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<title>1996: BCRA CSG report on GPS usage</title>
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<font size=-1>BCRA Cave Surveying Group
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"<a href="http://www.chaos.org.uk/survex/cp/cpinfo.htm">Compass Points</a>"
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15, March 1997, pp 11-15</font>
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<center><H2><A NAME="art4">GPS For Expedition cave location</A></H2>
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<I>Wookey</I></center>
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<P><I><b>GPS units are now cheap enough that cavers can afford to consider
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using them in the field. They are particularly useful where a proper surface
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survey is not possible due to constraints such as:
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<ul><li>shortage of manpower or time,
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<li>distance from, or absence of, known fixed points,
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<li>inhospitable terrain.</ul>
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<p>The disadvantage is that they aren't particularly accurate. In the summer
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of 1996 CUCC were able to borrow a unit and test it out in an area where
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checking back to known fixed points was possible and thus an idea of actual
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accuracy in the field could be formed. Here I describe what was done, the
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methods used, and the results obtained.</b></I>
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<P>I scrounged a Garmin GPS45 from my mate Ian Harvey (see his review in
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<A HREF="http://www.chaos.org.uk/survex/cp/cp11/cpoint11.htm#art4">Compass Points
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11</A>) to take on Cambridge UCC's annual jaunt to Austria. The expedition
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area is high up in the mountains in very unhelpful karst terrain where
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getting about is greatly impeded by dwarf pine and small cliffs, as well as
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an awful lot of holes in the ground. There are 200 logged caves in the area,
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about half of which have been found by CUCC. A depressingly small number of
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these have surface surveys to their entrances, and many have very little info
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indeed recorded about them, and thus are effectively 'lost'. This state of
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disorganisation has developed over the past 20 years of exploration, where
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for much of that time the imperative has been to find cave, not ponce about
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doing dull things like surface surveying in the pouring rain. Encouraged by
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the Austrian record-keepers' complaints about missing info, and a general
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modernisation of attitude, CUCC has been working to find out and record what
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actually <I>is</I> known and thus generate a list of lost caves enabling us
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to go and sort out what is where, what is marked, which numbers have been
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used twice, which caves have been numbered twice etc. By the start of the
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1996 expedition we finally had a pretty good list of caves by status.
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'Surveyed to', 'Known', 'Marked but Missing', 'Unmarked & Missing'. The
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'Surveyed to' list was embarrassingly small, but the 'Known' list was quite
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big.
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<P>This was where the GPS could potentially be very useful. It allows a
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searcher to wander freely about the place looking for caves according to
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whatever information is available and then simply record the position upon
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finding it, without having to do a long survey (potentially several
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kilometres) to the nearest fixed point/existing survey. The only downside is
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that the resulting fixes are likely to be significantly less accurate than a
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surface survey. In Theory, at least, this problem should be largely dealt
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with by logging at the entrance for some time and taking a geometric mean of
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the results. Information gleaned about GPS before I left suggested that
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averaging for an hour or so should give improved results, and if you could do
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it for several hours you should get pretty good results. The nominal accuracy
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(with SA on, see below) is about 100m (it would be 30m without SA). This
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'100m accuracy' means that the position given will be within 100m of the real
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location 95% of the time. Averaging for 12 hours will get you 10m accuracy,
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averaging for an hour should be around 60m. The problem here is that for the
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purposes of recording the positions of caves in order to return to them, we
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get hit by the inaccuracies twice, once when marking the original waypoint,
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and then again when trying to return to that waypoint. This can leave you
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200m from the cave in a bad case. This doesn't sound like much, but believe
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me in the terrain of the loser plateau this is pretty useless. You've
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generally got to be within 50m to have a hope of finding most of the caves,
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depending on the quality of the description of how to find it.
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<H4>In the field:</H4>
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<P>So, after playing with it all the way to Austria, (and thus saving
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ourselves from a disastrous wrong turn in Lille) we got a very pleasing line
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all the way across Europe with a little gap where the ferry was. The fact
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that the last 400km was on the back of a tow-truck is another story. On
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arrival I left the unit in the sunshine for 4 hours plotting the position of
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basecamp. Slowly a track was built up wandering around in a big blob around
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the campsite.
