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< title > CUCC Expo Surveying Handbook: GPS coordinate systems< / title >
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< h2 id = "tophead" > CUCC Expo Surveying Handbook< / h2 >
< h1 > GPS and coordinate systems< / h1 >
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< p > See also:< br >
< a href = "/eastings" > Troggle UTM data report< / a > < br >
< a href = "lasers.htm" > Geographical fixed points on Loser< / a > < br >
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< a href = "coord.htm" > Basic Coordinate Systems< / a > .< br >
< a href = "https://hoehle.org/downloads/SD_10_Handbuch.pdf" > SD 10 Handbook: Vergleich der Ö K 50 mit der neuen Ö K 50-UTM< / a > .
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< h3 id = "summary" > Summary - for 2023< / h3 >
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< p > Surveying in Austria does < em > not< / em > just use the latitude and longitude you may be familiar with from Google maps, your phone etc.
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< p > All our caves and locations in our survex files use the < a href = "https://hoehle.org/downloads/SD_10_Handbuch.pdf" > Austrian Caving national grid system< / a > , numbers which look like this:
< table class = "trad" >
< tr > < th > Northing< / th > < th > Easting< / th > < th > Altitude< / th > < th >
Description< / th > < / tr >
< tr > < td > 411563.49 < / td > < td > 5282622.35< / td > < td > 1867.95m < / td > < td >
p204a at the Stone Bridge< / td > < / tr >
< / table >
< p > These grid references are what we use to locate the entrances and to tie in the survey of a new cave in with the rest of the 180+ km of cave and surface surveys.
< p > Note that there are 6 digits starting 41... and 7 digits starting 52... This is not a mistake.
< p > However, when you prospecting and discover a new cave, you possibly only have the WGS84 latitude and logitude from your phone, e.g. < var > 47.690933 N 13.821467 E< / var > (degrees and decimals of degrees, set your phone to produce this: none of that degrees/minutes/seconds stuff, but if that's all that you have, we can work with that).
< p > So when you are recording the position of a completely new entrance, before you do any surveying, it will be the WGS84 lat/long that you will be writing down on the survey notes, and which will get scanned into the wallet back at base, and which you will enter into the < a href = "/handbook/survey/caveentry.html" > New Cave and Entrance forms< / a > on troggle (step 6 of the dave data processing guide) in the lat/long. data entry fields.
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< h3 > Coordinate systems< / h3 >
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< p > [The text from this point onwards is from before we had mobile phones, but it is correct.]
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< p > 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. The usual systems are either
Lat/Long with the WGS84 datum, which all GPSses support out of the box; or
the Austrian BMN (Bundesmeldnetz) system, which should be relative to the
Austrian MGI datum (Hermannskogel), which is what we use for our surveys. It
doesn't matter < i > too< / i > much what combination of parameters you actually use
as long as you record what they were, since we have conversion programs that
can convert coordinates between the systems; but remember to record < b > which
grid and which datum< / b > your GPS was set for when you copy the fix into the
survey file. (Besides the risk of introducing severe errors, it is an
extraordinarily tedious task to have to repeatedly try all the plausible
combinations of grid and datum that a given set of numbers might be in, convert
them all to dataset coordinates and see which ones give answers in vaguely the
right place, which is what someone will have to do if you don't write the
details down.)< / p >
< p > If you want to set up your GPS to use the same coordinates the survey data
set uses, which makes life a lot easier, then these are the runes to use as a
"User Grid" if the unit doesn't support BMN coordinates immediately (which
Garmin ones don't, for example):< / p >
< div style = "background-color: #BDB" >
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< tr >
< th > Ellipsoid:< / th > < td > Austrian (Bessel 1841)< br / >
a = 63377397.155m (Δ A = 739.845)< br / >
1/f = 299.1528128 (Δ f x 10,000 = 0.10037428)
< / td > < / tr >
< tr >
< th > Datum:< / th >
< td > Austria MGI (Hermannskogel)< / td > < / tr >
< tr > < th > Projection:< / th > < td > Transverse Mercator (BMN zone M31)< / td > < / tr >
< tr > < th > North reference:< / th > < td > Grid< / td > < / tr >
< tr > < th > Grid parameters:< / th > < td > Central meridian 13° 20'E< br / >
False easting 450km< br / >
No additional scaling< br / >
Grid boundaries at 11° 50' and 14° 50'< / td > < / tr >
< tr > < th > EFEC coordinate conversion< br / > equation with respect to WGS84:< / th >
< td > Offsets Δ x = -575m, Δ y = -93m, Δ z = -466m< br / >
Rotations ω < sub > x< / sub > = 5.1"; ω < sub > y< / sub > = 1.6", ω < sub > z< / sub > = 5.2"< br / >
Scaling -2.5ppm
< / td > < / tr > < / table >
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< p > Note: The table above used to incorrectly give the y rotation as 5.1".
But there's a slightly more accurate version in the
< a href = "coord.htm#ftnBMN" > Coordinate Systems section< / a > .< / p >
< p > (Technical note: the BMN grid is actually the same as Universal Transverse
Mercator zone 31, but setting your GPS for UTM will give rather different
coordinates, as the plateau is actually in zone 33. My understanding of the
situation is that the Austrians have found it more convenient to extend one
grid to cover the whole country, thus deviating from UTM for the areas of the
country which are just over the grid boundary. This is an important gotcha to
watch out for, since while all the other coordinate systems produce answers in
recognisably different formats, UTM 33 coordinates look like dataset
coordinates but are offset by a couple of kilometres. Having your GPS set to
the wrong datum produces even more subtle errors - the difference between BMN
grid + WGS84 datum and BMN grid + Austrian datum is an offset of around 500m to
the south and 50m in altitude.)< / p >
< p > A good way of testing that your GPS is correctly set up is to set it WGS84
Lat/Long and enter a waypoint for a point whose coordinates are known –
such as the 204a tag, at 47° 41.456'N 013° 49.288' – and then
change the settings again to use the user grid. It will now convert this point
into the new grid; if you check its coordinates, it should come out as
something close to 486697E, 5283699N, which are the BMN coordinates for 204a.
For use in the dataset we tend to subtract the 450km offset in the easting and
ignore the first two digits of the northing, giving 36697E 83699N.< / p >
< p > Write down the figure that the GPS gives for each waypoint < i > at the
time< / i > (just in case some failure loses the data from the GPS memory –
this has happened a couple of times in recent years, much to the annoyance of
everybody involved). 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 :-), and as a backup (expo computers break down surprisingly often
it seems) write it down by hand on one of the A5 cave info sheets with all the
other details of your cave and put that in the surveys ringbinder file. < / p >
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< h3 > Nitty Gritty< / h3 >
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< a href = "../i/meridianstreifen.jpg" > < img src = "../t/meridianstreifen.jpg" > < / a >
< br > < br > < figcaption > Austrian Meridians< a href = "https://hoehle.org/downloads/SD_10_Handbuch.pdf" > SD 10 Handbook< / a > 8.7 - click to enlarge< / figcaption >
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< p > 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 the
short introduction to < a href = "coord.htm" > coordinate systems< / a > , which briefly
says "it's horribly complicated and we use computer programs to do it properly".
(A rather outdated first attempt at this can also be found in Wookey's
< a href = "../../years/1996/gps.htm" > Compass Points Article< / a > from 1996, which
briefly says "it's horribly complicated and we don't really know how to do it
properly".) Overall things have significantly improved since the early days,
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particularly as without the fog of the
< a href = "https://www.gps.gov/systems/gps/modernization/sa/" > Selective Availability< / a >
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
the results. 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!< / p >
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