how to determine elevation?

I use this to plot bike rides around town and track elevation. If you punch in your address you can then follow roads by clicking around, and it posts the height change from point to point. Great tool.

http://www.heywhatsthat.com/profiler.html

To get around this, scientists try using tide gauges. A tide gauge is a large (1 foot [30 cm] or more in diameter), long pipe with a small hole below the water line. This pipe is often called a stilling well. Even though waves are changing the water level outside the gauge constantly, they have little effect inside the gauge. The sea level can be read relatively accurately inside this pipe. If read on a regular basis over a time span of years and then averaged, one can get a measurement of sea level.