Fit a linear regression to a 3D plot

Newbie
1

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ROOT Version: 6.30/06
Platform: ubuntu
Compiler: Not Provided

I save my data in an textfile and it it loos like this (2 21 0 4 13 1 15 31 2) i.e x1,y1,z1,x2,y2,z2,x3,y3,z3. I plotted this to a 3D histogram but I have trouble with fitting a linear regression through the data set. I attach my macro.
FitMuonTrajectory.C (1.6 KB)

2

Hi,

It is hard to understand what exactly the problem is. Can you reduce the example to a few lines, make explicit what you expect and expose what you get?

Cheers,
D

3

You need to use TF2 instead of TF1 because your fit function is two-dimensional, as you write in the formula and in the comment. That is a mistake for sure.

Try with TF2 and see if it works…

4

Thanks TF2 works fine. How could I modify this so that fit is linear regression in xy and xz plane?

5

You could create two TGraphs and fill them with the x, y, and x, z positions, respectively.
And then fit each independently with a linear function?

6

Yes but I thought I could all at once. I guess I will just have to create two TGraphs then. Once again thanks for the help.

7

In principle if you have the formula for the 3D line.

It is easy to manually get it. It is just a linear algebra exercise.

Set z=0, and it gives you the equation for the line in xy
Set y=0, and it gives you the equation for the line in xz

I just noticed that in your function, you actually use the equation for the plane, not the 3D line!

I guess that is not what you intend to do.

I googled, and I found a ROOT example that does 3D line fit. You can use that as an inspiration.
https://root.cern/doc/master/line3Dfit_8C.html

cheers

8

Woww!! That helps a lot. Thank you so much.!!!