Join Date: May 2011
Location: Central New Jersey
Not the voice of experience here and shootin' from the hip here. If I were tasked with creating an 18' straight edge I would laminate 1x4 three across (best wood available), joint it on the longest bed jointer I had. I still think a laser (level) would offer the best reference over that distance so I would check it by lining the laser up just above either end. If there was a crown you would know it right away. Any other error could be detected by drawing a white card across the edge looking for high/low spots. Any found would be removed by a jointer hand plane.