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<Center><IMG SRC="img171.png">
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<P>Five-hour GPS plot showing how the readings wander, and that the geometric
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mean is likely to be a better position estimate than any of the individual
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readings.</Center>
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<P>This 'apparent movement' effect is due primarily to SA, and also to the
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movement of the satellites and other inaccuracies of the system. Here is a
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bit more detail on these aspects of GPS. SA (Selective Availablity) is
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something of a misnomer for the deliberate degradation of the signals by the
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US military to stop everyone else using the system to their advantage. It
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basically means that the satellites are all lied to a bit, and thus so is
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your GPS unit. The net result is that when you are standing still your GPS
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will tell you that you are moving (typically at a slow walking pace) around
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randomly. Thus when the unit is left in one place, recording track
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information, you get the screen slowly filling up with scribble. The centre
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of this scribble is as good a reading of your actual position as you can get
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with this technology. Survey-grade GPS uses a different technique to produce
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sub-centimetre accuracy despite SA, and the Military have a second, coded
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channel to get much more accurate fixes.
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<P>I spent several days over the next few weeks wandering about the plateau
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looking for caves, finding a few lost ones and numerous 'known but not
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surveyed to' ones, as well as quite a few new ones. At all of these positions
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info was logged with the unit left at the entrance whilst I drew up the
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immediate vicinity, the cave itself, took bearings on surrounding mountains
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and wrote a description - i.e. collected as much information as possible to
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enable the cave to be properly recorded and re-found in future. This was
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using a handy feature of the GPS45. It can be set to record its position in a
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'track log' every n minutes, and has room to store 768 positions. Once these
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are full the unit starts overwriting the oldest data (unless it is set to
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stop when full). 5 minutes seems a sensible setting for cave-location use,
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although those who know about SA suggest that there is actual no point taking
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readings more often than every 15 minutes. 5 minute logging will take 64
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hours to fill up the memory, which was certainly plenty for a 3-week stint in
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Austria - obviously you adjust it to suit your circumstances. You can also
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take a simple 'Waypoint' position readings, by pushing the 'Mark' button,
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which records the current position, and invites you to give it a name for
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future reference. These names are restricted to 6 characters (which is a bit
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of a pain), and nearby ones are displayed on the map screen.
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<P>Getting fixes in these alpine conditions was no problem at all. Although
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the area is mountainous, you generally have a large area of sky visible and
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this meant that we nearly always had a full set of satellites visible. This
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means that you get altitude data as well as position data, although in
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general this was given as +/- 60m, (which is more like +/-120m allowing for
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SA). Also the waypoints and track log do not record altitude, so you have to
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write it down to get any record. I noticed that the position immediately
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after it got a fix could be way out (up to 1km), but after a couple of
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minutes it usually improved to be within the correct area. You can tell this
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once you have a few waypoints marked because you get a line on the map screen
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zooming in from way over to one side.
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<Center><IMG SRC="img173.png"><br>Figure 1
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showing how the first couple of readings after getting a fix can be
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several hundred metres off.</Center>
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<P>Battery usage is an issue. The GPS 45 uses batteries quite fast. It takes
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4 AA cells, and set of these alkalines will last about 10 hours use - more if
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it isn't continuous. For this sort of logging that means you can use a set up
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in a day or two, which is expensive and leaves you carrying an awful lot of
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batteries round with you. NiCads do work, but only give about 4 hours use. I
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got round this problem by using my FX5 as a power source. The GPS45 will take
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any input from 5 - 40V so any 5V or more battery pack should work. I stole
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the though-the-headset charger bits off my charger, and attached them to a
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backplate hastily manufactured from a couple of layers of Lilt-bottle
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plastic. This made a lightweight FX5 adapter, when soldered to the combined
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power/serial lead which already had a car ciggy-lighter connector and 9-pin D
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- makes a pretty strange-looking cable! The FX5 should power the unit for
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about 50 hours, although with external power at this voltage some units may
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be prone to shutting down. The GPS45 has a 'battery saver' mode which is
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appropriate for use where the unit is moving at constant velocity so it
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should be fine for entrance-logging. I did not know about this at the time,
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but it can give up to double the battery lifetime. When I find out how to
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enable it I'll give it a go.
