(a) will vary from aircraft to aircraft. Static port location is a major part of the certification test program - the alternate is permitted considerably more latitude in maximum error. Normally the alternate error corrections, while considerably more than for the primary system, are not overly great. There are some design standard requirements which require that the primary errors are tightly constrained.
(b) the ASI is a differential pressure gauge - not a speed gauge. Rather than calibrate the dial in pressure units which might confuse people, we use a standard equation to relate pressures to an equivalent speed value. However, this equivalence is accurate only at sea level for standard conditions. Elsewhere, the ASI tells fibs. One of the fibs is covered by the normal density correction using either a prayerwheel or the instrument equivalent adjustment ring. Another is due to errors associated with the compressible nature of air - if you don't allow for this error at higher speeds, then the instrument reading will include a not insignificant error. What speed you might get concerned about compressibility corrections is not all that critical but around 200-250 kt IAS at low level is probably a reasonable point to become interested.
If you want to check out the background, have a look at any standard undergrad aerodynamics textbook or else at some of the relevant sites listed in the Tech Log URL sticky. However, unless your maths is up to speed, you might find the details more confusing than illuminating. From a practical point of view, it is more important to be aware of the error's existence and correction, than the derivation from first principles ...
Last edited by john_tullamarine; 21st May 2003 at 09:32.