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2001-08-15 19:29:27 +01:00
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2004-04-22 19:13:46 +01:00
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<title>CUCC Expo Surveying Handbook: GPS</title>
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2004-04-21 10:35:34 +01:00
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2004-04-24 12:50:58 +01:00
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<h2 id="tophead">CUCC Expo Surveying Handbook</h2>
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<h1>Locating entrances by GPS</h1>
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<h3>Which point to fix</h3>
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<p>The entrance to a cave significant enough to get a number and a survey
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will eventually be marked by a numbered tag attached to a spit. This
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will then become the primary survey station - ie. the point where an
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underground survey will start, and the point to which a surface survey
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should go. It's worth thinking about where you would put such a tag right
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from the start. Unlike the first rigging bolt (often used as the first
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point of a survey in the past) it should be sited with a particular
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view to its visibility and accessibility without having to put on SRT
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kit. If such a point has a clear view of the majority of the sky, then
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this is the point to use for a GPS fix too.</p>
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<p>If however, you are dealing with a cave at the foot of a cliff, or
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otherwise with a restricted view of the sky, then choose instead a good
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landmark with a clear view, and within one (or maybe two) survey shots
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of the entrance. If you have found a group of caves close together, it
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might be better to GPS a central point rather than get quick (but less
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accurate) fixes on each entrance.</p>
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<h3>Taking the fix</h3>
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<p>Once you have chosen your point, mark it in some way (could be a spit hole
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or a cairn, for example - we aren't supposed to use paint any more) and place
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the GPS on the point. If you build a cairn, make it wide rather than high -
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tall cairns are knocked down by the depth of snow each winter. Give it a
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couple of minutes to get a fairly good fix (the first figure reported may be
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quite a way out, but after a couple of minutes things should settle). Then mark
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the point as a waypoint. If you're feeling really keen, you can set it up for
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<i>averaging</i>, which gives a more accurate fix – some GPS receivers
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support this automatically, and with others you can just leave it recording a
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track log, then record another waypoint at the same place just before you leave
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so it's clear to someone examining the track log when you actually left.
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(Averaging was once crucial to getting any kind of remotely close fix, but is
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somewhat less important these days now that Selective Availability has been
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turned off.)</p>
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<p>While the GPS is recording your location, you can do something useful
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(like rigging the cave, doing a surface survey from the GPS point to the
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marker spit, looking for other caves, or even having lunch!) Remember to stop
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the waypoint averaging before moving the unit or changing the display page.
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<b>Take a photo of your GPS point showing at least one of your cave entrances
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too.</b></p>
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<h3>Coordinate systems</h3>
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<p>It doesn't especially matter what display options are selected when you
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are getting the GPS fix, but it is important to use standard ones when writing
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down the reported position in the survey book. The usual systems are either
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Lat/Long with the WGS84 datum, which all GPSses support out of the box; or
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the Austrian BMN (Bundesmeldnetz) system, which should be relative to the
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Austrian MGI datum (Hermannskogel), which is what we use for our surveys. It
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doesn't matter <i>too</i> much what combination of parameters you actually use
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as long as you record what they were, since we have conversion programs that
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can convert coordinates between the systems; but remember to record <b>which
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grid and which datum</b> your GPS was set for when you copy the fix into the
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survey file. (Besides the risk of introducing severe errors, it is an
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extraordinarily tedious task to have to repeatedly try all the plausible
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combinations of grid and datum that a given set of numbers might be in, convert
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them all to dataset coordinates and see which ones give answers in vaguely the
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right place, which is what someone will have to do if you don't write the
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details down.)</p>
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<p>If you want to set up your GPS to use the same coordinates the survey data
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set uses, which makes life a lot easier, then these are the runes to use as a
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"User Grid" if the unit doesn't support BMN coordinates immediately (which
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Garmin ones don't, for example):</p>
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<div style="background-color: #BDB">
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<table>
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<tr>
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<th>Ellipsoid:</th><td>Austrian (Bessel 1841)<br />
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a = 63377397.155m (ΔA = 739.845)<br />
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1/f = 299.1528128 (Δf x 10,000 = 0.10037428)
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</td></tr>
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<tr>
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<th>Datum:</th>
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<td>Austria MGI (Hermannskogel)</td></tr>
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<tr><th>Projection:</th><td>Transverse Mercator (BMN zone M31)</td></tr>
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<tr><th>North reference:</th><td>Grid</td></tr>
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<tr><th>Grid parameters:</th><td>Central meridian 13°20'E<br />
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False easting 450km<br />
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No additional scaling<br />
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Grid boundaries at 11°50' and 14°50'</td></tr>
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<tr><th>EFEC coordinate conversion<br />equation with respect to WGS84:</th>
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<td>Offsets Δx = -575m, Δy = -93m, Δz = -466m<br />
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Rotations ω<sub>x</sub> = 5.1"; ω<sub>y</sub> = 1.6", ω<sub>z</sub> = 5.2"<br />
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Scaling -2.5ppm
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</td></tr></table>
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</div>
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2017-07-22 01:14:48 +01:00
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<p>Note: The table above used to incorrectly give the y rotation as 5.1".
