The accuracy of GPS altitude is pretty poor at the best of times, that’s why fancy ones have a barometer built in. This is unless you have a base station that your GPS can speak to help it resolve the atmospheric delay the signal experiences.

Anyone know what height measurements does Tracklogs use for it’s altitude? Is it based on OS contours/spot heights (which are pretty accurately derived from airphoto’s) or some thing like the freely available 90m SRTM DEM.