Creates a new data file that is spaced in a uniform grid from input experimental data file.
The Data Interpolation module is accessible from the Tools section of the main menu.
The purpose of the module is create new files containing data generated by performing cubic-splines interpolation of the input data.
Users supply an input data file containing three columns: q, I(q), and error of I(q) at each q-value. In addition, a value of I(0) and the error of I(0) usually obtained by Gunier analysis. The user also specifies the spacing of number of q-values to be generated by the interpolation algorithm.
Any data set with smoothly varying data can be interpolated. SANS, SAXS, X-ray or neutron reflectivity, etc. The output labels are q and I(q) by default and can not be changed in the browser view.
Do not attempt to use the algorithm to extrapolate data (i.e. estimate data outside the range of the input data set). Rather, choose a delta q value such that the first interpolated data point falls within the range of the input data.
All background correction and data reduction pertaining to the input data set have to be completed before using this module.
Two files are generated. The first is the interpolated data set given your desired number of q-values and q-range. The other is truncated at the point that the signal-to-noise ratio drops below 2.
More data points may not be mathematically justified. For SAS there is limited information content related to the size of the molecule measured in the experimental scattering data. 15 to 31 points are generally used. See BIOISIS for a theoretical and pratical reasoning regarding the number of points one should use.
The number of q-points, range of q, and the spacing of the q-points used to create the interpolated data files MUST match the input settings that you use in subsequent modules to calculate SAS profiles (Calculate) that are subsequently used to compare theoretical and experimental data (Chi-Square Filter).
Users do not have to generate interpolated data using this module to input experimental data in subsequent modules. Any third-party method can be used but the number of q-points, range of q, and the spacing of the q-points MUST match what is used in subsequent modules as mentioned above.
The final range of q will be from 0 to a maximum q-value of (number of q-values - 1) * delta q, where delta q is the spacing between adjacent q-values entered by the user.
This example generates interpolated data files for an example input data file containing 154 points from q = 0.01298 to 0.3978.
run name: user defined name of folder that will contain the results.
experimental data file: Name of input file with experimental data with at least three columns: q, I(q), and error in I(q)
output file name: Name of file that will contain the interpolated data.
I(0): Experimentally determined value of scattering intensity at q = 0.
I(0) error: Experimentally determined value of the error of the scattering intensity at q = 0.
new delta q: Desired spacing of q-values (1/Angstrom).
number of new q-values: Integer number of desired q-values.
The output will show a plot of the original and interpolated data as well as the name of the input file used as well as the path and names of the interpolated data file.
Note that roll-over help will indicate options to resize, zoom and reset the view of the plot.
Results will be written to a new directory within the given "run name". For example, in the figure it is noted that the structures and dimensions were saved files within the current project directory within the chosen "run name" directory. In addition, the q-values are reported to be used in subsequent SAS calculator module(s): see Calculate.
The cubic-splines algorithm may give sub-standard results for certain types of data sets. Make sure to compare the interpolated results with the original data.
Numerical Recipes: The Art of Scientific Computing (3rd ed.) W. H. Press, S. A. Teukolsky, W. T. Vetterling, B. P. Flannery, New York: Cambridge University Press. ISBN 978-0-521-88068-8 (2007). BIBTeX, EndNote, Plain Text
SASSIE: A program to study intrinsically disordered biological molecules and macromolecular ensembles using experimental scattering restraints J. E. Curtis, S. Raghunandan, H. Nanda, S. Krueger, Comp. Phys. Comm. 183, 382-389 (2012). BIBTeX, EndNote, Plain Text
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