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But there's a slightly more accurate version in the
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<a href="coord.htm#ftnBMN">Coordinate Systems section</a>.</p>
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2005-05-31 14:10:49 +01:00
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<p>(Technical note: the BMN grid is actually the same as Universal Transverse
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Mercator zone 31, but setting your GPS for UTM will give rather different
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coordinates, as the plateau is actually in zone 33. My understanding of the
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situation is that the Austrians have found it more convenient to extend one
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grid to cover the whole country, thus deviating from UTM for the areas of the
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country which are just over the grid boundary. This is an important gotcha to
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watch out for, since while all the other coordinate systems produce answers in
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recognisably different formats, UTM 33 coordinates look like dataset
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coordinates but are offset by a couple of kilometres. Having your GPS set to
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the wrong datum produces even more subtle errors - the difference between BMN
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grid + WGS84 datum and BMN grid + Austrian datum is an offset of around 500m to
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the south and 50m in altitude.)</p>
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2004-09-21 22:32:35 +01:00
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<p>A good way of testing that your GPS is correctly set up is to set it WGS84
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Lat/Long and enter a waypoint for a point whose coordinates are known –
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such as the 204a tag, at 47°41.456'N 013°49.288' – and then
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change the settings again to use the user grid. It will now convert this point
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into the new grid; if you check its coordinates, it should come out as
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something close to 486697E, 5283699N, which are the BMN coordinates for 204a.
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For use in the dataset we tend to subtract the 450km offset in the easting and
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ignore the first two digits of the northing, giving 36697E 83699N.</p>
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<p>Write down the figure that the GPS gives for each waypoint <i>at the
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time</i> (just in case some failure loses the data from the GPS memory –
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this has happened a couple of times in recent years, much to the annoyance of
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everybody involved). That's all you need to do at the cave. Get the GPS data
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downloaded to a computer next time you are in Base Camp (or Top Camp if someone
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has a laptop :-), and as a backup (expo computers break down surprisingly often
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it seems) write it down by hand on one of the A5 cave info sheets with all the
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other details of your cave and put that in the surveys ringbinder file. </p>
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<p>If you want to read about the nitty gritty of converting GPS coordinates to
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the ones used by the Kataster system, you can do no better than read the
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short introduction to <a href="coord.htm">coordinate systems</a>, which briefly
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says "it's horribly complicated and we use computer programs to do it properly".
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(A rather outdated first attempt at this can also be found in Wookey's
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<a href="../../years/1996/gps.htm">Compass Points Article</a> from 1996, which
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briefly says "it's horribly complicated and we don't really know how to do it
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properly".) Overall things have significantly improved since the early days,
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particularly as without the fog of the SA variation it's now easy to find out
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whether your GPS is set up right by just GPSsing a known point and comparing
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the results. However, the main point of having a GPS fix on an entrance is
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so we can find it again and be sure it is the same one!</p>
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<hr />
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<ul id="links">
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<li><b>Expedition Handbook:</b>
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<ul>
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<li>Expo Handbook <a href="../index.htm">Introduction</a></li>
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<li><b>Surveying:</b>
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<ul>
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<li>Surveying <a href="index.htm">Overview</a> and index of
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topics</li>
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<li><a href="what.htm">What is a cave survey?</a></li>
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<li><a href="why.htm">Why am I doing this?</a></li>
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<li>Methods: <a href="how.htm">underground</a></li>
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<li>Pitfalls to avoid, <a href="hints.htm">hints'n'tips</a> to
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make life easier</li>
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<li>Methods: <a href="ontop.htm">surface</a></li>
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<li>Base Camp: <a href="getin.htm">getting it in</a> to the
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computer</li>
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<li>Base Camp: <a href="drawup.htm">drawing it up</a>, writing
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the description</li>
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</ul></li>
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<li><a href="../look4.htm">Prospecting guide</a></li>
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<li><a href="../rescue.htm">Rescue guide</a></li>
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<li><a href="../rigit.htm">Rigging guide</a></li>
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<li><a href="../photo.htm">Photography guide</a></li>
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</ul></li>
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<li><a href="../../infodx.htm">Index to info/topics pages</a></li>
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<li><a href="../../indxal.htm">Full Index to area 1623</a>
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<ul>
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<li><a href="../../areas.htm">Area/subarea descriptions</a></li>
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</ul></li>
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<li><a href="../../index.htm">Back to Expedition Intro page</a></li>
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<li><a href="../../../index.htm">Back to CUCC Home page</a></li>
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</ul>
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2001-08-15 19:29:27 +01:00
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