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source: trunk/workshop-foss4g/projection_exercises.rst @ 14

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Section 16: Projection Exercises

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

Remember the online resources that are available to you:

Also remember the tables we have available:

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

Exercises

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

    SELECT Sum(ST_Length(the_geom))
  FROM nyc_streets;

    10418904.7172

  • "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",
GEOGCS["NAD83(HARN)",
  DATUM["NAD83 (High Accuracy Regional Network)",
    SPHEROID["GRS 1980", 6378137.0, 298.257222101, AUTHORITY["EPSG","7019"]],
    TOWGS84[-0.991, 1.9072, 0.5129, 0.0257899075194932, -0.009650098960270402, -0.011659943232342112, 0.0],
    AUTHORITY["EPSG","6152"]],
  PRIMEM["Greenwich", 0.0, AUTHORITY["EPSG","8901"]],
  UNIT["degree", 0.017453292519943295],
  AXIS["Geodetic longitude", EAST],
  AXIS["Geodetic latitude", NORTH],
  AUTHORITY["EPSG","4152"]],
PROJECTION["Lambert Conic Conformal (2SP)", AUTHORITY["EPSG","9802"]],
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],
AUTHORITY["EPSG","2831"]]

  • "What is the length of all streets in New York, as measured in SRID 2831?"

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

    10421993.706374

    Note

    The difference between the UTM 18 and the Stateplane Long Island measurements is (10421993 - 10418904)/10418904, or 0.02%. Calculated on the spheroid using :ref:`geography` the total street length is 10421999, which is closer to the Stateplane value. This is not surprising, since the Stateplane 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.

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  • "What is the KML representation of the point at 'Broad St' subway station?"

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

    <Point><coordinates>-74.010671468873468,40.707104815584088</coordinates></Point>

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

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