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Re: Dip-meter again
From: Richard B. Langley
Date: 2012 Apr 10, 10:01 -0300
From: Richard B. Langley
Date: 2012 Apr 10, 10:01 -0300
Thanks, Alex, but I was not talking about ordinary averaging but the use of parametric least squares, which is able to estimate the value of a bias along with the parameters of interest. So, if we have a series of observations for which we can assume that the bias was reasonably constant, then by simultaneously processing the complete set, one should be able to get a single estimate of position and the value of the bias (dip). -- Richard On 10-Apr-12, at 9:49 AM, Alexandre E Eremenko wrote: > Dear Richard, > > Unfortunately, no statistical method, including least squares > can help with dip. The reason is that dip can deviate from its > normal value for relatively long periods. > For example, if our much discussed observation with Bill B on lake > Michigan is explained by the dip (which a majority on the list seems > to believe), this anomalous dip persisted for several hours, > and was almost constant. (This is an extreme example of course). > What averaging (or least square) helps to eliminate is a > SUM of MANY small INDEPENDENT errors. > The error of the dip is not a "random" error but a "systematic" one. > And the only way to eliminate it is the use of some dip-meter device. > > However, we know that dip-meters were rarely used. > (Western manuals almost never mention the device, > Soviet ones do mention, and recommend, and it was a standard > equipment, > but the same manuals recognize that "people do not use it"). > > This only shows that navigators did not care about anomalous dip. > That high accuracy in celestial navigation was not needed, > and that large variations of the dip are probably rare. > > Alex. > > On Tue, 10 Apr 2012, Richard B. Langley wrote: > >> >> Warning: academic exercise follows ;-) >> >> Perhaps if one has sufficient redundant observations and uses least >> squares to estimate position, one could include dip as an additional >> quantity estimated simultaneously from the (biased) observations. The >> same procedure is used to process GPS measurements where one of the >> "nuisance" parameters is the offset of the receiver's clock from GPS >> System Time, which is generally unknown. >> >> -- Richard Langley >> >> On 10-Apr-12, at 1:31 AM, Antoine Cou�tte wrote: >> >>> Still, your observations once again point out that DIP is definitely >>> one "weak link" in the accuracy computation chain, since even under >>> (quite) good conditions, dip standard deviation was already close to >>> 0.15/0.20 arc minute. >>> >> >> ----------------------------------------------------------------------------- >> | Richard B. Langley E-mail: >> lang---ca | >> | Geodetic Research Laboratory Web: http://www.unb.ca/GGE/ >> | >> | Dept. of Geodesy and Geomatics Engineering Phone: +1 506 >> 453-5142 | >> | University of New Brunswick Fax: +1 506 >> 453-4943 | >> | Fredericton, N.B., Canada E3B >> 5A3 | >> | Fredericton? Where's that? See: http:// >> www.fredericton.ca/ | >> ----------------------------------------------------------------------------- >> >> >> >> >> >> : http://fer3.com/arc/m2.aspx?i=118883 >> >> ----------------------------------------------------------------------------- | Richard B. Langley E-mail: lang@unb.ca | | Geodetic Research Laboratory Web: http://www.unb.ca/GGE/ | | Dept. of Geodesy and Geomatics Engineering Phone: +1 506 453-5142 | | University of New Brunswick Fax: +1 506 453-4943 | | Fredericton, N.B., Canada E3B 5A3 | | Fredericton? Where's that? See: http:// www.fredericton.ca/ | -----------------------------------------------------------------------------
