Re: Expressing physical units (Was: Reprise: 'in out' parameters for functions)

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

Russ wrote:

> By the way, I work in the field of air traffic management (ATM), and
> I'd really like to see a practical way to guarantee units consistency.
> The general mks system is inappropriate for ATM. The traditional units
> for ATM are nautical miles (nmi) for horizontal length, and feet (ft)
> for altitude. That won't change in our lifetimes -- and probably
> never. As for time, it can be in seconds, minutes, or hours.
> Horizontal speed in usually given in terms of knots (kn), which is
> nmi/hr, but altitude rate is usually given in terms of ft/min. Heading
> is normally given in degrees.
> 
> In my experience, the most common units problem is confusion between
> degrees and radians. Radians are preferable for use inside programs,
> but degrees are preferable for I/O. This problem really needs to be
> licked once and for all.

It does not differ from [m] vs. [ft]. The dimension of radian is [1] =
m**0*A**0*s**0... The dimension of degree is [Pi/180]. No magic here. More
difficult is Celsius degree vs. K, which AFAIK cannot be solved using C++
templates.

> The next most common problem I find is that
> altitude is alternately expressed in terms of ft, hundreds of ft, or
> thousands of feet. Standard "Flight Levels" are at multiples of 1000
> ft, but they are expressed in units of 100 ft. For example, "FL310" is
> a pressure altitude of 31,000 ft.

See above.

-- 
Regards,
Dmitry A. Kazakov
www.dmitry-kazakov.de