+</section>
+
+<section><title>Empty <emphasis>Item</emphasis></title>
+<para>
+There are several options to quickly delete a subsection of data:
+</para>
+<itemizedlist>
+<listitem><para>Empty Realtime</para></listitem>
+<listitem><para>Empty Upload</para></listitem>
+<listitem><para>Empty Download</para></listitem>
+<listitem><para>Empty All</para></listitem>
+</itemizedlist>
+</section>
+
+</section><!-- End GPS -->
+
+<section><title>DEM (Digital Elevation Model) Layer</title>
+<para>
+This layer provides elevation data from the <ulink url="http://www2.jpl.nasa.gov/srtm/">Shuttle Radar Topography Mission (SRTM)</ulink>
+</para>
+<para>
+Elevation data can be useful in planning trips, as you many wish to avoid/minimise the hills encountered. [OSM Cyclemap is also useful this way too]
+</para>
+<para>
+To download the data use the DEM Download tool (shown on the toolbar), and then click on a area of the viewport. Once files have been downloaded they can then be loaded from disk via the DEM properties in future application runs.
+</para>
+<para>
+The current data server is: <ulink url="http://dds.cr.usgs.gov/srtm/version2_1/">NASA STRM 2.1</ulink>.
+</para>
+<para>
+ATM &appname; does not auto download DEM data. If you want to get lots of data blocks, you may wish to use some other program get such as <application>curl</application> or <application>wget</application> to download them for an area.
+</para>
+<note>
+<para>
+Using many DEMs is CPU/memory intensive. Depending on your computer's capability, &appname; will be less responsive when about 50+ DEM blocks are loaded. It is probably unwise to attempt using 200+ blocks, so trying to use DEMs covering large countries/continents (USA, Australia etc...) is unlikely to work. In these conditions under Linux, &appname; may be automatically stopped by the "<ulink url="http://en.wikipedia.org/wiki/Out_of_memory/">OOM</ulink> Killer"
+</para>
+</note>
+
+</section><!-- End DEM -->
+
+
+<section><title>Maps Layer</title>
+<para>
+This layer provides a single map resource, you may have multiple map layers but only top one (if enabled) will be visible.
+You will need an open internet connection when you are downloading maps, but once downloaded they are available from the hard disk cache. When map are avaliable from the disk cache it is much faster and can be used offline.
+</para>
+<para>
+Some maps are continually improving over time (e.g. OpenStreetMap) and so in order to not to have to (re)download the data all the time &appname; employs a timeout method - 'Tile Age' to determine whether to access the server. However a forced refresh for the current view can be made via the <guilabel>Reload All Onscreen Maps</guilabel> option.
+</para>
+<para>
+Inbuilt maps include various <ulink url="http://en.wikipedia.org/wiki/TerraServer-USA">Terraserver</ulink> (Primarily USA coverage) and <ulink url="http://openstreetmap.org/">OpenStreetMap (OSM)</ulink> ones and more:
+</para>
+<itemizedlist>
+<listitem><para>Terraserver Topos</para></listitem>
+<listitem><para>Terraserver Aerials</para></listitem>
+<listitem><para>Terraserver Urban Areas</para></listitem>
+<listitem><para>OpenStreetMap (Osmarender)</para></listitem>
+<listitem><para>OpenStreetMap (Mapnik) - This is the default</para></listitem>
+<listitem><para>OpenStreetMap (Maplint)</para></listitem>
+<listitem><para>OpenStreetMap (Cycle)</para></listitem>
+<listitem><para>NASA BlueMarble</para></listitem>
+<listitem><para>SpotMap (&appname; Version1.1+)</para></listitem>
+</itemizedlist>
+
+<para>
+&appname; can be configured to handle additional maps. See <link linkend="extend_viking">Extending &appname;</link> for further detail.
+</para>
+
+<section><title>Layer Operations</title>
+<section><title>Download Missing Onscreen Maps</title>
+<para>
+This requests map tiles for areas of the viewport that do not currently have one.
+</para>
+</section>
+
+<section><title>Download New Onscreen Maps</title>
+<para>
+This requests map tiles only if the maps are older than the Tile Age preference.
+</para>
+</section>
+
+<section><title>Reload All Onscreen Maps</title>
+<para>
+Force a refresh of all visible map tiles.
+</para>
+</section>
+</section>
+
+
+</section><!-- End Maps -->
+
+<section><title>Aggregrate Layer</title>
+<para>
+This layer is a container layer to hold other layers. There is always an initial and controlling 'Top Layer' that can not be removed, although it can be renamed.
+</para>
+<para>
+This layer type is useful for grouping other layers, especially TrackWaypoint layers, in whatever categories are relevant to you: such as by activity, location or date.
+Here are some suggestions:
+<itemizedlist>
+<listitem><para>Hiking</para></listitem>
+<listitem><para>Mountain Biking</para></listitem>
+<listitem><para>Road Cycling</para></listitem>
+<listitem><para>Car Trips</para></listitem>
+<listitem><para>Sailing</para></listitem>
+<listitem><para>Holidays</para></listitem>
+<listitem><para>UK Points of Interest</para></listitem>
+<listitem><para>USA Points of Interest</para></listitem>
+<listitem><para>etc...</para></listitem>
+</itemizedlist>
+</para>
+<para>
+Thus one can have a list of all your tracks, but groups can be shown or not using the relevant layers visibility checkbox.
+</para>