, spatialref): Transform geometry to new spatial reference system.

• AG.segmentize!(geom, maxlength): Modify the geometry such it has no segment longer than the given distance.

• AG.empty!(geom): Clear geometry information.

Export Formats

• AG.toWKB(geom)

• AG.toISOWKB(geom)

• AG.toWKT(geom)

• AG.toISOWKT(geom)

• AG.toGML(geom)

• AG.toKML(geom)

• AG.toJSON(geom)

Binary Operations

The following is an non-exhaustive list of binary operations available for geometries.

Predicates

The following predicates return a Bool.

• AG.intersects(g1, g2)

• AG.equals(g1, g2)

• AG.disjoint(g1, g2)

• AG.touches(g1, g2)

• AG.crosses(g1, g2)

• AG.within(g1, g2)

• AG.contains(g1, g2)

• AG.overlaps(g1, g2)

Immutable Operations

The following methods do not mutate the input geomteries g1 and g2.

• AG.intersection(g1, g2)

• AG.union(g1, g2)

• AG.difference(g1, g2)

• AG.symdifference(g1, g2)

Mutable Operations

The following methods modifies the first argument g1.

• AG.addgeom!(g1, g2)

• AG.addgeomdirectly!(g1, g2)