Sorry, I don't have the time to read the PQS documentation in-depth right now. Still, I'd like to make one suggestion. I think that handling of dimensional quantities and conversion factors are orthogonal concepts and should be separated. I suggest that for maximum transparency the library should *exclusively* handle quantities expressed in SI units. I'm aware that this "mildly forces" developers to adopt SI units. I consider this a Good Thing. Conversion factors between non-SI units and SI units should be constant dimensional quantities, e.g. (assuming constructors from double): const length foot = .3048 ; // Meter const power european_horse_power = 735.4987 ; // Watt const mass pound = 0.4535924; // Kilogram // ... This way, one could e.g. construct dimensional quantities like this: const power deux_chevaux = 2 * european_horse_power; Non-SI quantities would have to stay "out of the system" in normal floating point variables: length altitude; double altitude_ft = altitude / foot; Ignoring dimensionality, this is the notation Mathematica chooses. I have done some engineering simulations, written a flight simulation framework and used one that uses US units (D6; see www.bihrle.com). The D6 source code is riddled with magic constants. I've collected some conversion constants in units.h of cpp-lib, see http://gwesp.tx0.org/software/. Regards -Gerhard -- Gerhard Wesp ZRH office voice: +41 (0)44 668 1878 ZRH office fax: +41 (0)44 200 1818 For the rest I claim that raw pointers must be abolished.