How do you establish “elevation from sea level”?

Introduction: (Initial Observation)

One of the geographical information that we often see or hear about of towns is their elevation. Elevation is one of the factors that determines the climate of an area. Some cities are actually known by their elevation. For example, Denver, Colorado is known as the Mile-High City because it is a mile above sea level. For cities and mountains that are close to the ocean, elevation can be measured using general measurement tools and some calculations, but what if the location is many miles away from any ocean?

How do you measure elevation when the town is in the mountains and very far from the nearest ocean? In this project we will investigate different possible methods that can be used to measure the elevation of a place in relation to sea level.

Adult help and supervision is required.

In this project we want to find a way to measure the elevation of our town and possibly a mountain around in relation to sea level. Of course sea level is not the same all over the world, for example “The Pacific and Atlantic Oceans are not the same height in Central America. ”

In our experiment we will make an instrument to measure the air pressure. Since air pressure will reduce in higher elevations, we can use this to calculate the elevation in different areas. In our experiment elevation above sea level is determined by the geographic features or height above “mean sea level”-basically the average height of all of the oceans.

While this may be pretty good for determining the relative height of all geographic features worldwide, it’s really not all that accurate. So we continue our research about modern technologies in measuring elevation on the ground and even in the ocean.

Information Gathering:

Find out about the properties of an elevated area. Think about the physical variables such as air pressure, temperature, light, and others to see if any of such variables have a direct relation with elevation. Read books and magazines or ask professionals who might know in order to learn about the effect of elevation on measurable physical variables. Keep track of where you got your information from.

The air’s pressure is caused by the weight of the air pressing down on the Earth, the ocean and on the air below. Earth’s gravity, of course, causes the downward force that we know as “weight.” Since the pressure depends on the amount of air above the point where you’re measuring the pressure, the pressure falls as you go higher.

The air’s pressure also changes with the weather. Air pressure, in fact, is one of the important factors affecting the weather. For more on this see:

• Graphic: How high and low air pressure affect the weather
• Using barometer readings, wind as forecasting tools
• Storms and fronts index with links to more on air pressure and weather.

The information below tells you more about how air pressure is measured and its relation to air density. Below you’ll also find the mathematical formulas used to describe the air’s decreasing pressure with altitude.

Units of pressure

In the U.S., air pressure at the surface is reported in inches of mercury while air pressure aloft is reported in millibars, also known as hectopascals (hPa). Scientists, however, generally use pressures in hectopascals.

In the rest of the world, measurements are usually given in hectopascals although you will sometimes see them in centimeters of mercury, especially on older barometer.

The term “hectopascals” is replacing the term “millibars.” The hectopascal is a direct measure of pressure, like pounds per square inch, but in the metric system. Since the measurement is in the metric system, 1,000 millibars equals one bar. A bar is a force of 100,000 Newtons acting on a square meter, which is too large a unit to be a convenient measure of the Earth’s air pressure. Inches of mercury measure how high the pressure pushes the mercury in a barometer.

To convert between inches of mercury and millibars, one millibar is equal to 0.02953 inches of mercury. The El Paso, Texas, National Weather Service Office has a weather calculator posted on the web that can be used to make the conversion.

Centimeters of mercury can be translated into inches of mercury by dividing the number of centimeters by 2.54.

The use of direct pressure measurements goes back to the late 19th century when the great Norwegian meteorologist Vilhelm Bjerknes, a leader in making meteorology a mathematical science, urged weather services to use direct pressure measurements because they can be used in the formulas that describe the weather.

A sidelight: In the International System (SI) of measurements, the unit of pressure is the Pascal, named after Blaise Pascal, the 17th century scientist who made important discoveries about air pressure. The standard atmospheric pressure at the Earth’s surface of 1013.25 millibars is equal to 101,325 Pascals. To avoid large numbers, air pressure is reported in hectoPascals, which are the same as millibars. In many nations, you are now likely to hear reports such as, “air pressure, 1020.0 hectoPascals.” This is the same as 1020.0 millibars.

Pressure corrections

When you read a barometer, the reading directly from it is the “station pressure.”

Two things affect the barometer’s reading, the high or low air pressure caused by weather systems, and the air pressure caused by the station’s elevation, or how high it is above sea level. No matter what the weather systems are doing, the air’s pressure decreases with height. If you’re trying to draw a weather map of air pressure patterns, you need a way to remove the effects of the station’s elevation. That is, you want to see what the pressure would be at the station if it were at sea level.

You need to calculate, sea-level pressure, which is defined as: “A pressure value obtained by the theoretical reduction of barometric pressure to sea level. Where the Earth’s surface is above sea level, it is assumed that the atmosphere extends to sea level below the station and that the properties of that hypothetical atmosphere are related to conditions observed at the station.” To do this, you have to take into account the barometric reading at the station, the elevation above sea level, and the temperature.

Another kind of barometric reading is the altimeter setting, which aircrafts use. It’s defined as: “The pressure value to which an aircraft altimeter scale is set so that it will indicate the altitude above mean sea level of an aircraft on the ground at the location for which the value was determined.” For it, all you need is the station pressure and the elevation, you can ignore the temperature.