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<H4>Downloading</H4>
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<P>We were unable to download all this data from the GPS whilst in Austria,
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despite soldering up a cable and peering at specsheets and software
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downloaded from the very useful GPS pages on the net, as the software/serial
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interface simply refused to play. This may be because the serial port on the
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GPS45 is an NMEA (marine instruments) standard, not the more familiar RS232,
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thus the voltage level is only 5V, somewhat below the 9-12V normally used on
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computers. Most serial interfaces will work anyway, but yours, like ours, may
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not. Either get a different serial card, or build a step-up interface -
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again, details of what you need are in the GPS net docs. This setback was
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inconvenient, but not serious as the track log was big enough to hold all the
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data we needed, so I could do the download and analysis on return to the UK.
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<P>Unfortunately, when I got back the GPS couldn't be found anywhere! I had
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noticed during packing of a huge pile of stuff into Julian's van that I
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hadn't specifically noticed it going in. Concerned, this being borrowed kit,
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I asked Julian, who assured me that he had seen me put it in earlier. 'Are
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you sure?', 'Yes, definitely', he said. Given that all our stuff had gone
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into this van, and I'd had it the day before in my rucksack, I had to assume
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he was right, and back to the UK we drove. However, once back, a great deal
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of checking showed no sign of it, and an increasing air of perplexed
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embarrassment settled. It must have fallen out of my rucksack somewhere on
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the last day, or been stolen from the campsite pile of rucksacks that night.
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Then someone had the brilliant idea of leaving a message for one of the
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expedition members (the CSG's membership sec., Andy Atkinson, no less) who
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had gone on to the Imperial CCC expedition to Slovenia, and was returning via
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the campsite in Austria two weeks later. So a cryptic message about GPS units
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was left with the long-suffering Gasthof owner Karin, telling Andy to go and
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look for it on the mountain. He rang me at 6.30 one Saturday morning to get
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this bizarre message explained, and after the requisite grumble, set off up
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the mountain. Miraculously the GPS was sat quietly on the VD1 fixed point,
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the highest point of the route to our major cave, none the worse for it's
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two-week sojourn atop an Alp apart from an inexplicable cracked display. This
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was because four of us had left it there whilst chatting about glaciology on
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that last day, logging the VD1 position, and then had simply set off to go
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caving, forgetting about it entirely - ooops! So beware, that if distracted
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during the logging process, and if you are sufficiently dense, you may just
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leave the damn thing behind entirely! Garmin UK repaired the cracked screen
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for free, despite it being out of warranty, where it would normally cost 30
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quid, and thus it finally got back to its owner.
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<H4>Processing the Data</H4>
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<P>This little setback dealt with I was finally able to download the track
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data using the simple, free Gardown7 utility, a no-frills bit of DOS software
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that simply generates a text file of data. You give a command like GARDOWN7 T
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S2 to download Track data on Serial port 2. This gives a text file with a
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header and then one line per track log point, like this:
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<P><pre width=80>
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T N49 37.7525 E006 12.2740 Fri Jul 12 22:51:49 1996<BR>
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T N49 37.7441 E006 12.2804 Fri Jul 12 22:53:50 1996<BR>
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T N49 37.7635 E006 12.3152 Fri Jul 12 22:55:51 1996<BR>
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T N49 37.8224 E006 12.3338 Fri Jul 12 22:56:36 1996<BR>
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T N49 38.1552 E006 12.8984 Fri Jul 12 22:58:36 1996<BR>
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T N49 38.2060 E006 14.2361 Fri Jul 12 23:00:36 1996<BR>
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T N49 38.3663 E006 15.8547 Fri Jul 12 23:02:38 1996<BR>
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T N49 38.4481 E006 16.0234 Fri Jul 12 23:04:39 1996
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</pre>
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<P>This is: T for Track, Then the Northing and Easting (Lat. /Long) in
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Degrees and minutes, then Day, Month, Date, Time, Year.
