Topic:
Mistie Analysis correction algorithm (L1 and L2) explanation

Description:

Solution:
L1 Correction (Constant)

Process steps:
1.) The program computes the target correction for each line by subtracting the
    residual mistie multiplied by a dampification factor (0.5) from the computed
    mistie correction from the last iteration.

     -the residual mistie of a tie point is the difference between the estimated
      mistie at the point, and the computed mistie at the point.

     -the residual mistie of a line is the median of residuals for all of the
      tie points on the line.  If the median falls between two values, then the
      program picks the larger of the two.

In the beginning of the process, the computed correction for all the lines is
zero.  The process is iterative.

2.) The program finds the target corrections for all reference lines (assuming
    they are to be shifted).
    This is done by summing the computed corrections for each reference line 
    and dividing by the number of reference lines.

3.) Next the value obtained in step 2 is subtracted from the computed correction
    so that the net change to the reference lines is zero.  

The code iterates through a number of times with the goal of balancing the
changes across all the tie points.

Results - bulk shift of one balanced value applied to every trace of the
non-reference line.


L2 (Variable)

Process steps:
1.) The program computes the weightage of each line.  This is computed as the
    sum of the mistie estimates of all interactions associated with the line.
    This is then normalized across the line.

2.) The weighted average of the 'arrival time' for each intersection is computed
    using the estimates and weightages from step 1.  The 'arrival time' of an
    intersection is the maximum value of the mistie estimate from the
    crosscorrelation at the intersection.

For example:  If we have W(I) and W(J) as the computed weightage for lines I and
              J respectively.  G(IJ) is the 'arrival time' of line I at 
              intersection with line J, and G(JI) is the 'arrival time' of
              line J at intersection with line I, then the weight average of 
              mistie is:

              (G(IJ)*W(I) + G(JI)*W(J)) / (W(I) + W (J)).

3.) The program then computes the correction at each intersection as the
    difference of weighted 'arrival time' computed in step 2 and the estimated
    arrival time.  

Results - bulk shift of differing values applied to every trace of the
non-reference lines.



Last Modified on: 25-NOV-98