err, isn’t the easiest way by just looking at an OS map? Trickier in a town or city admittedly. But as bing maps has 1:50 and 1:25k you might be Ok. or if you have mapping software such as memory map or basecamp its really easy.
GPS will get your altitude from one of three sources
GPS, not very accurate as the geometry of GPS satellites is not conducive to accurate vertical positioning. the worse the view of the sky the worse the fewer satellites you can see, which means worse geometry and therefore less accurate height data. think +- 50 meters for GPS altitude.
Air pressure which varies constantly, those with ABC watched such as mountaineers will be familiar with manually setting your elevation when ever you reach a feature with a known height (calibration). think +- 10 meters depending on how recently you calibrated. Can be used as a powerful navigation technique in conjunction with a map as knowing your height on a mountain side will narrow down the number of places you might actually be.
From a loaded map or height database file. totally dependant on how good the data is.
Sea level in the UK can vary throughout the course of a day by up to 12 meters BTW, depending on location.
so if elevation is really important to you, set your altimeter manually based on the height a known location (ride start) referenced to a map and do it often, how often depends on the weather and how much faff you can put up with. But I suspect its not that important to most people.
