Re: How durable is your GPS?
"First, I thought GPS information included altitude"
They do, it's calculated along with the horzontal data, though, it requires four birds as a minimum rather than three just for location. Generally, you get many more than that all of which help with accuracy.
"do these generally use GPS altitude or the one out of a database?"
Always GPS. Elevation data requires a bit more number crunching, which is a challenge when they pick the smallest weakest processor to do the job. Also, good accurate elevation data is expensive, and only has limited coverage. As others have mentioned, using barometric pressure is a by far better way of doing it - if you need altitude to be close.
"With an accuracy of a few feet"
I'm nit picking here, but you only get 3 metre 95% accuracy (typically) with WAAS enabled systems. And since that only operates either on or around the US, most of the world is stuck with non-WAAS, which is about 15 meters 95%, and altitude would be around 23 metres 95%. I need to stress I'm just quoting widely available numbers, and leaving out some gotchas, but these numbers should be acceptable most of the time. From there, it gets really, really complicated really, really fast.
"GPS location does just seem to jitter around a bit"
That's normal, and some units have built-in code to prevent this, so it "looks" clean to the otherwise untrained eye, and a whole lot less annoying on maps... Some handle this gracefully, some not at all.
"Dead reckoning until the GPS gets better?"
I've only seen dead reconing being used in (some) car dash GPSs. But I don't have wide experience with that, so can't be sure.
"Can it use the accelerometer?"
On a bicycle? Don't think so. What would be the point? They're widley used elsewhere, but not on bikes.
"I'd be getting 90MPH speeds in no time hahaha."
Don't laugh, I did that once. One hell of a long and steep downhill, the whole time thinking, "If I blow a tyre now, they'll have to scoop me up with a wet&dry vacuum cleaner.". Scary.