Tracking head movement in outdoor activities is more challenging than in controlled indoor lab environments. Large-magnitude head scanning is common under natural conditions. Compensatory gaze (head and eye) scanning while walking may be critical for people with visual field loss. We compared the accuracy of two outdoor head tracking methods: differential inertial measurement units (IMU) and simultaneous localization and mapping (SLAM). At a fixed location experiment, a gaze aiming test showed that SLAM outperforms IMU in terms of error (IMU: 9.6°, SLAM: 4.47°). In an urban street walking experiment conducted with five patients with hemifield loss, the IMU drift, quantified by root-mean-square deviation, was as high as 68.1°, while the drift of SLAM was only 5.3°. However, the SLAM method suffered from data loss due to tracking failure (~10% overall, and ~ 18% when crossing streets). Our results show that the SLAM and IMU methods have complementary properties. Because of no data gaps, the differential IMU method may be desirable as compared to SLAM in settings where the signal drift can be removed in post-processing and small gaze estimation errors can be tolerated.