The entrance to a cave significant enough to get a number and a survey will eventually be marked by a numbered tag attached to a spit. This will then become the primary survey station - ie. the point where an underground survey will start, and the point to which a surface survey should go. It's worth thinking about where you would put such a tag right from the start. Unlike the first rigging bolt (often used as the first point of a survey in the past) it should be sited with a particular view to its visibility and accessibility without having to put on SRT kit. If such a point has a clear view of the majority of the sky, then this is the point to use for a GPS fix too.
If however, you are dealing with a cave at the foot of a cliff, or otherwise with a restricted view of the sky, then choose instead a good landmark with a clear view, and within one (or maybe two) survey shots of the entrance. If you have found a group of caves close together, it might be better to GPS a central point rather than get quick (but less accurate) fixes on each entrance.
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 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 your GPS supports averaging, then choose "average" and leave the GPS to do its own averaging for half an hour or more. If it doesn't support averaging, then fix another waypoint at the same location just before you leave. This shows someone examining the track log later that all the track points relate to the same spot, and they can then do the averaging themselves.
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 marker spit, looking for other caves, or even having lunch!) It does not matter (and may be slightly beneficial) if you leave the GPS longer than half an hour. But if you want to use it for anything else, remember to stop the waypoint averaging before moving the unit or changing the display page. Take a photo of your GPS point showing at least one of your cave entrances too.
It doesn't especially matter what display options are selected when you are getting the GPS fix, but it is important to use standard ones when writing down the reported position in the survey book. Currently we use "German" grid, "Austrian" datum, and "metric" units. All is not lost if you can't select this particular set, but it is very important that you write down what was actually used, since different grid systems can give results up to a couple of kilometres different!
If your GPS doesn't support Austrian coordinates out of the box, then you can set it up as a "User Grid" using some or all of the following cryptic runes:
| Ellipsoid: | Austrian (Bessel 1841) a = 63377397.155m (ΔA = 739.845) 1/f = 299.1528128 (Δf x 10,000 = 0.10037428) |
|---|---|
| Datum: | Austria MGI (Hermannskogel) |
| Projection: | Transverse Mercator (BMN zone M31) |
| Grid parameters: | Central meridian 13°20'E False easting 450km No additional scaling Grid boundaries at 11°50' and 14°50' |
| EFEC coordinate conversion equation with respect to WGS84: | Offsets Δx = -575m, Δy = -93m, Δz = -466m Rotations ωx = 5.1"; ωy = 5.1", ωz = 5.2" Scaling -2.5ppm |
Write down the figure that the GPS gives for each waypoint at the time (just in case some failure loses the data from the GPS memory). That's all you need to do at the cave. Get the GPS data downloaded to a computer next time you are in Base Camp (or Top Camp if someone has a laptop :-). This should be put in a file and a clear reference to it put in the notKH survey book. Don't alter the file in any way - it may be necessary to upload it to a GPS unit at some time to do coordinate conversion. Also copy your written down data to the survey book with all the other details of your cave.
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 Compass Points Article. Briefly, this says "it's horribly complicated and we don't really know how to do it properly". 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!