CreateTopoGeom
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## Name

CreateTopoGeom — Creates a new topo geometry object from topo element array - tg_type: 1:[multi]point, 2:[multi]line, 3:[multi]poly, 4:collection

## Synopsis

`topogeometry CreateTopoGeom(`varchar toponame, integer tg_type, integer layer_id, topoelementarray tg_objs`)`;

`topogeometry CreateTopoGeom(`varchar toponame, integer tg_type, integer layer_id`)`;

## Description

Creates a topogeometry object for layer denoted by `layer_id` and registers it in the relations table in the `toponame` schema.

`tg_type` is an integer: 1:[multi]point (punctal), 2:[multi]line (lineal), 3:[multi]poly (areal), 4:collection. `layer_id` is the layer id in the topology.layer table.

punctal layers are formed from set of nodes, lineal layers are formed from a set of edges, areal layers are formed from a set of faces, and collections can be formed from a mixture of nodes, edges, and faces.

Omitting the array of components generates an empty TopoGeometry object.

Availability: 1.1

## Examples: Form from existing edges

Create a topogeom in ri_topo schema for layer 2 (our ri_roads), of type (2) LINE, for the first edge (we loaded in `ST_CreateTopoGeo`).

    INSERT INTO ri.ri_roads(road_name, topo) VALUES('Unknown', topology.CreateTopoGeom('ri_topo',2,2,'{{1,2}}'::topology.topoelementarray);

## Examples: Convert an areal geometry to best guess topogeometry

Lets say we have geometries that should be formed from a collection of faces. We have for example blockgroups table and want to know the topo geometry of each block group. If our data was perfectly aligned, we could do this:

    -- create our topo geometry column --
    SELECT topology.AddTopoGeometryColumn(
    	'topo_boston',
    	'boston', 'blockgroups', 'topo', 'POLYGON');

    -- addtopgeometrycolumn --
    1

    -- update our column assuming
    -- everything is perfectly aligned with our edges
    UPDATE boston.blockgroups AS bg
    	SET topo = topology.CreateTopoGeom('topo_boston'
            ,3,1
            , foo.bfaces)
    FROM (SELECT b.gid,  topology.TopoElementArray_Agg(ARRAY[f.face_id,3]) As bfaces
    	FROM boston.blockgroups As b
                INNER JOIN topo_boston.face As f ON b.geom && f.mbr
            WHERE ST_Covers(b.geom, topology.ST_GetFaceGeometry('topo_boston', f.face_id))
                GROUP BY b.gid) As foo
    WHERE foo.gid = bg.gid;

    --the world is rarely perfect allow for some error
    --count the face if 50% of it falls
    -- within what we think is our blockgroup boundary
    UPDATE boston.blockgroups AS bg
    	SET topo = topology.CreateTopoGeom('topo_boston'
            ,3,1
            , foo.bfaces)
    FROM (SELECT b.gid,  topology.TopoElementArray_Agg(ARRAY[f.face_id,3]) As bfaces
    	FROM boston.blockgroups As b
                INNER JOIN topo_boston.face As f ON b.geom && f.mbr
            WHERE ST_Covers(b.geom, topology.ST_GetFaceGeometry('topo_boston', f.face_id))
    	OR
     (  ST_Intersects(b.geom, topology.ST_GetFaceGeometry('topo_boston', f.face_id))
                AND ST_Area(ST_Intersection(b.geom, topology.ST_GetFaceGeometry('topo_boston', f.face_id) ) ) >
                    ST_Area(topology.ST_GetFaceGeometry('topo_boston', f.face_id))*0.5
                    )
                GROUP BY b.gid) As foo
    WHERE foo.gid = bg.gid;

    -- and if we wanted to convert our topogeometry back
    -- to a denormalized geometry aligned with our faces and edges
    -- cast the topo to a geometry
    -- The really cool thing is my new geometries
    -- are now aligned with my tiger street centerlines
    UPDATE boston.blockgroups SET new_geom = topo::geometry;

## See Also

AddTopoGeometryColumn, toTopoGeom ST_CreateTopoGeo, ST_GetFaceGeometry, TopoElementArray, TopoElementArray_Agg

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