@alex Yeah, phone GPS. You might be right about it switching to cell location.
Thing is, at the moment, it's not a "little problem" (except in first-world problem sense that it's pretty inconsequential to my life as a whole). I've done eleven rides this week and six of them have had significant GPS problems, plus one more that was probably user error. This one and one more have had me jump across town but then jumped back, which is more or less recoverable; three others, it's jumped randomly and never recovered before I got to my destination;
one, I got bored of waiting for a GPS lock and set off without it. I guess the lesson here is that my phone has crappy GPS.
Well, I bought a Topeak bag/handlebar mount thing and my phone didn't glitch for the whole of the 7-mile ride I did. Small sample size but I'm keeping my fingers crossed.
I remember doing a ride up the beach, at sea level. at low tide, so no real change in sea position. Strava told me I did a hundred metres of climbing on that 15km length. At sealevel.... and the sea doesn't slope.