Questions tagged [curve-fitting]
For questions about determining parameters for particular functional form based on set of given data points. These points may be the exact values generated from some underlying function or possibly have some additional noise component.
49
questions
1
vote
0answers
53 views
Fitting data with a Voigt function
I have some data, (xrd data), that I would like peak fit with a pseudo-Voigt function, a combination of a Gaussian and a Lorentzian function. These are the functions
$G(x) = I \exp\left( -\frac{4\ln(2)...
4
votes
1answer
630 views
Why is my curve_fit not producing the covariance matrix and the correct values for the unknown variables?
I am trying to fit supernova data into a scipy.curve_fit function. However, when my code runs, the values of the unknown variables given by ...
1
vote
3answers
121 views
Fitting line to a staircase function
I have a staircase/step function $n(E)$. I know the points $\{E_i\}$ at which each "step" occurs and all steps are of constant height 1. I need to fit a line $a + bE$ to this function and ...
0
votes
2answers
136 views
Computing infinite series with iterated functions
I found this question (linked here) which asks to find what this infinite series converges to
$$ \sum_{n=1}^{\infty} \int_0^{\pi} f_n(x) dx $$
where $f_{n+1}(x) = \sin(f_n(x)) $ and $f_1 = \sin(x)$. ...
-1
votes
1answer
351 views
Fitting using curve_fit of scipy in python gives totally different answer for 1/t and t
I was trying to fit some data to a single degree exponential decay function but a*exp(-x*t) and a*exp(-x/t) gives completely ...
4
votes
0answers
58 views
Pade-like approximation, but force poles to be negative
Are there techniques to form a Pade approximation (or Pade-like approximation), except force the poles of the rational function to be negative?
I am trying to use Pade approximations to extrapolate a ...
2
votes
0answers
64 views
How to curve-fit the lower envelope of random sequence?
I'm more or less familiar with procedures and methods to fit a curve to experimental data, and I have done this many times using Matlab. However this time I have a problem that I'm not sure how to ...
3
votes
1answer
236 views
Good languages/packages for interior point optimization with non-linear constraints?
I'm currently using Python's scipy.optimize package to perform parameter estimation for a system of 10 ODEs. I have some observed data, and I'm trying to find the set of parameters which makes the ODE ...
0
votes
1answer
154 views
Dealing with arrays and fit function in Gnuplot
I want to evaluate the Birch-Murnaghan Equation of State (BM-EOS) for different volumes.
I tried declaring a 1D array A in which every element would be the answer ...
3
votes
0answers
55 views
Least-squares fit of explicit parabolic sheet to data points
For a given set of data points
$$\{(x_i, y_i, z_i)\}$$
there exists some
$$f_{ABC}(x,y)=Ax^2+Bxy+Cy^2$$
that minimizes
$$\sum_i(f_{ABC}(x_i,y_i)-z_i)^2$$
$A$, $B$, and $C$ can be found quickly ...
0
votes
1answer
78 views
Fit spline to point cloud with lowest energy
I am looking for a way to fit a spline of order 2 to a 2d image or point cloud.
The input will be an gray scale image. The start and end points are given as 2D coordinates.
The goal is to find a ...
2
votes
0answers
44 views
Good 3D surface fits for multiscale oscillatory surfaces
I have a 3D surface in $x$, $y$, and $z$. where $z$ is a function of $x$ and $y$ and my points are on a structured grid in $x$ and $y$. My function $z$ is highly oscillatory and irregular with ...
1
vote
0answers
36 views
Fitting a multivariate PDE (using Java)
I'm doing simulations of 2 coupled PDE's with Comsol Multiphysics. I want to fit some data (using the Application method, whose language is Java) to those simulations.
In order to answer my question ...
1
vote
0answers
15 views
Error on the fit parameters when several good fits exist
I am using the reduced chi-squared statistic to determine the goodness of fit. I run several simulations and determine that a parameter 'p' has a certain range of values that all give values between 0....
1
vote
1answer
230 views
What is a good library in Python for correlated fits in both the $x$ and $y$ data?
I have $x$ and $y$ data, both of which have their own covariance matrices. scipy.optimize.curve_fit will accept a covariance matrix for the $y$ data, called ...
0
votes
0answers
39 views
Fit exponential convergence
I'm working with a numerical algorithm whose output $y$ asymptotically approaches a certain unknown value $a$.
