SUBSCRIBE!
Home
Current Column
Archived Columns

by Teresa Martin

Frequent readers know that GPS is my newest toy. And this week I learned the pros and cons of coming to rely on ... and got to ponder the meaning of sea level -- and how one of the surprises of GPS has turned out to be how level the sea isn't.

But first things first. On one day, I found my way to Chatham Elementary School with ease. No loops around Chatham looking for Depot Road! On another, I found my way to a restaurant in West Harwich and when that friendly little voice told me to "turn left now" darned if it wasn't spot on and knew where I was better than I did.

And then, there was the next day. I was traversing through the woods in Dennis-Yarmouth, to a meeting at a summer camp. Merrily, merrily full of confidence, knowing I had my finder at my side. And then ... the receiver froze. A crash. Its little screen locked up.

But this is a computer and these things happen - surely this was nothing that turning it one and off for a reset wouldn't fix. Wrong.

The little screen went blank and stayed blank. I feel alone, abandoned. And quickly realized that in little more than a week I had so come to rely on my navigation partner that I even left it one to provide directions when I already knew where I was going. There was something oddly comforting in having a navigator in the passenger seat, even a digital disembodied one.

But it was gone. A cell phone call brought me in to land. My usual left-right/east-west confusion gave me the scenic route on the way out via a path that wasn't exactly what I expected ...

Several times I tried to reach my directionally skilled companion only to meet the blank screen. I then did the mature thing - I ignored it for 24 hours. At which point it mysteriously turned on again.

And so the second part of the week let me play around with latitude and longitude and, my favorite living on a low =lying peninsula - feet above sea level. And in doing so, learned that an unexpected consequence of GPS was the discovered that sea level isn't exactly level.

I got a hint of this when in a short half mile I went from 55 feet to 9 feet to 21 feet to -6 feet and back up to 13 feet. And that sent me to the web for an explanation.

And it seems that GPS has dramatically changed how altitude is measured. GSP uses something called a reference ellipsoid - basically the gravitational surface of a flattened version of earth. In theory this should have been close (and could serve as a substitute for) something called Mean Sea Level - or MSL, which is the imaginary surface of the top of the ocean wrapped around the plant. But, ooopsy! It turned out the two values were quite different.

In 1992, a satellite called TOPEX/POSEIDON was launched. Its goal was to gather precise altimetric measurements which showed that the GPS to MSL difference weren't human error - they reflect the surprising fact that the sea is not exactly level. Water masses actually accumulate more in some areas than other -- a sort of clumping of water.

It also let me into an exploration of elevation vs. altitude and a whole refreshed way of thinking about the more-or-less elliptical mass we call home. I remember being terribly bored with ninth grade earth science. It seemed totally unconnected to the earth upon which we spent our days. In the past week with this little navigation box has not only proved that I can move from A to B without random loops around W in the middle - but it has also reconnected the real earth to the science of earth.

Suddenly, I find myself aware that 41-51-22 N and 69-59-13 W is not just a disconnected set of numbers, but it is a very real place on Route 6. And that the shape of earth isn't just a disconnected abstract geography lesson, but a very tactile and uneven terrain that isn't always measured the same. And in place of my evergreen question - Where do I turn next? - I have a whole host of new questions about the very nature of our planet and the way its physical properties shape my every day driving world.

That is a very cool intersection of science, technology, and real down to earth pragmatics.