Here we have compared altimeter measurements with nearby tide gauges at two sites: Pohnpei in the western Pacific and Baltra in the eastern Pacific. Sea-level in these regions are very large during El Niño events (indicated by arrows). Sea-level is much lower than normal in the west and higher than normal in the east as will be explained later.
If the tide gauges are considered to be perfect (which they are not), the accuracy of the altimeter is about 3 cm. However, if a more realistic error is assumed for the tide gauges then the accuracy of the altimeter measurement is around 2 cm. Note that we are using the mean standard deviation as a measure of error; differences at some times can be up to 3 times this number. Also, comparisons at some tide gauges are better or worse. The average is about 2.5 cm.
Pohnpei in Western Pacific
Standard Deviation of Differences = 3 cm
Baltra in Eastern Pacific
Standard Deviation of Differences = 3.5 cm