added a new introduction to coordinate systems and conversions between them

handbook/survey/gps.htm

@@ -133,14 +133,17 @@ it seems) write it down by hand on one of the A5 cave info sheets with all the
 other details of your cave and put that in the surveys ringbinder file.  </p>
 
 <p>If you want to read about the nitty gritty of converting GPS coordinates to
-the ones used by the Kataster system, you can do no better than read Wookey's
-<a href="../../years/1996/gps.htm">Compass Points Article</a>.  Briefly, this
-says "it's horribly complicated and we don't really know how to do it
-properly". Things have improved a little since those days, particularly as
-without the fog of the SA variation it's now easy to find out whether your GPS
-is set up right by just GPSsing a known point and comparing the results.
-However, the main point of having a GPS fix on an entrance is so we can find it
-again and be sure it is the same one!</p>
+the ones used by the Kataster system, you can do no better than read the
+short introduction to <a href="coord.htm">coordinate systems</a>, which briefly
+says "it's horribly complicated and we use computer programs to do it properly".
+(A rather outdated first attempt at this can also be found in Wookey's
+<a href="../../years/1996/gps.htm">Compass Points Article</a> from 1996, which
+briefly says "it's horribly complicated and we don't really know how to do it
+properly".) Overall things have significantly improved since the early days,
+particularly as without the fog of the SA variation it's now easy to find out
+whether your GPS is set up right by just GPSsing a known point and comparing
+the results.  However, the main point of having a GPS fix on an entrance is
+so we can find it again and be sure it is the same one!</p>
 
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