In this study, we investigate the accuracy of the GPS based aircraft trajectory and subsequent ALS surface models derived from the 2011/12 season. We compare trajectories from a single typical survey flight, processed using a number of different GPS analysis strategies. We describe our approach used to obtain in situ data for ALS validation, and subsequently use these data to show agreement at the ~50 mm level (mean bias). Extensive crossover analysis using the full ALS swath shows we are able to achieve ALS precision of ~70 mm using the preferred GPS solution.