pgLatLon

diff README.html @ 42:1b9cd45e9e48

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Bugfix for type casts to ecluster; New "fair_distance" function
author jbe
date Tue Oct 25 18:44:43 2016 +0200 (2016-10-25)
parents f1a9a5de9a12
children 10640afbe2ea
line diff
     1.1 --- a/README.html	Fri Oct 21 12:57:46 2016 +0200
     1.2 +++ b/README.html	Tue Oct 25 18:44:43 2016 +0200
     1.3 @@ -1,5 +1,5 @@
     1.4 -<html><head><title>pgLatLon v0.8 documentation</title></head><body>
     1.5 -<h1>pgLatLon v0.8 documentation</h1>
     1.6 +<html><head><title>pgLatLon v0.9 documentation</title></head><body>
     1.7 +<h1>pgLatLon v0.9 documentation</h1>
     1.8  
     1.9  <p>pgLatLon is a spatial database extension for the PostgreSQL object-relational
    1.10  database management system providing geographic data types and spatial indexing
    1.11 @@ -40,8 +40,8 @@
    1.12  <p>It is also possible to compile and install the extension without GNU Make as
    1.13  follows:</p>
    1.14  
    1.15 -<pre><code>cc -Wall -O2 -fPIC -shared -I `pg_config --includedir-server` -o latlon-v0007.so latlon-v0007.c
    1.16 -cp latlon-v0007.so `pg_config --pkglibdir`
    1.17 +<pre><code>cc -Wall -O2 -fPIC -shared -I `pg_config --includedir-server` -o latlon-v0008.so latlon-v0008.c
    1.18 +cp latlon-v0008.so `pg_config --pkglibdir`
    1.19  cp latlon.control `pg_config --sharedir`/extension/
    1.20  cp latlon--*.sql `pg_config --sharedir`/extension/
    1.21  </code></pre>
    1.22 @@ -486,6 +486,40 @@
    1.23  
    1.24  <p>Same as <code>epoint(float8, float8)</code> but with arguments reversed.</p>
    1.25  
    1.26 +<h4><code>fair_distance(ecluster, epoint,</code> samples <code>int4 = 10000)</code></h4>
    1.27 +
    1.28 +<p>When working with user-generated content, users may be tempted to create
    1.29 +intentionally oversized objects in order to optimize search results in an
    1.30 +unfair manner. The <code>fair_distance</code> function aims to handle this by returning an
    1.31 +adjusted distance (i.e. distance increased by a penalty) if a geographic object
    1.32 +(the <code>ecluster</code>) consists of more than one point.</p>
    1.33 +
    1.34 +<p>The first argument to this function is an <code>ecluster</code>, the second argument is a
    1.35 +search point (<code>epoint</code>), and the third argument is an interger related to the
    1.36 +precision (higher precision will require more computation time).</p>
    1.37 +
    1.38 +<p>The penalty by which the returned distance is increased fulfills (at least) the
    1.39 +following properties:</p>
    1.40 +
    1.41 +<ul>
    1.42 +<li>For search points far away from the <code>ecluster</code> (i.e. distance approaching
    1.43 +infinity), the penalty approaches zero (i.e. <code>fair_distance</code> behaves the same
    1.44 +as <code>distance</code>).</li>
    1.45 +<li>If the <code>ecluster</code> consists of a set of points, the penalty for a search point
    1.46 +close to one of that points (closer than half of the minimum distance between
    1.47 +each pair of points in the <code>ecluster</code>) is chosen in such a way that the
    1.48 +adjusted distance is equal to the distance from the search point to the
    1.49 +closest point in the <code>ecluster</code> multiplied by the square root of the count of
    1.50 +points in the <code>ecluster</code>.</li>
    1.51 +</ul>
    1.52 +
    1.53 +<p>The function interally uses a Monte Carlo simulation to compute the result. The
    1.54 +third parameter (which defaults to 10000) can be used to adjust the number of
    1.55 +samples taken. It is ensured that the penalty is always positive, i.e. results
    1.56 +returned by the <code>fair_distance</code> function are always equal to or greater than
    1.57 +the results returned by the <code>distance</code> function regardless of stochastic
    1.58 +effects.</p>
    1.59 +
    1.60  <h4><code>GeoJSON_to_epoint(jsonb, text)</code></h4>
    1.61  
    1.62  <p>Maps a GeoJSON object of type "Point" or "Feature" (which contains a

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