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<P>I imported this data into Excel7 using the import text function to split
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this up into columns, and throw away the useless stuff like the T, N &
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E.. Having done this, I split the data up into sections by time, thus
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grouping all the logged points at one location together. At this point I
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realised a problem with my method. As the time is the only way you can tell
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which track log items were recorded where it is very important to turn the
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unit off between logging positions, otherwise you can't tell when you were
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standing still. It is possible to use the waypoint information that you have,
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as this is also timestamped, but basically be aware that if there are gaps in
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the datastream it is much easier to split up the sections. An alternative
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would be to record the GPS time (displayed by the GPS) on arrival and
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departure at a site, but you really do have to remember if keeping the unit
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on whilst wandering about. Keeping the unit on is useful because<br>
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a) it's fun to play with and watch your little squiggle wander about the
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place, and<br>
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b) you seem to get more accurate position data after the unit has had a fix
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for a few minutes.
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<P>There were 723 track log points, taken in 50 sections at 41 positions
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(some places were logged more than once to see what repeatability was like -
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(see Figure 2, page 15). For each section of logging the average position was
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calculated in X & Y. At this point I have to explain the complications of
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co-ordinate conversion. This is by far the biggest problem in using a GPS.
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Although the unit knows all about a whole host of co-ordinate systems and
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origins, the serial data downloaded is always in Lat./Long. format using the
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WGS84 spheroid. When you record positions they are likely to be in the
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locally useful co-ordinate system, and this is what you will need to plot
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positions on the map. In the UK this will be 'OSGB'. On an expedition you
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need to do a bit of research to find out what you should be using and how to
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do the necessary conversions. The OS publish a very useful couple of
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documents 'Transverse Mercator Projection - Constants, Formulae and Methods',
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and 'Conversion of Latitude and Longitude to/From National Grid'. I have
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photocopies of these if anyone needs them - they may now also be available on
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the net. These sums have been turned into a DOS program called 'CONVERT.EXE'
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(imaginative stuff!), and a RISCOS Basic program, both of which I also have.
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<H4>Grid Systems in Austria</H4>
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<P>The Austrian Grid system has proved to be something of a conundrum, and I
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still haven't reached a fully satisfactory answer. The maps themselves don't
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give any information about the Grid System in use, although they do have
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Lat./Long markings at the edges, but I have been given some information on
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the grid system in use, that Andy Waddington has discovered.
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<P>The co-ordinate system is called the Gauß + Krüger
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Landeskoordinatensystem, which is in fact a designed-to-confuse alternative
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name for Universal Transverse Mercator (UTM) and in this area is UTM zone 33.
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Within this system, there are a number of triangulation points surveyed
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purely for the purpose of locating caves: these are the
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Katastertriangulierungspunkte, or KT points. The X (or north) co-ordinate,
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when quoted in full, is actually the distance in metres north of the equator.
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<P>These Laser Rangefound points, use grid co-ordinates which can cause a
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great deal of confusion on several counts:
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<UL>
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<LI>Y means easting, X means northing, so these symbols
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are the opposite of what seems normal to British punters.
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<LI>a typical grid reference doesn't have the same
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number of digits in the easting and northing, because the origin
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is a long way south (its on the equator !), but not that far west,
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and no leading zeroes are added.
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<LI>not all leading digits of the grid co-ordinates
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are quoted anyway.
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<LI>Sometimes one version of the easting is quoted,
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with values in our area of around 36000, and sometimes a version
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from a system (BMN) where the equivalent is 486000. To be fair,
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the systems are not usually mixed in one area.
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</UL>
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<P>So we get points with co-ordinates like this (prefix
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(52) for complete X co-ord.):
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<P>KT114_96 36670.37 83317.43 1903.97 Hinterer Schwarzmooskogel
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Trig point
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<P>Laser0 36386.56 82804.16 1815.45 Hilfstandpunkt (hillock
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N of 161)
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<P>Laser0_1 36093.56 82337.43 1782.35 Near 156
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<P>This is fine as far as it goes, but the GPS45 gives UTM and 'German' as
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different Grid systems. I believe 'German' to be the Gauß &
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Krüger referred to above as the readout is prefixed 'GK'. These two
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systems give similar, but not identical results, and neither of them is much
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like the values above, which is what we have the local fixed points marked
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in.
