Nice research but please let me asure you that its not what you think. The Arc is so big that a person would struggle to find the exact centre that they would have to face for prayers. Also you are right that most older Mihrabs are not exactly correct and the intension is what counts, but as a land surveyor myself Obviosuly in this day and age with acurate survey & GPS the norm there is no excuss for new constructions having the direction wrong, the intention would then be false and it would not be a mehrab for prayer to mecca. Either way, you dont have a mehrab without a mosque and its not Islamic custom to make a house of worship to God out of trees.
I say this as a Muslim who strongly opposis any act of terrorism and belives in the True teachings of Islam, not the misinterprated and twisted to suit their objectives religion of terrorists.Let me just say I ain't buying your "I say this as a Muslim" bit, any more than your half-hearted claim to be a surveyor. If you were a surveyor on the ground there in Shanksville, you'd point your wing-stretched fingertips at the ends of the crescent and slap a 90 (real surveyor's field technique for establishing a surprisingly accurate perpendicular bisector). The architect could have been trying to incorporate an adjustment for magnetic declination, but I don't think so, and I bet you know better, too. His intention is called "plausible deniability." He needs to be able to tell one story ( I didn't design any real Islamic symbolism into it, because if I had, I'd have made it more precise) to the National Parks Service, and the opposite story (This is a Mosque, wherein the greater orientation is slightly off, but with enough interior elements to help you fine tune your direction) to those Muslims he wishes to appeal.
For anyone to suggest that this architect has benign intentions (don't forget that the focus of his grad school thesis was Middle Eastern influence in architecture), is an insult to anyone paying attention.
Also don't discount the presumed "value" for the architect later claiming that the "3-degree error" is reflective of his presumed American inferiority. His intent is to memorialize the terrorists, to symbolically complete their mission by martyring them on the ground in Shanksville. Anyone who claims otherwise is a fool. Anyone who vandalizes my blog posts with anonymous comments on old posts is a coward.
And if you really think you've "asured" me: Uh, no. Dipshit.
My assurance will come with the widespread exposure of this dispicable design and the project's cancellation.