At the risk of completely hijacking this thread...
Maps are created using these methods:
en.wikipedia.org
The whole "real time" triangulation method using multiple satellites seems a bit ambitious to me, and the results speak for themselves. +/- 30-40m!! Yikes. It's been a while, but I do seem to remember my Garmin PC software giving me the option to turn-on "elevation correction", or something like that to reference known data based on my route.
If I seem like I'm "crawling out of a hole" here with regard to cycling GPS stuff, I am. I actually tossed my Garmin off of the Tacoma Narrows Bridge 10 years ago, and quite frankly hadn't thought about it again until... well, today. Maybe riding with a computer was my REAL cycling gear pet peeve?