Prior to the launch of the Geoscience Laser Altimeter System (GLAS) on the Ice, Cloud and land Elevation Satellite (ICESat) in January 2003, topographic surveys were made by NASA's Airborne Topographic Mapper (ATM) over regions of the western United States and the Antarctic Dry Valleys to support calibration and validation of the range and pointing errors of GLAS lasers. Surveyed areas included terrain with large slopes, allowing pointing-bias estimation with as little as a few seconds of ICESat data. Range errors over sloping irregular surfaces are calculated by computing the expected GLAS return waveform and comparing it with the actual waveform. We conclude that the range bias is less than 2 cm and that pointing errors for the best available data set (Laser 2a) have rss errors less than 2 arcsec.