more reasons why we have raster overviews

git-svn-id: http://svn.osgeo.org/postgis/trunk@8425 b70326c6-7e19-0410-871a-916f4a2858ee

diff --git a/doc/using_raster_dataman.xml b/doc/using_raster_dataman.xml
index 7c51ee4157c4bce7c97697ca3c45f168620a5bca..1c0b4100ec8798340d5325f2069873eacfef73d8 100644 (file)
--- a/doc/using_raster_dataman.xml
+++ b/doc/using_raster_dataman.xml
@@ -291,11 +291,11 @@ psql -d gisdb -f elev.sql</programlisting>
         </sect2>
         <sect2 id="RT_Raster_Overviews">
                <title>Raster Overviews</title>
-               <para><varname>raster_overviews</varname> catalogs information about raster table columns used for overviews and additional information about them that is useful to know when utilizing overviews. Overview tables are cataloged in both <varname>raster_columns</varname> and <varname>raster_overviews</varname> because they are rasters in their own right but also serve an additional special purpose. These are generated along-side the main raster table when you use the <varname>-l</varname> switch in raster loading.</para>
+               <para><varname>raster_overviews</varname> catalogs information about raster table columns used for overviews and additional information about them that is useful to know when utilizing overviews. Overview tables are cataloged in both <varname>raster_columns</varname> and <varname>raster_overviews</varname> because they are rasters in their own right but also serve an additional special purpose of being a lower resolution caricature of a higher resolution table. These are generated along-side the main raster table when you use the <varname>-l</varname> switch in raster loading.</para>
                <para>Two main reasons for overviews are:</para>
                <orderedlist>
-                       <listitem>Low resolution representation of the core tables common used for fast mapping zoom-out.</listitem>
-                       <listitem>Computations are faster to do on them though the computations are not as accurate as the high-res tables, they can be suitable in many rule-of-thumb computations.</listitem>
+                       <listitem>Low resolution representation of the core tables commonly used for fast mapping zoom-out.</listitem>
+                       <listitem>Computations are generally faster to do on them than their higher resolution parents because there are fewer records and each pixel covers more territory.  Though the computations are not as accurate as the high-res tables they support, they can be suitable in many rule-of-thumb computations.</listitem>
                </orderedlist>
         </sect2>
    </sect1>