Hallo,

is it possible using the Curve and surface analysis module to fit a curve or surface to point data but to define a constrain in the adjustment, i.e., fix one or more parameters of the target curve or surface function?

To be specific, I am trying to fit a sphere of known radius to 3D point data. Therefore, the radius of the sphere would not be an unknown parameter in the adjustment, only coordinates of the sphere center.

If it is possible, in what way can it be defined in JUniForm module?

I would very much appreciate if you took the time to read and answer my question.

Thank you,
Antonio

Hello,

If it is possible, in what way can it be defined in JUniForm module?

Yes it is. Let the following points lying on a sphere

# Reiner Jäger, Tilman Müller, Heinz Saler, Rainer Schwäble: Klassische und robuste Ausgleichungsverfahren
# 1. Auflage, 2005
# ISBN: 3-87907-370-8
# Kugelausgleichung S. 331f

1 14.551  9.495  8.433
2  9.071  5.002 10.892
3 12.865  4.306  8.775
4  9.170  7.898 11.930
5  9.993  9.575 11.745
6 12.152  6.017 11.054

If a sphere is fitted using JUniForm one obtain the following results:

The radius is about r = 5 m. If the radius r is known (with superordinate accuracy), it is possible to restrict the parameters. Let r = 6 m be the known radius. To fix the parameter, go to Properties --> Parameter. Change the Processing type to Fixing, and the Initial guess to 6.

It is almost impossible to derive appropriate approximations, if user defined surfaces are defined. Therefore, you have to disable the option. Go to Adjustment --> Least-squares and uncheck the option Derive initial guess. (To get appropriate approximations, just fit the sphere without restriction beforehand, afterwards add the radius restriction as described above.)

The restricted results are

kind regards
Micha

Micha,

thank you very much for your reply. I understand now!
The unchecking of the Derive initial guess option was the missing piece.
It works perfectly .

Thank you,
Antonio

Hello Antonio,

The unchecking of the Derive initial guess option was the missing piece.

Maybe, I should add some kind of notice, to avoid confusions, something like e.g.:

For some options the automatic estimation of an initial guess must be disabled, and appropriate approximations have to be specified manually.

regards
Micha

Micha,

I agree.

Best wishes,
Antonio

Hello Antonio,

within the upcoming release, a message will be shown, i.e.,

Kind regards
Micha

Hello,

FYI: the new release JAG3Dv20210222 contains the notification to avoid confusion.

regards
Micha