Introduction to PostGIS

10. Geometry Exercises

Here’s a reminder of all the functions we have seen so far. They should be useful for the exercises!

  • sum(expression) aggregate to return a sum for a set of records
  • count(expression) aggregate to return the size of a set of records
  • ST_GeometryType(geometry) returns the type of the geometry
  • ST_NDims(geometry) returns the number of dimensions of the geometry
  • ST_SRID(geometry) returns the spatial reference identifier number of the geometry
  • ST_X(point) returns the X ordinate
  • ST_Y(point) returns the Y ordinate
  • ST_Length(linestring) returns the length of the linestring
  • ST_StartPoint(geometry) returns the first coordinate as a point
  • ST_EndPoint(geometry) returns the last coordinate as a point
  • ST_NPoints(geometry) returns the number of coordinates in the linestring
  • ST_Area(geometry) returns the area of the polygons
  • ST_NRings(geometry) returns the number of rings (usually 1, more if there are holes)
  • ST_ExteriorRing(polygon) returns the outer ring as a linestring
  • ST_InteriorRingN(polygon, integer) returns a specified interior ring as a linestring
  • ST_Perimeter(geometry) returns the length of all the rings
  • ST_NumGeometries(multi/geomcollection) returns the number of parts in the collection
  • ST_GeometryN(geometry, integer) returns the specified part of the collection
  • ST_GeomFromText(text) returns geometry
  • ST_AsText(geometry) returns WKT text
  • ST_AsEWKT(geometry) returns EWKT text
  • ST_GeomFromWKB(bytea) returns geometry
  • ST_AsBinary(geometry) returns WKB bytea
  • ST_AsEWKB(geometry) returns EWKB bytea
  • ST_GeomFromGML(text) returns geometry
  • ST_AsGML(geometry) returns GML text
  • ST_GeomFromKML(text) returns geometry
  • ST_AsKML(geometry) returns KML text
  • ST_AsGeoJSON(geometry) returns JSON text
  • ST_AsSVG(geometry) returns SVG text

Also remember the tables we have available:

  • nyc_census_blocks
    • blkid, popn_total, boroname, geom
  • nyc_streets
    • name, type, geom
  • nyc_subway_stations
    • name, geom
  • nyc_neighborhoods
    • name, boroname, geom

10.1. Exercises

  • “What is the area of the ‘West Village’ neighborhood?”

    SELECT ST_Area(geom)
      FROM nyc_neighborhoods
      WHERE name = 'West Village';


    The area is given in square meters. To get an area in hectares, divide by 10000. To get an area in acres, divide by 4047.

  • “What is the area of Manhattan in acres?” (Hint: both nyc_census_blocks and nyc_neighborhoods have a boroname in them.)

    SELECT Sum(ST_Area(geom)) / 4047
      FROM nyc_neighborhoods
      WHERE boroname = 'Manhattan';


    SELECT Sum(ST_Area(geom)) / 4047
      FROM nyc_census_blocks
      WHERE boroname = 'Manhattan';
  • “How many census blocks in New York City have a hole in them?”

    SELECT Count(*)
      FROM nyc_census_blocks
      WHERE ST_NumInteriorRings(ST_GeometryN(geom,1)) > 0;


    The ST_NRings() functions might be tempting, but it also counts the exterior rings of multi-polygons as well as interior rings. In order to run ST_NumInteriorRings() we need to convert the MultiPolygon geometries of the blocks into simple polygons, so we extract the first polygon from each collection using ST_GeometryN(). Yuck!

  • “What is the total length of streets (in kilometers) in New York City?” (Hint: The units of measurement of the spatial data are meters, there are 1000 meters in a kilometer.)

    SELECT Sum(ST_Length(geom)) / 1000
      FROM nyc_streets;
  • “How long is ‘Columbus Cir’ (Columbus Circle)?

    SELECT ST_Length(geom)
      FROM nyc_streets
      WHERE name = 'Columbus Cir';
  • “What is the JSON representation of the boundary of the ‘West Village’?”

    SELECT ST_AsGeoJSON(geom)
      FROM nyc_neighborhoods
      WHERE name = 'West Village';
        [583276.81990686338,4509378.825446927], ...

    The geometry type is “MultiPolygon”, interesting!

  • “How many polygons are in the ‘West Village’ multipolygon?”

    SELECT ST_NumGeometries(geom)
      FROM nyc_neighborhoods
      WHERE name = 'West Village';


    It is not uncommon to find single-element MultiPolygons in spatial tables. Using MultiPolygons allows a table with only one geometry type to store both single- and multi-geometries without using mixed types.

  • “What is the length of streets in New York City, summarized by type?”

    SELECT type, Sum(ST_Length(geom)) AS length
    FROM nyc_streets
    GROUP BY type
    ORDER BY length DESC;
                           type                       |      length
     residential                                      | 8629870.33786606
     motorway                                         | 403622.478126363
     tertiary                                         | 360394.879051303
     motorway_link                                    | 294261.419479668
     secondary                                        | 276264.303897926
     unclassified                                     | 166936.371604458
     primary                                          | 135034.233017947
     footway                                          | 71798.4878378096
     service                                          |  28337.635038596
     trunk                                            | 20353.5819826076
     cycleway                                         | 8863.75144825929
     pedestrian                                       | 4867.05032825026
     construction                                     | 4803.08162103562
     residential; motorway_link                       | 3661.57506293745
     trunk_link                                       | 3202.18981240201
     primary_link                                     | 2492.57457083536
     living_street                                    | 1894.63905457332
     primary; residential; motorway_link; residential | 1367.76576941335
     undefined                                        |  380.53861910346
     steps                                            | 282.745221342127
     motorway_link; residential                       |  215.07778911517


    The ORDER BY length DESC clause sorts the result by length in descending order. The result is that most prevalent types are first in the list.

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Next: 11. Spatial Relationships

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