Fitting a Logarithmic Spiral to Empirical Data With Displaced Origin
7 Pages Posted: 25 Apr 2006
Date Written: April 20, 2006
Logarithmic spirals are abundantly observed in nature. Gastropods/cephalopods (such as nautilus, cowie, grove snail, thatcher, etc.) in the mollusca phylum have spiral shells, mostly exhibiting logarithmic spirals vividly. Spider webs show a similar pattern. The low-pressure area over Iceland and the Whirlpool Galaxy resemble logarithmic spirals.Many materials develop spiral cracks either due to imposed torsion (twist), as in the spiral fracture of the tibia, or due to geometric constraints, as in the fracture of pipes. Spiral cracks may, however, arise in situations where no obvious twisting is applied; the symmetry is broken spontaneously. It has been found that the rank size pattern of the cities of USA approximately follows logarithmic spiral.
The usual procedure of curve-fitting fails miserably in fitting a spiral to empirical data. The difficulties in fitting a spiral to data become much more intensified when the observed points z = (x, y) are not measured from their origin (0, 0), but shifted away from the origin by (cx, cy). We intend in this paper to devise a method to fit a logarithmic spiral to empirical data measured with a displaced origin. The method is also be tested on numerical data.
It appears that our method is successful in estimating the parameters of a logarithmic spiral. However, it may be noted that the range specification is very important. We have observed that if ranges are guessed unduly large (in which the shift parameters lie), the efficiency of the method is marred. Moreover, estimated values of the parameters of a logarithmic spiral (a and b in r = a*exp(b(theta+2*pi*k) highly sensitive to the precision to which the shift parameters (cx and cy) are correctly estimated.
Keywords: Logarithmic Spiral, Growth Spiral, Bernoulli Spiral, Equiangular Spiral, Cartesian Spiral, Empirical data, Shift in origin, change of origin, displaced pole, polar displacement, displaced origin, Curve Fitting, Spiral fitting, Box Algorithm, Non-linear Programming, multi-modality, Rank size rule
JEL Classification: C15, C63
Suggested Citation: Suggested Citation