The sunburned tourist leaned forward, dripping sweat on my clean glass counter and sized me up through inch-thick spectacles. He then asked me the stupidest question in the world. Or so I thought.
"Vat elewation ish sea level?"
He was sweating because it was June in Death Valley; the thermometer read 122 degrees Fahrenheit. He was asking me, in particular, because I was a ranger and Death Valley is home to the lowest place in the Western Hemisphere: Badwater, 282 feet below sea level.
Barely choking down a "You’ve got to be kidding!," I explained that sea level was, well, just sea level, you know – the mathematical mean between high and low tides at the seashore, "zero feet." Simple.
"Bhat how ish dis determined? Vrom vhere do zay measure?," he began to shake his finger at me. I saw then that I had fallen prey to a man skilled in the time-honored National Park sport called "Stump the Ranger." He continued to grind in his advantage.
"Ish not za Pacific Ocean higher zan za Atlantic? Don’t zay haf to update eferyzing ven dere iz an ers-quake und zee ground shifts?"
I was nailed. It was my turn to look sunburned.
He was right, of course, as I later found out. The truth is, there’s no absolute sea level elevation for the entire world, the entire U.S. or even California. Mean sea level isn’t one of those absolutes of nature, like the speed of light or pi.
Lake Tahoe, for instance has its own version of sea level which doesn’t necessarily match up with the mathematical tidal mean along the California coast. The 6,223-foot-elevation "rim" of Lake Tahoe, measured from the dam in Tahoe City, isn’t really 6,223 feet above mean sea level, according to Gary Stone, Tahoe’s federal water master. The rim elevation is a convenient "Lake Tahoe Datum" used to keep modern water measurements in sync with historical lake level numbers.
The rise and fall of the lake surface has been measured against the Lake Tahoe Datum since 1912, when the dam was built, Stone said. In 1929 U.S. Coast and Geodetic Survey engineers "carried" sea level into the area and found that the Lake Tahoe Datum was more than a foot off, according to Jim Mullen, hydrologist for the USGS in Carnelian Bay.
As a result, the brass bar sticking out of the concrete dam marks the Lake Tahoe Datum. About 200 feet away, on a bridge is the 1929 USC&GS benchmark. The two don’t match.
But it doesn’t really matter, because sea level is in the calculator of the beholder. Along the coast of California sea level has been revised at least three times this century. That’s why maps made in different years show topographic milestones like Mount Whitney towering at slightly different heights and Badwater glowering at different depths.
Not only do new surveys change the elevation numbers, but the earth we are trying to measure is stretching, scrunching, sinking and rising.
The earthquake in Southern California measurably raised mountains, and the pumping of huge quantities of water from beneath the Sacramento and San Joaquin valleys has caused valley floors to sink several feet.
Then there’s the possibility the ice caps will melt and raise sea levels or another ice age will start and freeze water at the poles, thus lowering sea levels.
The closer you look at sea level, the less constant it becomes. By hanging onto the long-held – if imaginary – Lake Tahoe Datum, local hydrologists have kept things simple.
Speaking for outwitted rangers everywhere, I’d say they’re onto something.