Subject: Units in FITS files

From: "Clive Page, Leicester University, UK" <[email-masked]>
Date: Wed, 21 Apr 93 06:32:04 -0400

My thoughts, for what they are worth, are as follows:

(a) Pixels are units of both length and area:
I don't think the ambiguity is a serious problem, but it's a pity we 
can't come up with a suitable name for the side of a pixel.  After all 
the term "voxel" as the 3-d analogue of the pixel is now well 
established.  

(b) Units raised to negative powers: 
One problem with the use of forms such as "Hz**(-1)" is that the
resulting strings can be rather long: 8 chars compared to 3 for the form
"/Hz".  An important use for units strings is to put them in table
headings: when these are displayed on a screen, or printed out, you
won't be able to get many columns across the screen if the units
strings take up so much space. Note that "W m**(-2) Hz**(-1)" takes up
nearly a fifth of my screen-width.  Even worse, some bits of software
may decide to truncate the strings at some arbitrary limit, say 16
chars. If the parentheses are removed, as in "Hz**-1" this reduces the
length a bit, but on the whole I prefer the original form on the grounds
of simplicity and brevity. 

(c) Numerical prefixes:  
I think the average user is going to be annoyed and will think the
programmer somewhat stupid if the units string appeares as something as
complicated as "10**(3) eV" when what is meant is just "keV", a term
that all high-energy physicists are instantly familiar with.  The former
requires a few seconds though to be interpreted correctly, and is also
harder for the machine to parse.  I see no advantage at all in using it.

Similarly I don't think it is a good idea to use powers of 10 in the
units strings until you get beyond the range for which standard prefixes
are defined (10**-18 to 10**+18 ??). 

Clive Page