From Wikipedia: http://en.wikipedia.org/wiki/Arch
Isometric view of a typical arch
An arch requires all of its elements to hold it together, raising the question of how an arch is constructed. One answer is to build a frame (historically, of wood) which exactly follows the form of the underside of the arch. This is known as a centre or centring. The voussoirs are laid on it until the arch is complete and self-supporting. For an arch higher than head height, scaffolding would in any case be required by the builders, so the scaffolding can be combined with the arch support. Occasionally arches would fall down when the frame was removed if construction or planning had been incorrect. (The A85 bridge at Dalmally, Scotland suffered this fate on its first attempt, in the 1940s). The interior and lower line or curve of an arch is known as the intrados.
Old arches sometimes need reinforcement due to decay of the keystones, known as bald arch.
The gallery shows arch forms displayed in roughly the order in which they were developed.
Round arch or Semi-circular arch
Unequal round arch or Rampant round arch
Equilateral pointed arch
Shouldered flat arch -see also jack arch
Three-foiled cusped arch
Reverse ogee arch
Catenary or Parabolic arch
The most common arch is just the radius of a circle, using the chord as the base and a height determined by the design/designer. An elliptical arch may be most suited to the design over a simple chord and the video shows a neat way to construct one. Thanks for posting the link!