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) 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;
10418904.7172
What is the WKT definition of SRID 2831?
SELECT srtext FROM spatial_ref_sys WHERE SRID = 2831;
Or, via https://epsg.io/2831
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(geom,2831))) FROM nyc_streets;
10421993.706374
Nota
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.
How many streets cross the 74th meridian?
SELECT Count(*) FROM nyc_streets WHERE ST_Intersects( ST_Transform(geom, 4326), 'SRID=4326;LINESTRING(-74 20, -74 60)' );
223
The «74th meridian» is a fancy way of saying «a vertical line in geographics where the X value is -74». We can construct such a line and then compare it to the streets, projected into geographics also. Projecting the line into UTM and comparing it there will return a slightly different answer. To get the same answer, you need to «segmentize» it, so it has more points, before transforming.
SELECT Count(*) FROM nyc_streets WHERE ST_Intersects( geom, ST_Transform(ST_Segmentize('SRID=4326;LINESTRING(-74 20, -74 60)'::geometry,0.001), 26918) );