OK, having studied the link from KISS, I think I now understand the problem.
The measuring tool will measure one leg of a multi-leg earthing system, but the measured resistance for that one leg will be the leg resistance in series with all the other legs connected in parallel.
This may seem an odd way of measuring, but if the measured resistance of a leg is below the maximum resistance allowed for that leg, then the leg is within specification. The measurement technique will always read a higher resistance that the resistance of the individual leg which is measured.
That sounds very complicated, but I cannot explain it any simpler.
What Grossel wants to do, is to calculate the true resistance of each leg.
This can get messy.
If we start the analysis with a simple arrangement of three resistors, and we measure R1, we effectively measure (R1 + R2//R3) which we will call RT1.
Using a bit of maths, we can make an expression for RT1 in terms of R1, R2 and R3
This can be manipulated to give R1 in terms of RT1, R2 and R3.
We can also derive expressions for R2 and R3.
If we try and use these expressions as simultaneous equations to find R1, R2 and R3, we end up going around in circles and getting no where.
We need to use a bit of mathematical guess work and repetition known as iteration.
This is best done with a computer program or we can use something like MS Excel.
In this spreadsheet,
the measured resistance values are in the red cells
the guesstimated true resistance values for R1, R2 and R3 are in the green cells
the blue cells show the true values for R1, R2 and R3 at each iteration.
The first line of the iteration (iteration number 1) takes the red values of RT1 to RT3 and the green values of R1 to R3, and calculates new values for R1 to R3.
The second line of the iteration (iteration number 2) takes the red values of RT1 to RT3 and the blue values of R1 to R3 from the first line of iteration, and calculates new values for R1 to R3.
The third line of the iteration (iteration number 3) takes the red values of RT1 to RT3 and the blue values of R1 to R3 from the second line of iteration, and calculates new values for R1 to R3.
And so on down to iteration number 20.
Looking at the graph, we see that the values of R1, R2 and R3 oscillate about a bit and then settle down to 5, 6 and 7 (Ohms).
These are the values I used to calculate the 8.32, 8.91 and 9.72 measured values for RT1 to RT3.
This method can easily be expanded up to N resistors, by having more columns for RT1 to RTn and R1 to Rn, with the appropriate expressions for R1 etc as written above but with terms up to Rn.
I hope that this almost makes sense.
JimB