observatory-extension/doc/boundary_functions.md

Boundary Functions

Use the following functions to retrieve Boundary data. Data ranges from small areas (e.g. US Census Block Groups) to large areas (e.g. Countries). You can access boundaries by point location lookup, bounding box lookup, direct ID access and several other methods described below.

You can access boundaries through the CartoDB Editor. The same methods will work if you are using the CartoDB Platform to develop your application. We encourage you to use table modifying methods (UPDATE and INSERT) over dynamic methods (SELECT).

OBS_GetBoundariesByGeometry(polygon geometry, geometry_id text)

The OBS_GetBoundariesByGeometry(geometry, geometry_id) method returns a set of boundary geometries that intersect a supplied geometry. This can be used to find all boundaries that are within or overlap a bounding box. You have the ability to choose whether to retrieve all boundaries that intersect your supplied bounding box or only those that fall entirely inside of your bounding box.

Arguments

Name	Description
polygon	a bounding box or other WGS84 geometry
geometry_id	a string identifier for a boundary geometry
timespan (optional)	year(s) to request from ('NULL' (default) gives most recent)
overlap_type (optional)	one of 'intersects' (default), 'contains', or 'within'.

Returns

A table with the following columns:

Column Name	Description
the_geom	a boundary geometry (e.g., US Census tract boundaries)
geom_refs	a string identifier for the geometry (e.g., geoids of US Census tracts)

If geometries are not found for the requested polygon, geometry_id, timespan, or overlap_type, then null values are returned.

Example

Insert all Census Tracts from Lower Manhattan and nearby areas within the supplied bounding box to a table named manhattan_census_tracts which has columns the_geom (geometry) and geom_refs (text).

INSERT INTO manhattan_census_tracts(the_geom, geom_refs)
SELECT *
FROM OBS_GetBoundariesByGeometry(
       ST_MakeEnvelope(-74.0251922607,40.6945658517,
                      -73.9651107788,40.7377626342,
                      4326),
       'us.census.tiger.census_tract')

Errors

• If a geometry other than a point is passed as the first argument, an error is thrown: Invalid geometry type (ST_Polygon), expecting 'ST_Point'
• If an overlap_type other than the valid ones listed above is entered, then an error is thrown

OBS_GetPointsByGeometry(polygon geometry, geometry_id text)

The OBS_GetPointsByGeometry(polygon, geometry_id) method returns point geometries and their geographical identifiers that intersect (or are contained by) a bounding box polygon and lie on the surface of a boundary corresponding to the boundary with same geographical identifiers (e.g., a point that is on a census tract with the same geoid). This is a useful alternative to OBS_GetBoundariesByGeometry listed above because it returns much less data for each location.

Arguments

Name	Description
polygon	a bounding box or other geometry
geometry_id	a string identifier for a boundary geometry
timespan (optional)	year(s) to request from (NULL (default) gives most recent)
overlap_type (optional)	one of 'intersects' (default), 'contains', or 'within'.

Returns

A table with the following columns:

Column Name	Description
the_geom	a point geometry on a boundary (e.g., a point that lies on a US Census tract)
geom_refs	a string identifier for the geometry (e.g., the geoid of a US Census tract)

If geometries are not found for the requested geometry, geometry_id, timespan, or overlap_type, then NULL values are returned.

Example

Insert points that lie on Census Tracts from Lower Manhattan and nearby areas within the supplied bounding box to a table named manhattan_tract_points which has columns the_geom (geometry) and geom_refs (text).

INSERT INTO manhattan_tract_points (the_geom, geom_refs)
SELECT *
FROM OBS_GetPointsByGeometry(
       ST_MakeEnvelope(-74.0251922607,40.6945658517,
                       -73.9651107788,40.7377626342,
                       4326),
       'us.census.tiger.census_tract')

Errors

• If a geometry other than a point is passed as the first argument, an error is thrown: Invalid geometry type (ST_Point), expecting 'ST_MultiPolygon' or 'ST_Polygon'

OBS_GetBoundary(point_geometry, boundary_id)

The OBS_GetBoundary(point_geometry, boundary_id) method returns a boundary geometry defined as overlapping the point geometry and from the desired boundary set (e.g. Census Tracts). See the Boundary ID glossary table below. This is a useful method for performing aggregations of points.

Arguments

Name	Description
point_geometry	a WGS84 polygon geometry (the_geom)
boundary_id	a boundary identifier from the Boundary ID glossary table below
timespan (optional)	year(s) to request from (NULL (default) gives most recent)

Returns

A boundary geometry. If no value is found at the requested boundary_id or timespan, a null value is returned.

Value	Description
geom	WKB geometry

Example

Overwrite a point geometry with a boundary geometry that contains it in your table

UPDATE tablename
SET the_geom = OBS_GetBoundary(the_geom, 'us.census.tiger.block_group')

Errors

• If a geometry other than a point is passed, an error is thrown: Invalid geometry type (ST_Line), expecting 'ST_Point'

OBS_GetBoundaryId(point_geometry, boundary_id)

The OBS_GetBoundaryId(point_geometry, boundary_id) returns a unique geometry_id for the boundary geometry that contains a given point geometry. See the Boundary ID glossary table below. The method can be combined with OBS_GetBoundaryById(geometry_id) to create a point aggregation workflow.

