"68 longitude, 42 latitude" translates to a point in the North Atlantic Ocean about 150 miles east of Boston, Massachusetts. Washington, D.C. is approximately 77 degrees West longitude, 39 degrees North latitude.
Near the end of the movie when they are at the quicksand, you can clearly see the cable attached to the lead truck gradually lowering it into the quicksand.
The dialog has someone saying "Set the missile to detonate at 32,000 feet" - but the computer graphic on the monitor shows it set to detonate at "32.000 miles above sea level." That height (32 miles) would be too far up to cause significant damage - it is beyond most of the atmosphere. It is also more than 160,000 feet, not 32,000 feet.