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128 lines
5.2 KiB
Plaintext
128 lines
5.2 KiB
Plaintext
This contrib package contains two different approaches to calculating
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great circle distances on the surface of the Earth. The one described
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first depends on the contrib/cube package (which MUST be installed before
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earthdistance is installed). The second one is based on the point
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datatype using latitude and longitude for the coordinates. The install
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script makes the defined functions executable by anyone.
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Make sure contrib/cube has been installed.
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make
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make install
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make installcheck
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To use these functions in a particular database as a postgres superuser do:
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psql databasename < earthdistance.sql
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-------------------------------------------
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contrib/cube based Earth distance functions
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Bruno Wolff III
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September 2002
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A spherical model of the Earth is used.
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Data is stored in cubes that are points (both corners are the same) using 3
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coordinates representing the distance from the center of the Earth.
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The radius of the Earth is obtained from the earth() function. It is
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given in meters. But by changing this one function you can change it
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to use some other units or to use a different value of the radius
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that you feel is more appropiate.
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This package also has applications to astronomical databases as well.
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Astronomers will probably want to change earth() to return a radius of
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180/pi() so that distances are in degrees.
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Functions are provided to allow for input in latitude and longitude (in
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degrees), to allow for output of latitude and longitude, to calculate
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the great circle distance between two points and to easily specify a
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bounding box usable for index searches.
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The functions are all 'sql' functions. If you want to make these functions
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executable by other people you will also have to make the referenced
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cube functions executable. cube(text), cube(float8), cube(cube,float8),
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cube_distance(cube,cube), cube_ll_coord(cube,int) and
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cube_enlarge(cube,float8,int) are used indirectly by the earth distance
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functions. is_point(cube) and cube_dim(cube) are used in constraints for data
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in domain earth. cube_ur_coord(cube,int) is used in the regression tests and
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might be useful for looking at bounding box coordinates in user applications.
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A domain of type cube named earth is defined.
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There are constraints on it defined to make sure the cube is a point,
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that it does not have more than 3 dimensions and that it is very near
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the surface of a sphere centered about the origin with the radius of
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the Earth.
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The following functions are provided:
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earth() - Returns the radius of the Earth in meters.
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sec_to_gc(float8) - Converts the normal straight line (secant) distance between
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between two points on the surface of the Earth to the great circle distance
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between them.
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gc_to_sec(float8) - Converts the great circle distance between two points
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on the surface of the Earth to the normal straight line (secant) distance
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between them.
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ll_to_earth(float8, float8) - Returns the location of a point on the surface
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of the Earth given its latitude (argument 1) and longitude (argument 2) in
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degrees.
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latitude(earth) - Returns the latitude in degrees of a point on the surface
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of the Earth.
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longitude(earth) - Returns the longitude in degrees of a point on the surface
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of the Earth.
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earth_distance(earth, earth) - Returns the great circle distance between
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two points on the surface of the Earth.
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earth_box(earth, float8) - Returns a box suitable for an indexed search using
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the cube @> operator for points within a given great circle distance of a
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location. Some points in this box are further than the specified great circle
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distance from the location so a second check using earth_distance should be
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made at the same time.
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One advantage of using cube representation over a point using latitude and
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longitude for coordinates, is that you don't have to worry about special
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conditions at +/- 180 degrees of longitude or near the poles.
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Below is the documentation for the Earth distance operator that works
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with the point data type.
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---------------------------------------------------------------------
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I corrected a bug in the geo_distance code where two double constants
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were declared as int. I also changed the distance function to use
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the haversine formula which is more accurate for small distances.
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Bruno Wolff
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September 2002
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---------------------------------------------------------------------
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Date: Wed, 1 Apr 1998 15:19:32 -0600 (CST)
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From: Hal Snyder <hal@vailsys.com>
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To: vmehr@ctp.com
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Subject: [QUESTIONS] Re: Spatial data, R-Trees
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> From: Vivek Mehra <vmehr@ctp.com>
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> Date: Wed, 1 Apr 1998 10:06:50 -0500
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> Am just starting out with PostgreSQL and would like to learn more about
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> the spatial data handling ablilities of postgreSQL - in terms of using
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> R-tree indexes, user defined types, operators and functions.
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>
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> Would you be able to suggest where I could find some code and SQL to
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> look at to create these?
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Here's the setup for adding an operator '<@>' to give distance in
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statute miles between two points on the Earth's surface. Coordinates
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are in degrees. Points are taken as (longitude, latitude) and not vice
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versa as longitude is closer to the intuitive idea of x-axis and
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latitude to y-axis.
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There's C source, Makefile for FreeBSD, and SQL for installing and
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testing the function.
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Let me know if anything looks fishy!
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