From 195fd5b180fa919722890be03882a8437aee11df Mon Sep 17 00:00:00 2001
From: Philip Sargent <philip.sargent@gmail.com>
Date: Mon, 7 Jul 2025 17:04:32 +0100
Subject: [PATCH] tightend up text on using GPS for loop closure. - online edit
 of handbook/survey/ententry.html

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 handbook/survey/ententry.html | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/handbook/survey/ententry.html b/handbook/survey/ententry.html
index f358f3fdb..d44bd64aa 100644
--- a/handbook/survey/ententry.html
+++ b/handbook/survey/ententry.html
@@ -90,7 +90,11 @@ This page outlines step 7 of the survey production process. Each step is documen
   <p>There is a lot more to say about how to record the best GPS data, and how to link GPS with survey points, e.g. see <a href="gps.htm">Getting a GPS fix</a>
     <p>The altitude is not nearly as vital as the lat/long numbers. GPS altitudes are still pretty bad, we are waiting for cheaper <a href="https://en.wikipedia.org/wiki/Real-time_kinematic_positioning">RTK</a> systems for survey-quality altitudes. (If you take the altitude from a track while moving the altitude can easily be 15m wrong.)
     
-      <p>In practice, for us on the plateau, we get repeat measurements of the same spot by different teams in different years to be accurate to only about 3m. This is fine for finding entrances, and for checking whether two different teams have found the same cave, but it is not adequate for loop-closure. That requires particular care with averaging the reading over 2 minutes, and use of a location with good view of the sky, away from vertical rock, and surface survey using instruments to get from the GPS point to the actual cave entrances. To get a decent <em>altitude</em> measurement requires averaging over 10 minutes, on 2 or 3 separate days, and it is still not good enough - much better to use lat/lon and a topographic laser map (<a href="https://en.wikipedia.org/wiki/Real-time_kinematic_positioning">RTK</a> will change this).
+      <p>In practice, for us on the plateau, we get repeat measurements of the same spot by different teams in different years to be accurate to only about 3 - 5m. This is fine for finding entrances, and for checking whether two different teams have found the same cave, but it is not adequate for loop-closure. Nothing currently is good enough for loop-closer using GPS: we need a surface survey between entrances. Even taking care with averaging the reading over 2 minutes, and use of a location with good view of the sky, away from vertical rock, is not good enough (ionosphere issues).
+
+There is absolutely no way to get a decent <em>altitude</em> measurement, even averaging over 10 minutes, on 2 or 3 separate days,  is still not good enough. We have to use lat/lon and a topographic laser map (<a href="https://en.wikipedia.org/wiki/Real-time_kinematic_positioning">RTK</a> will change this).
+        
+        <p>Use a surface survey with instruments to get from the GPS point (with a good view of the sky) to the actual cave entrance tag point.
   
 <p>In previous decades the location of an entrance was the <em>output</em> of a whole lot of surveying and position fixing (e.g. see <a href="lasers.htm">laser points</a>). Today, an initial location of an entrance is available by GPS at the <em>beginning</em> of the process. So we have these fields to record the data. [We don't yet have the code to automatically add the *fix statements into the <var>fixedpts</var> data, or to the <var>essentials.gpx</var> download to be used for prospecting though.]
 <h2>List of New Entrance/Entrance_data fields</h2>