Navigating by GPS
April 24, 1998: As movie archaeologist Indiana Jones warned, "X never, ever marks the spot." And even if it does, the map it is on may be next to worthless.
Early maps were usually works of art - not science - drawn from human impressions of distances and of terrain and shorelines. And over time, land itself changes.
Not much can be done to straighten out an old map - unless it has a good marker like Mount Olympus that has moved little over the last few thousand years. But archaeologists now have a tool that can mark their location with precision.
Like truckers, sea captains, and even rental cars, archaeologists are turning to the Global Positioning System (GPS) developed by the U.S. Air Force. GPS can determine your position within a few centimeters and your speed within a few meters per seconds (a few mph).
For more details on how GPS benefits Earth science, check the University NAVSTAR Consortium.
The principle behind GPS is quite simple, although the execution is complex and elegant: synchronized clocks.
Think of it this way. You're standing on the 10-yard line in a football field. Four referees are standing on the rim of the stadium above you. Everyone has an accurate watch. At exactly high noon, each fires a starter's pistol. You faithfully write down the times that the sound arrives from each gun. Knowing the speed of sound and the referees' locations, and a bit of math, you can calculate that you are really on the 10.2-yard line.
GPS works much the same way with a constellation of 24 satellites 17,742 km (11,000 miles) up. Each carries a precise atomic clock and broadcasts its identity, location, and time. (The Air Force works constantly to recompute the satellites' positions and to adjust signals for variations in Earth's atmosphere.) GPS locators - in trucks, ships, planes, and archaeologist hands - have radio receivers, clocks, and small computers. They do the math and tell you your position. Russia has a similar system called GLONASS.
All good GPS receivers are accurate, but their precision varies. That is, you will get a correct position, but the uncertainty may be greater.
Because it was developed for military purposes, the most precise part of the signals is scrambled. But commercial units available even at map stores can use most of the signal to measure your position within 30 meters (98 feet). Much of the United States is covered by Differential GPS, ground stations that broadcast a correction signal to make your location more accurate. DGPS, as it is called, is used in precision agriculture, aviation, and ship navigation, and other fields.
What this does for archaeologists is take away a lot of uncertainty. Satellite images are great, but only if you know where you and that intriguing but faint straight line in the image are located. Otherwise, you could blunder past your target and never know it.
Once an archaeologist has found a lost site, GPS is great for surveying. Conventional surveying involves "chain gangs" using special telescopes (transits) to build a series of measured, connected triangles in a sophisticated game of "connect the dots."
GPS speeds the surveying work. You still have to go to each location, but once there, you record the location - including the height above sea level - and then move on to the next one.
In addition to letting archaeologists map a site, GPS also lets them establish precise boundaries so sites can be defined and protected. Increasingly, remote sensing instruments are incorporating GPS receivers so an image comes with a built-in caption that tells exactly where the corners and center of a picture are located.
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