doc/src/sgml/earthdistance.sgml

   1 <!-- $PostgreSQL$ -->
   2
   3 <sect1 id="earthdistance">
   4  <title>earthdistance</title>
   5
   6  <indexterm zone="earthdistance">
   7   <primary>earthdistance</primary>
   8  </indexterm>
   9
  10  <para>
  11   The <filename>earthdistance</> module provides two different approaches to
  12   calculating great circle distances on the surface of the Earth. The one
  13   described first depends on the <filename>cube</> package (which
  14   <emphasis>must</> be installed before <filename>earthdistance</> can be
  15   installed). The second one is based on the built-in <type>point</> datatype,
  16   using longitude and latitude for the coordinates.
  17  </para>
  18
  19  <para>
  20   In this module, the Earth is assumed to be perfectly spherical.
  21   (If that's too inaccurate for you, you might want to look at the
  22   <application><ulink url="http://www.postgis.org/">PostGIS</ulink></>
  23   project.)
  24  </para>
  25
  26  <sect2>
  27   <title>Cube-based earth distances</title>
  28
  29   <para>
  30    Data is stored in cubes that are points (both corners are the same) using 3
  31    coordinates representing the x, y, and z distance from the center of the
  32    Earth.  A domain <type>earth</> over <type>cube</> is provided, which
  33    includes constraint checks that the value meets these restrictions and
  34    is reasonably close to the actual surface of the Earth.
  35   </para>
  36
  37   <para>
  38    The radius of the Earth is obtained from the <function>earth()</>
  39    function. It is given in meters. But by changing this one function you can
  40    change the module to use some other units, or to use a different value of
  41    the radius that you feel is more appropiate.
  42   </para>
  43
  44   <para>
  45    This package has applications to astronomical databases as well.
  46    Astronomers will probably want to change <function>earth()</> to return a
  47    radius of <literal>180/pi()</> so that distances are in degrees.
  48   </para>
  49
  50   <para>
  51    Functions are provided to support input in latitude and longitude (in
  52    degrees), to support output of latitude and longitude, to calculate
  53    the great circle distance between two points and to easily specify a
  54    bounding box usable for index searches.
  55   </para>
  56
  57   <para>
  58    The following functions are provided:
  59   </para>
  60
  61   <table id="earthdistance-cube-functions">
  62    <title>Cube-based earthdistance functions</title>
  63    <tgroup cols="3">
  64     <thead>
  65      <row>
  66       <entry>Function</entry>
  67       <entry>Returns</entry>
  68       <entry>Description</entry>
  69      </row>
  70     </thead>
  71     <tbody>
  72      <row>
  73       <entry><function>earth()</function></entry>
  74       <entry><type>float8</type></entry>
  75       <entry>Returns the assumed radius of the Earth.</entry>
  76      </row>
  77      <row>
  78       <entry><function>sec_to_gc(float8)</function></entry>
  79       <entry><type>float8</type></entry>
  80       <entry>Converts the normal straight line
  81        (secant) distance between between two points on the surface of the Earth
  82        to the great circle distance between them.
  83       </entry>
  84      </row>
  85      <row>
  86       <entry><function>gc_to_sec(float8)</function></entry>
  87       <entry><type>float8</type></entry>
  88       <entry>Converts the great circle distance between two points on the
  89        surface of the Earth to the normal straight line (secant) distance
  90        between them.
  91       </entry>
  92      </row>
  93      <row>
  94       <entry><function>ll_to_earth(float8, float8)</function></entry>
  95       <entry><type>earth</type></entry>
  96       <entry>Returns the location of a point on the surface of the Earth given
  97        its latitude (argument 1) and longitude (argument 2) in degrees.
  98       </entry>
  99      </row>
 100      <row>
 101       <entry><function>latitude(earth)</function></entry>
 102       <entry><type>float8</type></entry>
 103       <entry>Returns the latitude in degrees of a point on the surface of the
 104        Earth.
 105       </entry>
 106      </row>
 107      <row>
 108       <entry><function>longitude(earth)</function></entry>
 109       <entry><type>float8</type></entry>
 110       <entry>Returns the longitude in degrees of a point on the surface of the
 111        Earth.
 112       </entry>
 113      </row>
 114      <row>
 115       <entry><function>earth_distance(earth, earth)</function></entry>
 116       <entry><type>float8</type></entry>
 117       <entry>Returns the great circle distance between two points on the
 118        surface of the Earth.
 119       </entry>
 120      </row>
 121      <row>
 122       <entry><function>earth_box(earth, float8)</function></entry>
 123       <entry><type>cube</type></entry>
 124       <entry>Returns a box suitable for an indexed search using the cube
 125        <literal>@&gt;</>
 126        operator for points within a given great circle distance of a location.
 127        Some points in this box are further than the specified great circle
 128        distance from the location, so a second check using
 129        <function>earth_distance</> should be included in the query.
 130       </entry>
 131      </row>
 132     </tbody>
 133    </tgroup>
 134   </table>
 135
 136  </sect2>
 137
 138  <sect2>
 139   <title>Point-based earth distances</title>
 140
 141   <para>
 142    The second part of the module relies on representing Earth locations as
 143    values of type <type>point</>, in which the first component is taken to
 144    represent longitude in degrees, and the second component is taken to
 145    represent latitude in degrees.  Points are taken as (longitude, latitude)
 146    and not vice versa because longitude is closer to the intuitive idea of
 147    x-axis and latitude to y-axis.
 148   </para>
 149
 150   <para>
 151    A single operator is provided:
 152   </para>
 153
 154   <table id="earthdistance-point-operators">
 155    <title>Point-based earthdistance operators</title>
 156    <tgroup cols="3">
 157     <thead>
 158      <row>
 159       <entry>Operator</entry>
 160       <entry>Returns</entry>
 161       <entry>Description</entry>
 162      </row>
 163     </thead>
 164     <tbody>
 165      <row>
 166       <entry><type>point</> <literal>&lt;@&gt;</literal> <type>point</></entry>
 167       <entry><type>float8</type></entry>
 168       <entry>Gives the distance in statute miles between
 169        two points on the Earth's surface.
 170       </entry>
 171      </row>
 172     </tbody>
 173    </tgroup>
 174   </table>
 175
 176   <para>
 177    Note that unlike the <type>cube</>-based part of the module, units
 178    are hardwired here: changing the <function>earth()</> function will
 179    not affect the results of this operator.
 180   </para>
 181
 182   <para>
 183    One disadvantage of the longitude/latitude representation is that
 184    you need to be careful about the edge conditions near the poles
 185    and near +/- 180 degrees of longitude.  The <type>cube</>-based
 186    representation avoids these discontinuities.
 187   </para>
 188
 189  </sect2>
 190
 191 </sect1>