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<P>For example the point VD1 looks like this in the three systems:
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<P>UTM: 33T 0411506 5280946
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<P>G&K: 5411471 5283059
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<P>Laser: 36093 5282491
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<P>Fortunately all of these systems are a metric Grid, so assuming that they
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are co-linear within the area in question, then only an offset is required to
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convert from one to another., and the offsets from one system to another can
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be determined by knowing one point in these two co-ordinate systems. More
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complexity is added when you consider the Datum used. The above were done
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using the Potsdam datum, but if you change it you will get slightly different
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results. It is noticeable that the Northings all agree to within 2.1km, but
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the Eastings are a bit more different. Ignoring the 5million km figure then
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the UTM & G&K are the same to within 35m, but the Austrian Laser
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points seem to be completely unrelated.
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<P>A solution to all this confusion seemed to be the software 'GEO2', written
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by Anton Helm at the university of Wien. This software converts between
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Lat./Long and Gauß & Krüger co-ordinates. In fact it is
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smarter than that and can do any conversion between spheroidal co-ordinates
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(e.g. Lat./Long), and any Grid System, given suitable parameters. It is
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rather 'UNIXy' software, being a script-processing language where variables
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are assigned to points and various operation can be done on these variables
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to achieve conversions. I have to admit that I have not fully investigated
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its results, but they seem to give yet another set of different answers!
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<P>Given these discrepancies I have taken a rather pragmatic approach to
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doing the conversion from one co-ordinate system to another. I have assumed
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that over the 3km are that I am dealing with I can take Lat./Long. to be
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linearly mapped to the G&K grid, which means I simply have to find the
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offset and scale for the conversion. If I have two points for which I know
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both the Lat./Long. co-ordinates and the G&K co-ordinates then a simple
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interpolation will provide the conversion for all the other points. This will
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only give reasonable accuracy if the two known points are at opposite corners
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of the area in question. In fact Excel will let you give it a whole series of
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pairs of points and return the slope and intercept values of the best-fit
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line through them., so this is what I have done. To get 'correct' conversion
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from Lat./Long. to G&K, I have converted (rather laboriously) each
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averaged result for a point of interest into G&K using the GPS45. This is
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done by entering a waypoint in Lat./Long. mode, then changing the co-ordinate
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system to 'German', and looking at the waypoint again. This is rather a lot
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of button-pushing on the GPS, but it kept me entertained for a couple of long
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winter evenings over Xmas in the Pen-y-Pass youth hostel. Using all these
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pairs to get the slope & intercept values then comparing the calculated
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results with the GPS results, shows that the process gives results accurate
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to 15m across the area we are concerned with - good enough.
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<P>The final part of getting a set of cave entrances that I can plot on the
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area map is the mysterious offset from GPS-reported G&K co-ordinates to
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Austrian Laser Rangefound co-ordinates. To do this I needed a laser point
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that I had reasonably good GPS values for. I chose the one on Brauning
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Näse as the GPS was left there for 8 readings, making it the most
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reliable Laser point data I had. The problem with using a single point for
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the offset is that any error in that GPS location is applied to all the
|
|
others too by being included in the offset from G&K (Garmin) to
|
|
G&K(Austrian Cavers). I still need to do further investigation to
|
|
determine the effects of this. The results could be improved by taking
|
|
multiple places where we have both G&K (Austrian Cavers) co-ordinates and
|
|
GPS locations and taking a mean of the various offsets, or by doing better
|
|
long-term GPS locations (e.g. for a couple of days) at one of these points
|
|
next year. We have what should be a very good dataset for point VD1 (where
|
|
the GPS was left accidentally), as it logged for 5.5 hours before giving up
|
|
due to flat batteries. Unfortunately this is not an Austrian Laserpoint,
|
|
although it is joined to one by a surface survey. Using this point might well
|
|
give better results, but it depends how good the surface survey is.
|
|
|
|
<P>So, after a great deal of head-scratching I have a pair of columns on my
|
|
spreadsheet giving co-ordinates in G&K (Austrian Cavers) co-ordinates. It
|
|
looks like this, showing the actual data and (boxed) the averaged
|
|
results:<IMG SRC="img174.png">The co-ordinate pairs are: decimalised
|
|
Lat./Long., interpolated G&K, offset 'Austrian' G&K, GPS calculated
|
|
G&K (note X&Y reversed on these, cos that's the way the GPS displays
|
|
them).
