Introduction to PostGIS

17. Projection Exercises

Here’s a reminder of some of the functions we have seen. Hint: they should be useful for the exercises!

  • sum(expression) aggregate to return a sum for a set of records
  • ST_Length(linestring) returns the length of the linestring
  • ST_SRID(geometry, srid) returns the SRID of the geometry
  • ST_Transform(geometry, srid) converts geometries into different spatial reference systems
  • ST_GeomFromText(text) returns geometry
  • ST_AsText(geometry) returns WKT text
  • ST_AsGML(geometry) returns GML text

Remember the online resources that are available to you:

Also remember the tables we have available:

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

17.1. Exercises

  • “What is the length of all streets in New York, as measured in UTM 18?”

    SELECT Sum(ST_Length(geom))
      FROM nyc_streets;
  • “What is the WKT definition of SRID 2831?”

    SELECT srtext FROM spatial_ref_sys
    WHERE SRID = 2831;

    Or, via prj2epsg

    PROJCS["NAD83(HARN) / New York Long Island",
        DATUM["NAD83 (High Accuracy Regional Network)",
          SPHEROID["GRS 1980", 6378137.0, 298.257222101,
          TOWGS84[-0.991, 1.9072, 0.5129, 0.0257899075194932, -0.009650098960270402, -0.011659943232342112, 0.0],
        PRIMEM["Greenwich", 0.0,
        UNIT["degree", 0.017453292519943295],
        AXIS["Geodetic longitude", EAST],
        AXIS["Geodetic latitude", NORTH],
      PROJECTION["Lambert Conic Conformal (2SP)",
      PARAMETER["central_meridian", -74.0],
      PARAMETER["latitude_of_origin", 40.166666666666664],
      PARAMETER["standard_parallel_1", 41.03333333333333],
      PARAMETER["false_easting", 300000.0],
      PARAMETER["false_northing", 0.0],
      PARAMETER["scale_factor", 1.0],
      PARAMETER["standard_parallel_2", 40.666666666666664],
      UNIT["m", 1.0],
      AXIS["Easting", EAST],
      AXIS["Northing", NORTH],
  • “What is the length of all streets in New York, as measured in SRID 2831?”

    SELECT Sum(ST_Length(ST_Transform(geom,2831)))
      FROM nyc_streets;


    The difference between the UTM 18 and the State Plane Long Island measurements is (10421993 - 10418904)/10418904, or 0.02%. Calculated on the spheroid using Geography the total street length is 10421999, which is closer to the State Plane value. This is not surprising, since the State Plane Long Island projection is precisely calibrated for a very small area (New York City) while UTM 18 has to provide reasonable results for a large regional area.

  • “What is the KML representation of the point at ‘Broad St’ subway station?”

    SELECT ST_AsKML(geom)
    FROM nyc_subway_stations
    WHERE name = 'Broad St';

    Hey! The coordinates are in geographics even though we didn’t call ST_Transform, why? Because the KML standard dictates that all coordinates must be in geographics (ESPG:4326, in fact) so the ST_AsKML function does the transformation automatically.

Continue Reading

Previous: 16. Projecting Data

Next: 18. Geography

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