I expect an exponential convergence, i.e. the data $y$ given by my algorithm should be ...
1
vote
2answers
168 views
identifying peaks in data
I have data with peaks on some background, for example:
The two prominent peaks at ~390 and ~450, as well as the much smaller peak at ~840. What are some options to programmatically find the position ...
2
votes
1answer
143 views
How to check experimental data against a theoretical curve? (Python)
I am trying to check the agreement of a dataset against a theoretical curve, specifically a bandstop filter in an RLC circuit.
I have generated a function which describes the curve we expect from the ...
1
vote
1answer
952 views
Gnuplot: How can I determine the maxima of a fit function in gnuplot?
I have a set of data data.txt which can be fit to a Gaussian function, f(x). I want to determine the coordinates of the point of ...
1
vote
1answer
4k views
Gnuplot: How can I fit a range of points (out of the entire data) to a function?
I have a set of data obtained for the I-V characteristics of an LED.
...
2
votes
2answers
512 views
What equation should I fit this set of data points to?
I have done an experiment Estimation of silver nitrate by potentiometric titration with standard KCl solution.
A plot of $\dfrac{\Delta E}{\Delta V}$ versus Volume of KCl solution gives the ...
0
votes
1answer
2k views
Numpy.polyfit with regularization
I am trying to use the numpy polyfit method to add regularization to my solution.
My non-regularized solution is
...
0
votes
2answers
63 views
Finding parameters numerically
I suspect that a function $f(x,y)$ is of the form $f(x,y)=a(bx+c)^{dy+e}$. I have access to several values of $f(x,y)$. How do I proceed numerically to find the parameters $\{a,b,c,d,e\}$?
By ...
1
vote
0answers
51 views
Fit constant term in LM algorithm
I'm using the Levenberg-Marquardt algorithm to fit my data with a Gaussian function:
$$
f(x)=a\cdot e^{-\frac{(x-c)^2}{2\sigma^2}}+f_0
$$
$a$, $c$, $\sigma$ and $f_0$ are the fitting parameters. The ...
1
vote
1answer
156 views
Least square with rectangular function
I have the function $c(t) = A \cdot \cos \left(\dfrac{2\pi}{\tau} \cdot t + \phi \right) $
which is used to define
$ T(t) =
\begin{cases}
M + c(t), & c(t) > 0 \\
M, &c(t) \leq 0.
\end{...
15
votes
3answers
976 views
Fitting Implicit Surfaces to Oriented Point Sets
I have a question regarding quadric fit to a set of points and corresponding normals (or equivalently, tangents). Fitting quadric surfaces to point data is well explored. Some works are as follows:
...
1
vote
1answer
559 views
matlab lsqcurvefit parameter estimation journey
The lsqcurvefit solution in matlab converges at different solutions depending upon the initial guess:
Surface represents the error (SSE) between model and data at various combinations of parameters ...
3
votes
2answers
197 views
Optimization of known function with respect to two unknown function arguments
I have a data set, composed of points $(x_i, y_i)$ for $i=1,N$. I also have a known function $F$, which maps these points $x_i$ to $y_i$ as such $F(x_i, a(x_i),b(x_i)) = y_i$, where $a(x_i)$ and $b(...
0
votes
3answers
201 views
Choice of solver/software for global optimisation of cheap black-box function with known derivatives
I am trying to estimate a few unknown parameters of my continuous non-linear PDAE model (simulated through finite-volume method spatial discretisation, and time-stepping through method-of-lines). I am ...
2
votes
4answers
3k views
Fit best polygon to a discrete contour
I have a discrete contour represented by a set of points. The contour looks like a polygon but if you zoom you see that the edges are rugged (that's because it was obtained while working on a finite ...
4
votes
1answer
95 views
Post-processing the noisy results of numerical simulation
I have the following curve, which is calculated on a large number of points and shows smooth behaviour when viewed from distance.
However, the derivative (shown below) exhibits artificial ...
1
vote
2answers
154 views
Finding an enclosing parabola for a set of points
I am looking for an algorithm that fits a parabola to a set of data points. However, no data point may be below the parabola. All points must be above the parabola. Is there an standard algorithm for ...
