*fix and GPS

handbook/survey/ententry.html

@@ -45,10 +45,53 @@ This page outlines step 7 of the survey production process. Each step is documen
   </p>
 
    <h3>Edit Entrance form</h3>
-   <p>New in 2021 are fields for the latitude and logitude (WGS84 - the same as your GPS displays). These should be entered in degrees with decimals for fractions of a degree, e.g. 42.357 (not 42 degress 21 minutes 25 seconds, or 42 degrees 21.42 minutes).
-     <p>These fields exist for newly-discovered cave entrances for which we currently have no location information except for a GPS reading by the original discoverer. These discoveries have a habit of getting completely lost: so enter it so that someone can find it again to properly survey it. In due course, proper surveys will be done and the cave will be linked in to the GPX and survex survey system. If this has been done, then that is authoritative and you do not need to enter location informaiton on this form.
-   <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 these to the <var>essentials.gpx</var> download to be used for prospecting though.]
+<ul>
+<li>There are labels on the fields which explain what they mean - read the labels.
+<li>There is popup information on some of the fields if you hover your mouse over them, try it.
+  <li>If you still don't understand the purpose or meaning of a field, read <a href="ententryfields.html">the documentation</a>. 
+  </ul>
 
+    <h4>WGS84 fields on the form</h4>
+<p>New in 2021 are fields for the latitude and logitude (WGS84 - the same as your GPS displays). <b>Unless</b> someone has already created the *fix point for the cave, these should be entered in the form. IF the *fix <em>has</em> been done, then <em>leave the WGS84 fields blank</em>.
+  <p>We always write degrees with decimals for fractions of a degree, e.g. 42.357 (not 42 degrees 21 minutes 25 seconds, or 42 degrees 21.42 minutes).
+<p>These WGS84 fields exist for newly-discovered cave entrances for which we currently have no location information except for a GPS reading by the original discoverer. These discoveries have a habit of getting completely lost: so enter it so that someone can find it again to properly survey it. In due course the cave will be linked in to the survex survey system using *fix statements. If this *fix work has been done, then that is authoritative and     <em>     you do not need</em> to enter location information on this form.
+   
+         
+       
+<h3 id="fix">More details about *fix statements</h3>
+<p>When a nerd creates a *fix statement, in the right place, the nerd will manually delete the WGS84 values on the Entrance.
+  
+<p>
+  The survex location uses a *fix statement and it looks like this:
+<code>
+*cs LONG-LAT<br>
+*fix p2023-js-02 reference 13.80841 47.69055 1745<br>
+*entrance p2023-js-02
+</code>
+  <p>If you are doing this for the first time, it might be a good idea to put it in the same survex file temporarily. A nerd will move it to the right place later. 
+    
+<div style="margin-left: 3em; margin-right: 4em"><em>The 'right place' is in the <var>fixedpts</var> part of the <var>:loser:</var> repository, but is very different depending on whether the cave is in the 1623 or the 1626 area - for historic reasons
+  </em> 
+  <p>For the 1623 area it is easy. Each year has a different file. For 2019 there is a file <var>gps/gps19.svx</var> which contians all the *fix statements for new locations that year.
+    <p>For the 1626 area it has got horribly complicated and you should talk to Wookey and Becka. Most likely it will have to go into <var>fixedpts/1626-no-schoenberg-hs-not-tied-to-caves.svx</var></div>
+
+<p>And in your main survex file you will need to make the connection between the survey points in the cave and the external location. This would be something like this (not real data), where survey station "1" is the tag at the entrance of the cave, and outside the begin/end section it is called "2023-js-02.1"
+<code>
+*equate p2023-js-02 2023-js-02.1<br />
+<br />
+*begin 2023-js-01<br />
+*export 1<br />
+...<br />
+*data normal from to length bearing gradient ignoreall<br />
+1	2	4.46	099.3	-54<br />
+...<br />
+*end 2023-js-01<br />
+  </code>
+  <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>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, and it is still not good.
+  
+<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>
 <p>The full list of fields is documented: <a href="ententryfields.html">the full list of data-entry fields</a> when creating an entrance.