Apple Watch SE (2nd generation) GPS distance or route is inaccurate

GPS on the Apple Watch SE (2nd generation) fails or drifts mostly because of conditions and calibration, and Apple's pages say how to fix both. Location Services must be on, and Motion Calibration and Distance must be on in System Services, since the watch uses them to refine your pace. Calibrate the watch by walking or running outdoors for about twenty minutes at your normal pace in an open area with a clear sky, which teaches it your stride length at different speeds. Tall buildings, trees and tunnels degrade the signal, so wait for the GPS signal before you start a workout. A watch that is loose on the wrist or worn over a sleeve can give poor data.

If distances are off by a lot, reset the calibration data in the iPhone's Settings, General, Transfer or Reset, Reset Location and Privacy, and calibrate again. For routes drawn in the Fitness app, a route that jumps across streets usually reflects a poor signal in the city, while a route that is a straight line means GPS never locked. Indoor workouts use the accelerometer, not GPS, so make sure you selected Outdoor Run or Walk and not Indoor. Cellular models can use the network to speed up a location fix, while GPS-only models rely on satellites alone, so a GPS-only watch needs a longer open-sky wait at the start of an outdoor workout.