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Episode 5 - Treasure Hunt

by Jonathan Hare

What is altitude?
Air pressure
How can we measure changes in air pressure?
Making a water altimeter
Other ways to calibrate an altimeter
Using our altimeter

The challenge
Treasure map We were given a treasure map plus some very basic instructions and told to find buried treasure on the slopes of a nearby mountain. We were supposed to start on a mountain trail and hike up 80 meters (260 feet) in altitude until we found a 'Punga Island' - where we would find the place to start our search for buried treasure!

We were supposed to build a device to measure altitude, use it to calculate our position along the rising track, and then break out our metal detector (which we built in the first episode, "Gold Rush") to locate the treasure.

What is altitude?
Altitude is the vertical height above a point, usually measured from sea level. Mountains are, of course, much higher than sea level, while some inland lakes (such as the Dead Sea) are actually below sea level. To find the treasure in this Rough Science challenge we needed to accurately measure a change of altitude, so we had to build an altimeter, which is a device to measure changes in altitude.

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Air pressure
Jonathan explains the physics to Ellen Our planet is covered with a thick layer of air, called the atmosphere. We aren't normally aware of it, but we are constantly weighed down by the air above us, a condition known as air pressure. If we stand on a mountain there is slightly less air above our heads than at lower altitudes. So there will be slightly less air pressure higher on top of a mountain than in a valley. We can measure air pressure quite easily. It also fluctuates with the weather, so the exact pressure is constantly changing.

How can we measure changes in air pressure?
One way is to make a U-shape out of plastic tube, fill it halfway with water and hold it upright, so the water makes a U shape in the tube and can't flow out.

If both ends of the tube are open to air, then there's an equal pressure of air above each water surface, and the water on each side will be at the same level.

Jonathan straps the altimeter to Ellen's backIf we cover one side of the tube, trapping the air pressure on that side, but leave the other side open, the levels will stay the same. But if we go up in altitude - for example, if we climb a mountain holding our tube of water - there will be less pressure on the water at the open end, while (in theory, at least) the closed end will still have the larger pressure of the air below. Due to this difference of pressure between the two surfaces, the water will move around to compensate. The levels now stand at different heights, and indicate both the difference in air pressure and the height we've risen up the mountain.

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So now we have a simple altimeter. A similar device, which measures air pressure due to weather changes but stays in a fixed position, like your house, is called a barometer.

Calibrating the altimeterWe can calibrate the altimeter by taking it up a known height and measuring the change in the levels of the water. We can then use the calibration to estimate the change in height and calculate how far we have gone up or down in altitude. We calibrated our Rough Science altimeter by taking it up Sentinel Point, which, according to the maps, is about 130 feet (40 meters) higher in altitude than the surroundings.

Making a water altimeter
Attaching the plastic tube to the wooden boardTake about 3-4 meters, around 10-12 feet, of thin (about -inch diameter) clear plastic tubing and attach it to a wooden board to form a U shape with one side about 12" long. Drill holes in the board and use wire or string to attach the tube. Make the U very narrow so that the two sides are only an inch or so apart, so it will be easier to see any changes in the water level.

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On the long side, coil up the extra tubing (important: don't cut off the extra length!) and attach a clamp onto the end so that it can be closed by tightening the clamp. Leave the other (short) end open. With the board vertical and both tube ends open, carefully fill the U tube halfway with colored water (you could soak a teabag in the water for 10 minutes to make it dark brown) so that it is easy to see.

Pouring liquid into the U tube Other ways to calibrate an altimeter
Elevators in high-rise buildings are ideal places to calibrate an altimeter! Try going up half the total number of floors, mark off the level changes, and check that you get roughly twice the change for the highest floor (assuming the elevator starts at the bottom and ends near the top). If you can ask the building manager for the building plans, you can get the building's exact height and calibrate the altimeter properly.

Using our altimeter
Ellen and Jonathan checking their altitudeIt was pouring with rain when Ellen and I headed off with Chris, our guide, to hunt for the treasure. The mountain streams were overflowing from the immense amount of water running down the hills. You could almost watch them rising! Chris took us to the start of the mountain track and then told us to lead the way and navigate the trail using our altimeter. Before starting we reset the altimeter to read zero at that level, then walked along the track. After about 130 feet Ellen stopped and asked Chris to check his professional altimeter. He read the dial but wouldn't tell us how our measurement compared (he told us later that we were very close).

Checking altimeter readingAfter about 150 feet we arrived at a mountain river forming a gorge leading up. We followed the riverbank for sometime, having to cross the bubbling stream in order to find a dry place to walk. At about 250 feet there was a clearing and we could see that the river split into two and formed an island in the middle - an island covered in Punga trees!

Our altimeter had navigated us successfully to the hidden treasure.

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Photo: Rough Scientists at work
Metal Detector Interactive