1
vote
1answer
1k views
Curve fitting for oscillating data
This is my first question. I have the following data that I'd like to approximate as a parametric function:
\begin{align}
y = a + (bx_1 + cx_2 + dx_3 + ex_1x_2 + fx_1x_3 + gx_2x_3 + hx_1x_2x_3 + i)*(...
5
votes
3answers
1k views
Fitting with a linear combination of exponentials
I want to elaborate on a statement I read in Acton's "Numerical methods that work", paragraph "Exponential fitting", page 252.
Computationally we are being asked to fit only the parameters $A$ and $...
2
votes
1answer
519 views
Pseudo Code for non linear power function fit needed
I am struggling finding pseudo Code for a non-linear fit of the following function:
$y = a\, x^b$
Package NLS in R does perform well, but utilizing external software is not practicable in my program ...
1
vote
1answer
185 views
Fit curve with rectangles
I have a one-dimensional set of points, i.e. $(n,y_n), 1\leq n \leq N$. I want to fit them with a linear combination of $k$ rectangular functions in a least-squared-error sense. Each rectangle is ...
4
votes
1answer
202 views
Question on speed and accuracy comparisons of different 2D curve fitting methods
This may be a trivial question, and I apologize if so. Consider the following simple problem: We have a 2D, regular grid of points (say $X = [0,5000] \times [0,5000]$) spaced uniformly by units of 1 (...
7
votes
1answer
654 views
Problems Implementing the Remez Algorithm
So first off:
*** This code is not being used in production software.
It is a personal project of mine, trying to understand
approximation theory and advanced curve fitting.
In other words, I'm ...
2
votes
0answers
52 views
Inverted value is not consistent with expectation
We have a group of observations
$$y = f(x_1, x_2, x_3) \enspace .$$
We have also a forward model $y = f(x_1, x_2)$. The forward model does not include $x_3$ because $x_3$ might include dozens of ...
0
votes
0answers
144 views
1D fit on a surface in matlab
I have a problem fitting a $N\times N$ ($32\times 32$ in this case) matrix in Matlab.
Here is the deal
I have this image, and I want to create 1 dimensional fit between the points and then be able ...
1
vote
1answer
261 views
Curve Fitting problem MATLAB
I have a system of differential equations with some unknown parameters and I need to find the optimal parameter set that fits best with the data I have.
For each parameter set choice (using for loops)...
0
votes
1answer
596 views
Power series regression linear fit in VBA excel
I wrote a program that calculates the best fit in VBA excel for the following model
$$
y_k=c_1x_k+c_0+c_{-1}(x_k)^{-1}
$$
solving for the best fit parameters $c_1$, $c_0$, and $c_{-1}$. However I ...
1
vote
0answers
115 views
How to curve fit an unknown function?
I have data which can be described by $y=f(x,z)$ where $z$ varies from 170 ~ 154. Now values given by $ks$ are known sample values that equals value given in the table header, $uks$ are unknown ...
1
vote
1answer
209 views
Conceptual question about fitting of scattered data
What are the problems that arise when fitting (2D or 3D) a set of scattered data? (non uniformly distributed)
I had some data I had to fit and I solved the problem using the ...
1
vote
0answers
2k views
Piecewise linear approximation of an experimental data curve. Knots position constrained
I need to fit a curve, obtained from experimental data, with a piecewise linear model (4 knots and therefore 3 lines).
I tried using the MATLAB function
...
2
votes
1answer
274 views
How to connect two fitted B-spline curve?
I use B-spline curve fitting to obtain one smooth curve. If I obtain two smooth B-spline , how can I connect then smoothly. For example, I have 59 points((x0,y0,z0),...,(x58, y58, z58)) and I have two ...
0
votes
1answer
897 views
Number of control points for B-spline curve
I am trying to use B spline curve fitting. The order of B-spline curve is 4. When I have many control points, it works well.
However if the number of control points is small such as two, my program ...
2
votes
0answers
157 views
Produce one smooth curve on one triangle mesh
I hope to get one smooth curve on one triangle mesh. I get one path on the mesh at first. The path consists of vertices of the mesh. I can see the path from the image below.
Each one green dot ...
4
votes
1answer
100 views
Finding self-similar solutions
Is there a general approach to finding self-similar solutions; i.e. collapsing several functions onto a single function by some transformation?
I have data from some experiments, and the functions ...
