Add small section about mapping.

docs/mapping.md

@@ -0,0 +1,21 @@
+# Mapping
+
+One of the strengths of Polaric-webapp is that it can leverage mapcache to create a local cache of maps on the server - a Beaglebone in our case. This means that even without an Internet connection, you will still be able to run the Web app with actual maps.
+
+## Online maps
+
+When you have an Internet connection, there is no reason not to use OpenstreetMaps or Google maps. The map configuration in [/etc/polaric-webapp/mapconfig.js](https://github.com/elafargue/aprs-box/blob/master/config/etc/polaric-webapp/mapconfig.js) defines OpenstreetMaps, and multiple Google Maps layers by default.
+
+Google Maps, in particular, does not allow you to cache its contents, so you will always need to be online to use those maps.
+
+## Online/offline maps
+
+Polaric-webapp (the OpenLayer subsystem of Webapp, really) is configured to leverage "mapcache". Mapcache requires a source that can talk the WMS protocol, and is able to locally proxy and cache the contents of maps, making them available offline - indefinitely if the expiry date of the cache is set to zero.
+
+For North America, there are several map providers that work great with Mapcache, and are pre-configured in ```mapconfig.js``` : the USGS topo and satellite maps.
+
+The OpenStreetMaps wiki [mentions WMS](http://wiki.openstreetmap.org/wiki/WMS) but I had very limited success with any of the providers mentioned there.
+
+## Overall performance
+
+On a Beaglebone black, map serving and caching, while not great, is perfectly adequate for a couple of users.

mkdocs.yml

@@ -7,5 +7,6 @@ pages:
     - Overview: software.md
     - Direwolf: direwolf.md
     - Polaric Server: polaric.md
+    - Local maps: mapping.md
     - Admin configuration: admin.md