Background: Mobile devices are increasingly used to collect location-based information from individuals about their physical activities, dietary intake, environmental exposures, and mental well-being. Such research, which typically uses wearable devices or smartphones to track location, benefits from the growing availability of fine-grained data regarding human mobility. However, little is known about the comparative geospatial accuracy of such devices. Objectives: In this study, we compared the data quality of location information collected from two mobile devices which determine location in different ways -a GPS watch and a smartphone with Google's Location History feature enabled. Methods: Twenty-one chronically ill participants carried both devices, which generated digital traces of locations, for 28 days. A smartphone-based brief ecological momentary assessment (EMA) survey asked participants to manually report their location at four random times throughout each day. Participants also took also part in qualitative interviews and completed surveys twice during the study period in which they reviewed recent phone and watch trace data to compare the devices' trace data to their memory of their activities on those days. Trace data from the devices were compared on the basis of: (1) missing data days; (2) reasons for