MD80 Flight Crew experiences heading discrepancies during the LGA 2 departure with the Whitestone climb off Runway 13. ATC is advised and the First Officer (FP) maintains wings level until reaching VMC. The First Officer's compass is slewed to the magnetic compass and all systems return to normal.

Narrative

Departing LGA; we were cleared to fly the LGA 2; Whitestone climb; first fix ELOIT from Rwy 13. After Take Off; the First Officer called for HDG sel 180 at 400'. The FMA indicated CLMP; Take Off; Take Off and when the Captain tried to go to HDG Sel; the FMA was frozen at Take Off; Take Off. Also; the Captain's and First Officer's flight directors had disappeared. The aircraft entered IMC approx 1500'. At this point; the First Officer rolled out on a heading of 180 on his ND. This was 30 degrees off from the Captain's ND and the First Officer's compass card which both showed 150. The First Officer elected to fly wings level to the assigned altitude of 5000'. The CA told ATC that they were experiencing heading difficulties and that they would fly present heading until fixing the problem. ATC copied and at some point asked the crew to turn left to 360. We were VMC at this point; so we began a left turn while still diagnosing which compass card was bad. On the northern track we were able to compare both compass cards to the magnetic compass. The First Officer's compass card was 30 degrees off the mag compass so he spun it to match the Captain's compass card and the mag compass. At this point all FMA and FD functions returned to normal.