Arguments

Name	Description
point_geometry	a WGS84 point geometry (the_geom)
boundary_id	a boundary identifier from the Boundary ID glossary table below
timespan (optional)	year(s) to request from (NULL (default) gives most recent)

Returns

A TEXT boundary geometry id. If no value is found at the requested boundary_id or timespan, a null value is returned.

Value	Description
geometry_id	a string identifier of a geometry in the Boundaries

Example

Write the US Census block group geoid that contains the point geometry for every row as a new column in your table.

UPDATE tablename
SET geometry_id = OBS_GetBoundaryId(the_geom, 'us.census.tiger.block_group')

Errors

• If a geometry other than a point is passed, an error is thrown: Invalid geometry type (ST_Line), expecting 'ST_Point'

OBS_GetBoundaryById(geometry_id, boundary_id)

The OBS_GetBoundaryById(geometry_id, boundary_id) returns the boundary geometry for a unique geometry_id. A geometry_id can be found using the OBS_GetBoundaryId(point_geometry, boundary_id) method described above.

Arguments

Name	Description
geometry_id	a string identifier for a Boundary geometry
boundary_id	a boundary identifier from the Boundary ID glossary table below
timespan (optional)	year(s) to request from (NULL (default) gives most recent)

Returns

A boundary geometry. If a geometry is not found for the requested geometry_id, boundary_id, or timespan, then a null value is returned.

Key	Description
geom	a WGS84 polygon geometry

Example

Use a table of geometry_ids (e.g., geoid from the U.S. Census) to select the unique boundaries that they correspond to and insert into a table called, overlapping_polygons. This is a useful method for creating new choropleths of aggregate data.

INSERT INTO overlapping_polygons (the_geom, geometry_id, point_count)
SELECT
  OBS_GetBoundaryById(geometry_id, 'us.census.tiger.county') As the_geom,
  geometry_id,
  count(*)
FROM tablename
GROUP BY geometry_id

OBS_GetBoundariesByPointAndRadius(point geometry, radius numeric, boundary_id text)

The OBS_GetBoundariesByPointAndRadius(point, radius, boundary_id) method returns boundary geometries and their geographical identifiers that intersect (or are contained by) a circle centered on a point with a radius.

Arguments

Name	Description
point	a WGS84 point geometry
radius	a radius (in meters) from the center point
geometry_id	a string identifier for a boundary geometry
timespan (optional)	year(s) to request from (NULL (default) gives most recent)
overlap_type (optional)	one of 'intersects' (default), 'contains', or 'within'.

Returns

A table with the following columns:

Column Name	Description
the_geom	a boundary geometry (e.g., a US Census tract)
geom_refs	a string identifier for the geometry (e.g., the geoid of a US Census tract)

If geometries are not found for the requested point and radius, geometry_id, timespan, or overlap_type, then null values are returned.

Example

Insert into table denver_census_tracts the census tract boundaries and geom_refs of census tracts which intersect within 10 miles of downtown Denver, Colorado.

INSERT INTO denver_census_tracts(the_geom, geom_refs)
SELECT *
FROM OBS_GetBoundariesByPointAndRadius(
  CDB_LatLng(39.7392, -104.9903), -- Denver, Colorado
  10000 * 1.609, -- 10 miles (10km * conversion to miles)
  'us.census.tiger.census_tract')

Errors

• If a geometry other than a point is passed, an error is thrown. E.g., Invalid geometry type (ST_Line), expecting 'ST_Point'

OBS_GetPointsByPointAndRadius(point geometry, radius numeric, boundary_id text)

The OBS_GetPointsByPointAndRadius(point, radius, boundary_id) method returns point geometries on boundaries (e.g., a point that lies on a Census tract) and their geographical identifiers that intersect (or are contained by) a circle centered on a point with a radius.

Arguments

Name	Description
point	a WGS84 point geometry
radius	radius (in meters)
geometry_id	a string identifier for a boundary geometry
timespan (optional)	year(s) to request from (NULL (default) gives most recent)
overlap_type (optional)	one of 'intersects' (default), 'contains', or 'within'.

Returns

A table with the following columns:

Column Name	Description
the_geom	a point geometry (e.g., a point on a US Census tract)
geom_refs	a string identifier for the geometry (e.g., the geoid of a US Census tract)

If geometries are not found for the requested point and radius, geometry_id, timespan, or overlap_type, then null values are returned.

Example

Insert into table denver_tract_points points on US census tracts and their corresponding geoids for census tracts which intersect within 10 miles of downtown Denver, Colorado, USA.

INSERT INTO denver_tract_points(the_geom, geom_refs)
SELECT *
FROM OBS_GetPointsByPointAndRadius(
  CDB_LatLng(39.7392, -104.9903), -- Denver, Colorado
  10000 * 1.609, -- 10 miles (10km * conversion to miles)
  'us.census.tiger.census_tract')

Errors

• If a geometry other than a point is passed, an error is thrown. E.g., Invalid geometry type (ST_Line), expecting 'ST_Point'