From 91c941ad56e83795882cc8ed7ba394ed6fd62eb2 Mon Sep 17 00:00:00 2001 From: Expo on server Date: Sun, 30 Jun 2024 16:43:40 +0100 Subject: [PATCH] altitude, link to rtk - online edit of handbook/survey/ententry.html --- handbook/survey/ententry.html | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/handbook/survey/ententry.html b/handbook/survey/ententry.html index 7e8b3cad7..f358f3fdb 100644 --- a/handbook/survey/ententry.html +++ b/handbook/survey/ententry.html @@ -88,9 +88,9 @@ This page outlines step 7 of the survey production process. Each step is documen *end 2023-js-01

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 Getting a GPS fix -

The altitude is not nearly as vital as the lat/long numbers. GPS altitudes are still pretty bad, and even the new Galileo system won't promise anything better than ± 0.4m in 2030. (If you take the altitude from a track while moving the altitude can easily be 15m wrong.) +

The altitude is not nearly as vital as the lat/long numbers. GPS altitudes are still pretty bad, we are waiting for cheaper RTK systems for survey-quality altitudes. (If you take the altitude from a track while moving the altitude can easily be 15m wrong.) -

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 altitude measurement requires averaging over 10 minutes, and it is still not good. +

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 altitude 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 (RTK will change this).

In previous decades the location of an entrance was the output of a whole lot of surveying and position fixing (e.g. see laser points). Today, an initial location of an entrance is available by GPS at the beginning 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 fixedpts data, or to the essentials.gpx download to be used for prospecting though.]

List of New Entrance/Entrance_data fields