Today I recorded an awfull walk tracked with Strava that so far had great results. WiFi/cell positioning was turned off in Location settings, but everything else was turned on (WiFi/3G/Bluetooth/..). First I walked 10 minutes before I got signal, even though I used GPS 5 minutes before that and all was fine (I just went inside a building for a few minutes, than back out). Next when I did get signal my route was tracked very sloppy, like I was walking zig-zag and/or in circles and such, even though I went straight down the same road. This was 20 minute walk, and like I've said 10 minutes without track, and next 10 were real bad. Thing is, the ride I did just before that, before entering the building - all fine! Wasn't 100% accurate, but was within some 5-10 meters perhaps at worst, but mostly +/-2-3 meters from wher eI passed. And that was in car. And literally I went through the same part of town, once in one direction in car (all fine), and than back on foot (terrible). There were no restarts, reboots, or any settings changed in the meantime.
Oh wait! I just remembered something! This just came to my mind while I was typing last few words of above paragraph. There was one big difference. I was USING PHONE while I was walking! I was actually reading this thread, than went to take a look at that free OpenStreetMaps navigation, than downloaded the app, and so on. So: a) was in my hand all the time, b) screen on, c) CPU was under some load I guess, d) mobile data was running all the time. Now remember how I said earlier that when I have wifi/3G completely off that it's all fine? And that everyone say Google Maps gives tham bad results (and it's an online tool downloading maps all the timae in bgnd while you drive)? What if there is something in that after all?
I'm kinda one step back now, maybe a few, but I'll continue testing with all radios off (WiFi/3G/Bluetooth) and see if I encounter bad days like today's. I did notice once that while I was speaking on phone while tracking I also had off results during those 100-200 meters, but I can't test with Airplane mod, after all - it's phone :P
As for those saying that how should they use GPS offline when they "need" data... I've got 2 suggestions. 1) Not all apps need data connection. Tracking apps don't use data because they just record info from satellites, so everything is offline. And navigation apps can have maps stored offline (like iGO MyWay/Primo that I use, or MapFactor Navigator that Doman uses, but these are just a few from many). 2) for fast lock, you just need data in the start, you get a lock with mobile data/WiFi, than you can turn it off, from there on phone knows (or should know) where you are, and shouldn't have problems finding you again. I'm not saying it's perfect, you won't get WhatsApp and Facebook messages while driving :P But on the other hand... maybe better you don't, eh?