Fits independent trends to data lines that are parallel to an axis


This routine fits trends to all lines of data in an NDF that lie parallel to a chosen axis. The trends are characterised by polynomials of order up to 15, or by cubic splines. The fits can be restricted to use data that only lies within a series of co-ordinate ranges along the selected axis.

The ranges may be determined automatically. There is a choice of tunable approaches to mask regions to be excluded from the fitting to cater for a variety of data sets. The actual ranges used are reported in the current co-ordinate Frame and pixels, provided they apply to all lines being fitted.

Once the trends have been determined they can either be stored directly or subtracted from the input data. If stored directly they can be subtracted later. The advantage of that approach is the subtraction can be undone, but at some cost in efficiency.

Fits may be rejected if their root-mean squared (rms) residuals are more than a specified number of standard deviations from the the mean rms residuals of the fits. Rejected fits appear as bad pixels in the output data.

Fitting independent trends can be useful when you need to remove the continuum from a spectral cube, where each spectrum is independent of the others (that is you need an independent continuum determination for each position on the sky). It can also be used to de-trend individual spectra and perform functions like debiassing a CCD which has bias strips.


mfittrend in axis ranges out
order knots=?



If TRUE, the ranges that define the trends are determined automatically, and Parameter RANGES is ignored. [FALSE]
The axis of the current co-ordinate system that defines the direction of the trends. This is specified using one of the following options.
  • Its integer index within the current Frame  of the input NDF (in the range 1 to the number of axes in the current Frame).

  • Its Symbol  string such as "RA" or "VRAD".

  • A generic option where "SPEC" requests the spectral axis, "TIME" selects the time axis, "SKYLON" and "SKYLAT" picks the sky longitude and latitude axes respectively. Only those axis domains present are available as options.

A list of acceptable values is displayed if an illegal value is supplied. If the axes of the current Frame are not parallel to the NDF pixel axes, then the pixel axis which is most nearly parallel to the specified current Frame axis will be used. AXIS defaults to the last axis. [!]

CLIP() = _REAL (Read)
Array of standard-deviation limits for progressive clipping of outlying binned (see NUMBIN) pixel values while determining the fitting ranges automatically. It is therefore only applicable when AUTO=TRUE. Its purpose is to exclude features that are not part of the trends.

Pixels are rejected at the ith clipping cycle if they lie beyond plus or minus CLIP(i) times the dispersion about the mediaan of the remaining good pixels. Thus lower values of CLIP will reject more pixels. The normal approach is to start low and progressivley increase the clipping factors, as the dispersion decreases after the exclusion of features. The source of the dispersion depends on the value the METHOD parameter. Between one and five values may be supplied. Supplying the null value (!), results in 2, 2.5, and 3 clipping factors. [2,2,2.5,3]

The type of fit. It must be either "Polynomial" for a polynomial or "Spline" for a bi-cubic B-spline. ["Polynomial"]
Set this TRUE if the data may contain spectral data with many lines–-a line forest–-when using the automatic range mode (AUTO=TRUE). A different approach using the histogram determines the baseline mode and noise better in the presence of multiple lines. This leads to improved masking of the spectral lines and affords a better determination of the baseline. In a lineforest the ratio of baseline to line regions is much reduced and hence normal sigma clipping, when FOREST=FALSE, is biased. [FALSE]
The number of interior knots used for the cubic-spline fit along the trend axis. Increasing this parameter value increases the flexibility of the surface. KNOTS is only accessed when FITTYPE="Spline". See INTERPOL for how the knots are arranged. The default is the current value.

For INTERPOL=TRUE, the value must be in the range 1 to 11, and 4 is a reasonable value for flatish trends. The initial default is 4.

For INTERPOL=FALSE the allowed range is 1 to 60 with an initial default of 8. In this mode, KNOTS is the maximum number of interior knots.

The upper limit of acceptable values for a trend axis is no more than half of the axis dimension. []

IN = NDF (Read & Write)
The input NDF. On successful completion this may have the trends subtracted, but only if SUBTRACT and MODIFYIN are both set TRUE.
The type of spline fit to use when FITTYPE="Spline".

If set TRUE, an interpolating spline is fitted by least squares that ensures the fit is exact at the knots. Therefore the knot locations may be set by the POSKNOT parameter.

If set FALSE, a smoothing spline is fitted. A smoothing factor controls the degree of smoothing. The factor is determined iteratively between limits, hence it is the slower option of the two, but generally gives better fits, especially for curvy trends. The location of of the knots is decided automatically by Dierckx’s algorithm, governed where they are most needed. Knots are added when the weighted sum of the squared residuals exceeds the smoothing factor. A final fit is made with the chosen smoothing, but finding the knots afresh.

The few iterations commence from the upper limit and progress more slowly at each iteration towards the lower limit. The iterations continue until the residuals stabilise or the maximum number of interior knots is reached or the lower limit is reached. The upper limit is the weighted sum of the squares of the residuals of the least-squares cubic polynomial fit. The lower limit is the estimation of the overall noise obtained from a clipped mean the standard deviation in short segments that diminish bias arising from the shape of the trend. The lower limit prevents too many knots being created and fitting to the noise or fine features.

The iteration to a smooth fit makes a smoothing spline somewhat slower. [FALSE]

MASK = NDF (Write)
The name of the NDF to contain the feature mask. It is only accessed for automatic mode and METHOD="Single" or "Global". It has the same bounds as the input NDF and the data array is type _BYTE. No mask NDF is created if null (!) is supplied. [!]
The method used to define the masked regions in automatic mode. Allowed values are as follows.
  • "Region" –- This averages trend lines from a selected representative region given by Parameter SECTION and bins neighbouring elements within this average line. Then it performs a linear fit upon the binned line, and rejects the outliers, iteratively with standard-deviation clipping (Parameter CLIP). The standard deviation is that of the average line within the region. The ranges are the intervals between the rejected points, rescaled to the original pixels. They are returned in Parameter ARANGES.

