next up previous
Next: Contiguous and Non-Contiguous Pixels
Up: THE AXIS COORDINATE SYSTEM
Previous: Pixel Positions and Dimensions

Default Axis Array Values   

An important feature of each axis array is a set of default values which serve to define the axis coordinate system in the absence of complete information. In the simplest case (i.e. no information), this reduces to the pixel coordinate system discussed in §[*]. The following describes how default values are obtained for each axis array:

Centre:
If values are required for an axis centre array and none have been provided, then its values are set equal to $i-\frac{1}{2}$, where i is the pixel's index in the relevant dimension. Thus, if an NDF had pixel-index bounds (3:10) in a particular dimension, the default axis centre array values for this dimension would be:
2.5, 3.5, 4.5, 5.5, 6.5, 7.5, 8.5, 9.5

Width:
If values are required for an axis width array and none have been defined, then its values are derived from the corresponding axis centre array by forming differences between the centre coordinates of the neighbouring pixels, i.e. the default width values are obtained as follows:[*]

$W_{n}(i) = \frac{1}{2} \vert C_n(i+1) - C_{n}(i-1)\vert$

This means that the default pixel widths match the local average spacing between pixel centres, which is usually appropriate. Note, however, that this does not guarantee that the pixels will be contiguous (i.e. that their edges will meet exactly) except in cases where the pixel centres are uniformly spaced (see §[*]).

Variance:
If no axis variance array values have been defined, then they default to zero, implying no uncertainty in the pixel centre positions.



next up previous
Next: Contiguous and Non-Contiguous Pixels
Up: THE AXIS COORDINATE SYSTEM
Previous: Pixel Positions and Dimensions


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