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|
|
|
<P>The easiest way to get this into Survex for plotting is to put in a column
|
|
that says *FIX <stationname> in front of these columns, and putting an
|
|
altitude value in at the end. Then by hiding all the data lines, and all the
|
|
columns except the G&K (Austrian) ones, this section of the spreadsheet
|
|
can then be exported to a text file and processed directly. By including the
|
|
waypoints marked in the field (and given a similar treatment to the track log
|
|
data), it is possible to see whether the averaged results are noticeably
|
|
better than the waypoint results.
|
|
|
|
<P>Here is typical example of the results:
|
|
|
|
<P><IMG SRC="img175.png"><br>Figure 2
|
|
showing 5 fixes for entrance 161d, and the 5 corresponding averaged
|
|
positions.
|
|
|
|
<P>The averaged positions for locations 1,2 and 5 are all within about 25m of
|
|
the surveyed point 161d, whereas the waypoints 1,2 & 5 are all 50-100m
|
|
away. However the 3 & 4 averages are slightly further away than the
|
|
corresponding waypoints. Location set 4 lasts for 25 minutes. After 20
|
|
minutes the position suddenly zooms off 500m to the E. It is this sort of odd
|
|
behaviour that means a GPS reading alone can be thoroughly misleading. The
|
|
bar on the left shows the 100m limit, and the readings are nearly all with in
|
|
this, although not quite.
|
|
|
|
<P><IMG SRC="img176.png"><br>Figure 3, showing the area
|
|
of the plateau near top camp with surface surveys and GPS positions.
|
|
|
|
<P>I have selected a number of locations, the corresponding GPS waypoint, and
|
|
the averaged track log data for it. As you can see the averaged data is
|
|
closer to the actual point than the waypoint for Laser7_11, p90 and p164,
|
|
although in the case of p164 there is hardly any difference. These locations
|
|
were averaged for only about 20 minutes. The B11 data illustrates the
|
|
importance of examining the data graphically, and checking suspicious stuff.
|
|
The point marked GPSavB11 Was one where the track log had clearly rested
|
|
somewhere for a few minutes, and the timing (at the end of a day), and known
|
|
route that day suggested that it was B11, done on return to top camp. This
|
|
was what got written down (with a question mark in the spreadsheet). After
|
|
all the grid processing and displaying the results in Survex I note that the
|
|
waypoint at B11 is much closer to the known position of the cave than the
|
|
GPSavB11 location. A quick check back to the track and waypoint data shows
|
|
that the waypoint was done two days later, and that there was a question mark
|
|
next to the GPSavB11 name. This all means that this isn't B11 at all, but a
|
|
short log at top camp whilst waiting for the tea to brew. Again - record
|
|
keeping is essential to avoid this sort of mistake.
|
|
|
|
<H4>Summary:</H4>
|
|
|
|
<P>GPS is definitely useful in the field, but you need to understand its
|
|
limitations. To get the best out of it you need to be diligent about how and
|
|
when you record information so that you can get the data out reliably
|
|
afterwards, and you need to understand about the co-ordinate system in use in
|
|
the area, and on the maps you have. Averaging is a useful technique, but can
|
|
be a rather hit-and-miss unless you can stay at a spot for at least an hour,
|
|
and for good results you need to be logging for at least 4 hours, which means
|
|
you need sufficient battery power, and may not always be convenient in the
|
|
field. Also beware turning your GPS on, taking a waypoint immediately and
|
|
walking off - it could be a <I>long</I> way out. If Differential GPS
|
|
transmissions are available in the area, then that will be much more
|
|
effective in giving accurate answers quickly - but of course it costs about
|
|
twice as much money, even if you don't have to pay for the data as well as
|
|
the receiver. As more of the far-flung caves in our area are surveyed-to, we
|
|
will be able see how it compares in accuracy to very long surface surveys.
|
|
And some time in the next ten years the US should turn off SA which will make
|
|
life significantly easier for this sort of work.
|
|
|
|
<HR>
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