Linear optics errors in the linac, the transport line, and the undulator section of a free electron laser (FEL) can significantly impact the FEL performance. In this paper we propose two methods to measure and correct the linear optics of one-pass systems using trajectory scan data. The methods have been successfully applied to data taken on the Linac Coherent Light Source (LCLS) to find the quadrupole errors and BPM gains.