Hi Bruce, I also tried to customize the quality of the measurements a while ago to be able to take into account important external loops that are less accurate than other, and more importantly to weight correctly different GPS and cartographic reference points.
The underlying idea is to assume that the measurements follow a Gaussian law with standard deviation sigma. In the statistical setting, you can show that in this case, you have to replace least-square estimations in the loop closure correction by weighted least-squares, where the weights are actually the (square of the) inverse of the the standard deviation. Although this is not documented in the Therion-book, this is described shortly in the survex manual (see below) and in different books on cave survey (A reference book in French is Speleographie by Faucher and Mautref, but I think this is also detailed in On station and other English survey book, although I am less familiar with them). In practice, the 3-sigma rule says that 99.7% of your measurements are within the bounds [exact value -3*sigma ; exact value + 3* sigma]. This is why a tape measure which is assumed to be within ±0.1 meter should have a std dev of 0.033m. One can also use the fact that about 95% of the values are within two standard deviations. The default definition of the grades BCRA3 and BCRA5 are in the file therion/lib/grades.th. These measures closely follow survex' ones. The value retained by survex and therion correspond to 3*sigma (i.e. 99.7%) although it is said to be 95% in the comments. They do correspond to the BCRA definition of these grades (http://bcra.org.uk/surveying/index.html) except for the angular measures in grade 3 which is 10 degrees instead of 2.5 degrees: * Grade 3: A rough magnetic survey. Horizontal & vertical angles measured to ±2.5º; distances measured to ±50 cm; station position error less than 50cm. * Grade 5: A Magnetic survey. Horizontal and vertical angles measured to ±1º; distances should be observed and recorded to the nearest centimetre and station positions identified to less than 10cm. Compass Point #14 (http://www.chaos.org.uk/survex/cp/CP14/CPoint14.htm) gives the explanation of this discrepancy: > How does the Survex file get from +/- 2.5 degrees to a standard deviation of 3.33 degrees? > [Wookey replies: ah, yes, well spotted. That's a mistake, caused by me forgetting to check the spec!. It should indeed read > 0.83 degrees for compass & clino SD, and 0.17 for tape and position SD. > Anyone using this feature in Survex v 0.70 should amend their BCRA3.SVX file.] Thus it seem that Therion's grade 3 definition should be updated (and 95% changed to 99.7% in the comments). In practice, however, I found that using this definition of grade 3 for unreliable surveys is quite good as the tape reading is usually very easy (it is resonnable to assume that it is within +/- 0.5m) while compass and clino measurements can be easliy wrong by 5 to 10 degrees for untrained people. Thus, I use in practice: * grade 3 for unreliable surveys, * grade 5 for relieable ones, * sd of 1 1 5 meters of SD for fixed points which coordinates are determined from digital geo-referenced maps, * sd of 10 10 30 to 30 30 100 meters for GPS points with my old Garmin 12 (sd of 2 times the FOM in X and Y and 3 times larger in Z because of the triangulation errors). For even older GPS measurements, I used 3 times the FOM in X and Y and 9 times in Z). Hope this helps. Xavier Survex manual explanation of std dev: -------------------------------------- Syntax: *sd <quantity list> <standard deviation> Example: *sd tape 0.15 metres Description: *sd sets the standard deviation of a measurement. <quantity> is one of TAPE|COMPASS|CLINO|COUNTER|DEPTH|DECLINATION|DX|DY|DZ <standard deviation> must include units and thus is typically "0.05 metres", or "0.02 degrees". See *units below for full list of valid units. To utilise this command fully you need to understand what a standard deviation is. It gives a value to the 'spread' of the errors in a measurement. Assuming that these are normally distributed we can say that 95.44% of the actual lengths will fall within two standard deviations of the measured length. i.e. a tape SD of 0.25 metres means that the actual length of a tape measurement is within + or - 0.5 metres of the recorded value 95.44% of the time. So if the measurement is 7.34m then the actual length is very likely to be between 6.84m and 7.84m. This example corresponds to BCRA grade 3. Note that this is just one interpretation of the BCRA standard, taking the permitted error values as 2SD 95.44% confidence limits. If you want to take the readings as being some other limit (e.g. 1SD = 68.26%) then you will need to change the BCRA3 and BCRA5 files accordingly. This issue is explored in more detail in various surveying articles. Bruce Mutton a écrit : > > Time has come to assign my least accurate survey data a lower accuracy! > > Iâm curious as to how Therion deals with the std deviations. Had a > look, but can not find much more than is in the Therion book. > > First, what are the default values adopted by Therion? Currently I > have only one survey in a large(ish) dataset that has been allocated > sdâs. From a casual look at the distortions it clearly works â in that > a nice result is obtained with more distortion in the lower accuracy > area. I guess that if the default values are zero, then all the > distortion would be forced to an area given non-zero sdâs; Is this how > it works? > > In this case it would seem that if one survey parameter has a sd > setting, all the surveys should all have their respective settings > allocated for this parameter to more accurately model a realistic > distribution of corrections. > > Second, has anyone made any attempt to associate sd values with > recognised cave survey grades (bcra grades for example). > > Here is my off the cuff assessment of what in NZ some people call > bcra5 but is really well short. > > grade NZBCRA4 # Bruces assessment of the BEST typical NZ surveying > practice MIGHT achieve > > tape length 0.1 meter # ie -0.2m to +0.2m for measurement to chest to 0.1m > > compass bearing 1 deg # ie -2 deg to +2 deg for sighting to light of > person on station to 1 deg > > gradient clino 1 deg # ie -2 deg to +2 deg for sighting to light of > person on station and sometimes accounting for 'eye to light' offset > to 1 deg > > endgrade > > I suspect the sdâs may often be larger than this. The actual sd for > polar measurements probably depends a lot on the average length of the > legs I expect > > Thanks > > Bruce > > ------------------------------------------------------------------------ > > _______________________________________________ > Therion mailing list > Therion at speleo.sk > http://mailman.speleo.sk/mailman/listinfo/therion > -------------- next part -------------- An embedded and charset-unspecified text was scrubbed... Name: grades.th URL: <http://mailman.speleo.sk/pipermail/therion/attachments/20081029/d06947fd/attachment.ksh> -------------- next part -------------- A non-text attachment was scrubbed... Name: Xavier_Pennec.vcf Type: text/x-vcard Size: 166 bytes Desc: not available URL: <http://mailman.speleo.sk/pipermail/therion/attachments/20081029/d06947fd/attachment.vcf>