    This is best suited to a low dispersion along the trend axis and a single concentrated region containing the bulk of the signal to be excluded from the trend fitting.

  • "Single" –- This is like "Region" except there is neither averaging of lines nor a single set of ranges. Each line is masked independently. The dispersion for the clipping of outliers within a line is the standard deviation within that line.

    This is more appropriate when the features being masked vary widely across the image, and significantly between adjacent lines. Some prior smoothing or background tracing (CUPID: FINDBACK) will usually prove beneficial.

  • "Global" –- This is a variant of "Single". The only difference is that the dispersion used to reject features using the standard deviation of the whole data array. This is more robust than "Single", however it does not perform rejections well for lines with anomalous noise.


Whether or not to subtract the trends from the input NDF. It is only used when SUBTRACT is TRUE. If MODIFYIN is FALSE, then an NDF name must be supplied by the OUT parameter. [FALSE]
The number of bins in which to compress the trend line for the automatic range-determination mode. A single line or even the average over a region will often be noisy; this compression creates a better signal-to-noise ratio from which to detect features to be excluded from the trend fitting. If NUMBIN is made too large, weaker features will be lost or stronger features will be enlarged and background elements excluded from the fitting. The minimum value is 16, and the maximum is such that there will be a factor of two compression. NUMBIN is ignored when there are fewer than 32 elements in each line to be de-trended. [32]
The order of the polynomials to be used when trend fitting. A polynomial of order 0 is a constant and 1 a line, 2 a quadratic etc. The maximum value is 15. ORDER is only accessed when FITTYPE="Polynomial". [3]
OUT = NDF (Read)
The output NDF containing either the difference between the input NDF and the various trends, or the values of the trends themselves. This will not be used if SUBTRACT and MODIFYIN are TRUE (in that case the input NDF will be modified).
The co-ordinates of the interior knots for all trends. KNOTS values should be supplied, or just the null (!) value to request equally spaced knots. The units of these co-ordinates is determined by the axis of the current world co-ordinate system of the input NDF that corresponds to the trend axis. Supplying a colon ":" will display details of the current co-ordinate Frame. [!]
Only used if SUBTRACT is FALSE. If PROPBAD is TRUE, the returned fitted values are set bad if the corresponding input value is bad. If PROPBAD is FALSE, the fitted values are retained. [TRUE]
Pairs of co-ordinates that define ranges along the trend axis. When given these ranges are used to select the values that are used in the fits. The null value (!), causes all the values along each data line to be used. The units of these ranges is determined by the axis of the current world co-ordinate system  of the input NDF that corresponds to the trend axis. Supplying a colon ":" will display details of the current co-ordinate Frame. Up to ten pairs of values are allowed. This parameter is not accessed when AUTO=TRUE. [!]
The number of standard deviations exceeding the mean of the root-mean-squared residuals of the fits at which a fit is rejected. A null value (!) means perform no rejections. Allowed values are between 2 and 15. [!]
The region from which representative lines are averaged in automatic mode to determine the regions to fit trends. It is therefore only accessed when AUTO=TRUE, METHOD= "Region", and the dimensionality of the input NDF is more than 1. The value is defined as an NDF section, so that ranges can be defined along any axis, and be given as pixel indices or axis (data) co-ordinates. The pixel axis corresponding to parameter AXIS is ignored. So for example, if the pixel axis were 3 in a cube, the value "3:5,4," would average all the lines in elements in columns 3 to 5 and row 4. See Section 9 for details.

A null value (!) requests that a representative region around the centre be used. [!]

Whether not to subtract the trends from the input NDF or not. If not, then the trends will be evaluated and written to a new NDF (see also Parameter PROPBAD). [FALSE]
Value for the title of the output NDF. A null value will cause the title of the NDF supplied for Parameter IN to be used instead. [!]
If TRUE and the input NDF contains variances, then the polynomial or spline fits will be weighted by the variances.

Results Parameters

This parameter is only written when AUTO=TRUE, recording the trend-axis fitting regions determined automatically. They comprise pairs of pixel co-ordinates.


mfittrend in=cube axis=3 ranges="1000,2000,3000,4000" order=4 out=detrend
This example fits cubic polynomials to the spectral axis of a data cube. The fits only use the data lying within the ranges 1000 to 2000 and 3000 to 4000 Ångstroms (assuming the spectral axis is calibrated in Ångstroms and that is the current co-ordinate system). The fit is evaluated and written to the data cube called detrend.
mfittrend in=cube axis=3 auto clip=[2,3] order=4 out=detrend
As above except the fitting ranges are determined automatically with 2- then 3-sigma clipping.
mfittrend in=cube axis=3 auto clip=[2,3] fittype=spline out=detrend interpol
As the previous example except that interpolation cubic-spline fits with four equally spaced interior knots are used to characterise the trends.
mfittrend m51 3 out=m51_bsl mask=m51_msk auto fittype=spl
This example fits to trends along the third axis of NDF m51 and writes the evaluated fits to NDF m51_bsl. The fits use a smoothing cubic spline with the placement and number of interior knots determined automatically. Features are determined automatically, and a mask of excluded features is written to NDF m51_msk.
mfittrend cube axis=3 auto method=single order=1 subtract out=cube_dt mask=cube_mask
This fits linear trends to the spectral axis of a data cube called cube, masking spectral features along each line independently. The mask pixels are recorded in NDF cube_mask. The fitted trend are subtracted and stored in NDF cube_dt.


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