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Information about an NDF's axis coordinate system is stored in its
axis components, which are conveniently categorised as follows:
Axis character components: |
LABEL |
-- |
Axis labels |
|
UNITS |
-- |
Axis units |
Axis array components: |
CENTRE |
-- |
Pixel centre coordinates |
|
WIDTH |
-- |
Pixel width values |
|
VARIANCE |
-- |
Variance estimates for pixel
positions |
As with the main components of an NDF, the names of these axis
components are significant,
since they are used by the NDF_ routines to identify the
component(s) to which certain operations should be applied.
Axis component names are specified in the same way as those of the
main components of an NDF, including the use of abbreviations, mixed case
and comma-separated lists where appropriate (see
§
for details).
Access to an axis component must also specify the number of the NDF
axis to be used.
This is normally an integer lying between 1 and the number of NDF
dimensions, but many routines will also accept a value of zero, indicating
that an operation is to be applied to all of an NDF's axes.
This additional item of information means that a separate set of routines
must be provided for accessing axis components.
Nevertheless, many of the principles described in earlier sections for
accessing other NDF components are also applicable here, so the descriptions
given below are relatively brief.
References to more complete descriptions are given where appropriate.
The following describes the purpose and interpretation of each axis
component in slightly more detail.
Axis Character Components:
- LABEL -
- This is a character string, whose value is intended for
general use for such things as labelling the axes of graphs or as a heading
for columns in tabulated output; e.g. `Scanner X offset'.
There is a separate axis label value for each NDF dimension.
- UNITS -
- This is a character string, whose value describes the
physical units of the quantity measured along an NDF's axis;
e.g. `micron'.
There is a separate axis units value for each NDF dimension.
Axis Array Components:
- CENTRE -
- This is a 1-dimensional array which holds the
coordinates of the pixel centres as described in
§
. The values in this array should either increase
or decrease monotonically with position in the array. There is a
separate 1-dimensional axis centre array for each dimension of
an NDF, whose size matches the size of the corresponding NDF dimension.
- WIDTH -
- This is a 1-dimensional array which holds a set of
non-negative width values for the NDF's pixels as described in
§
. There is a separate 1-dimensional axis
width array for each dimension of an NDF, whose size matches the
size of the corresponding NDF dimension. These width values
should be such that no point can lie inside more than two NDF pixels
simultaneously (i.e. although pixels are allowed to overlap with
their neighbours, they may not overlap with more distant pixels).
- VARIANCE -
- This is a 1-dimensional array which holds a set of
non-negative variance estimates representing any statistical
uncertainty in the value of the corresponding pixel centre
coordinate, as described in §
. There is a
separate 1-dimensional axis variance array for each dimension
of an NDF, whose size matches the size of the corresponding NDF
dimension.
Next: Axis Component States
Up: AXIS COMPONENTS
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Starlink User Note 33
R.F. Warren-Smith
11th January 2000
E-mail:rfws@star.rl.ac.uk
Copyright © 2000 Council for the Central Laboratory of the Research Councils