2017-AMS-01 and ent fixes

handbook/survey/gps.htm

@@ -30,14 +30,13 @@ accurate) fixes on each entrance.</p>
 
 <p>We now (2018) have differential GPS which is much more accurate than
 in the past (e.g. Wookey's 1996 article) but altitudes are often very inaccurate
-and GPOS devices don't tell you how inaccurate the altitude is.
+and GPS devices don't generally tell you how inaccurate the altitude is.
 
 <h3>Taking the fix</h3>
 
 <p>Once you have chosen your point, mark it in some way (could be a spit hole
 or a cairn, for example - we aren't supposed to use paint any more) and place
-the GPS on the point. If you build a cairn, make it wide rather than high -
-tall cairns are knocked down by the depth of snow each winter. Give it a
+the GPS on the point. Don't build a cairn, they don't last for decades and we have survey stations that old. Give the GPS device a
 couple of minutes to get a fairly good fix (the first figure reported may be
 quite a way out, but after a couple of minutes things should settle). Then mark
 the point as a waypoint. If you're feeling really keen, you can set it up for
@@ -45,9 +44,7 @@ the point as a waypoint. If you're feeling really keen, you can set it up for
 support this automatically, and with others you can just leave it recording a
 track log, then record another waypoint at the same place just before you leave
 so it's clear to someone examining the track log when you actually left.
-(Averaging was once crucial to getting any kind of remotely close fix, but is
-somewhat less important these days now that Selective Availability has been
-turned off.)</p>
+Averaging the readings like this is vital if you want an even vaguely accurate altitude reading.</p>
 
 <p>While the GPS is recording your location, you can do something useful
 (like rigging the cave, doing a surface survey from the GPS point to the