Short-circuit Logic

[Roy Smith]

In article <pan.2013.05.31.01.09.55.109000 at nowhere.com>,
 Nobody <nobody at nowhere.com> wrote:

> On Thu, 30 May 2013 19:38:31 -0400, Dennis Lee Bieber wrote:
> 
> > 	Measuring 1 foot from the 1000 foot stake leaves you with any error
> > from datum to the 1000 foot, plus any error from the 1000 foot, PLUS any
> > azimuth error which would contribute to shortening the datum distance.
> 
> First, let's ignore azimuthal error.
> 
> If you measure both distances from the same origin, and you have a
> measurement error of 0.1% (i.e. 1/1000), then the 1000' measurement will
> actually be between 999' and 1001', while the 1001' measurement will be
> between 1000' and 1002' (to the nearest whole foot).
> 
> Meaning that the distance from the 1000' stake to the 1001' stake could be
> anywhere between -1' and 3' (i.e. the 1001' stake could be measured as
> being closer than the 1000' stake).
> 
> This is why technical drawings which include regularly-spaced features
> will normally specify the positions of features relative to their
> neighbours instead of (or as well as) relative to some origin.

Not to mention "Do not scale drawing" warnings.  Do they still put that 
on drawings?  It was standard practice back when I was learning drafting.

> When you're dealing with relative error, the obvious question is
> "relative to what?".

Exactly.  Most programmers are very poorly training in these sorts of 
things (not to mention crypto, UX, etc).  I put myself in that camp too.  
I know just enough about floating point to understand that I don't 
really know what I'm doing.  I would never write a program where 
numerical accuracy was critical (say, stress analysis of a new airframe 
or a nuclear power plant control system) without having somebody who 
really knew that stuff on the team.