How pressure decreases with altitude

As you go higher in the air, the atmospheric pressure decreases. The  Constant pressure surfaces page on a University of Illinois web site explains how air temperature affects upper air pressures. USA TODAY online graphics on upper-air ridges and upper-air troughs explain more about pressure systems above the surface.

The exact pressure at a particular altitude depends of weather conditions, but a couple of rules of thumb (approximations) and a formula give you a general idea of how pressure decreases with altitude.

A rule of thumb for the altimeter correction is that the pressure drops about 1 inch of mercury for each 1,000 foot altitude gain. If you’re using millibars, the correction is 1 millibar for each 8 meters of altitude gain. These rules of thumb work pretty well for elevations or altitudes of less than two or three thousand feet.

Now a new technology known as global positioning system or GPS is used to determine distances and elevations with high accuracy.

Click here for more information about global positioning system.

You may also get more information about GPS and how it works in educational websites about GPS.

Also visit http://www.ball.com/aerospace/icesat.html for a new elevation measuring satellite.

http://www.ball.com/aerospace/icesat.html

California elevation map

Topographic maps show elevation contour.

Question/ Purpose:

What do you want to find out? Write a statement that describes what you want to do. Use your observations and questions to write the statement.

The purpose of this project is to find a method that can help us to establish the elevation from sea level. Since air pressure is lower in higher elevations, we want to use the reduction in air pressure to establish elevation.

Identify Variables:

When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other.

Elevation is an independent variable and air pressure is a dependent variable. In other words we want to see the effect of elevation on air pressure and come up with data that can help us to use a barometer to calculate elevation.

Hypothesis:

Based on your gathered information, make an educated guess about what types of things affect the system you are working with. Identifying variables is necessary before you can make a hypothesis.

My hypothesis is that the elevation can be calculated using the air pressure. Of course elevation is not the only factor that affects air pressure. Special weather conditions will also affect the air pressure and that will reduce the accuracy of our measurements.

Experiment Design:

Design an experiment to test each hypothesis. Make a step-by-step list of what you will do to answer each question. This list is called an experimental procedure. For an experiment to give answers you can trust, it must have a “control.” A control is an additional experimental trial or run. It is a separate experiment, done exactly like the others. The only difference is that no experimental variables are changed. A control is a neutral “reference point” for comparison that allows you to see what changing a variable does by comparing it to not changing anything. Dependable controls are sometimes very hard to develop. They can be the hardest part of a project. Without a control you cannot be sure that changing the variable causes your observations. A series of experiments that includes a control is called a “controlled experiment.”

Experiment: Measuring the elevation from sea level

This experiment needs a safe and easy accessible height. Do it with adult supervision, next to a tall staircase or tall building with open safe access to the sides such as balconies. The process is that you fill up a long tube with water and then close one end of it. Then while the open end of the tube is still in water, we start lifting the closed end up to about 35 feet. You will reach to a point that as the tube goes up, the water does not go up any more and you will see the water level.

At that point measure the vertical distance between the water level in the tube and the water level in your bucket of water. That will be the air pressure at the water level of your bucket. You will later use the air pressure to calculate the elevation.

Procedure:

Connect a heavy object such as a stone to one end of the tube that is supposed to remain in water. Place this end with the attached weight in the bucket of water.

Gradually enter more tube in the water making sure that the tube is filling up with water. You should not get any air bubbles inside the tube. Sucking the air from the other end of the PVC tube may help this process.

Continue this until the entire tube is filled with water. At this time close the end of the tube with the help of a nail or any similar plug. Do this while the end is still in water and make sure no air will enter the tube.

Lift the closed end and pull it towards the elevation using a string or by walking up the stairs. Continue this until some empty space will form at the top of the tube and you see the water level. Measure and record the water level from the level of water in the bucket.

What happened?

Initially while you pull the tube up you will see no empty space because the pressure of the air on the surface of the bucket is forcing the water up the tube. When the column of water is tall enough, the pressure of the water column will exceed the air pressure and water does not go any higher.

How to calculate?

If you measured 403 inches, then your air pressure is 403 inches of water. In other words air pressure is equivalent to the pressure of a column of water 403 inches high. But practically, the unit of atmospheric pressure is not inches of water. Atmospheric pressure is measured in two units, hectopascals (hPa) (which actually is Centimeters of water) and inches of mercury (Hg). The hectopascal is the international unit of measure for atmospheric pressure and inches of mercury are used in the U.S. for aviation purposes. Note: Pressure was formerly reported as millibars (mb). The hectopascal is numerically equivalent to the millibar. If you measured the height of water column in the metric system and it was 1013 cm, then the air pressure is 1013 hPa.

1 inch Hg = 33.86389 hPa

1 millibar = 0.02952998 In Hg

(standard atmosphere having a pressure at sea level of 29.921 inch Hg/1013.25 hPa).

By searching the net we found this information that helps you to calculate the height using pressure.
The table below uses metric units, which scientists use. (You may find a version using feet, Fahrenheit temperatures and pressures in inches of mercury.)

By comparing the first two rows of the above table we can see that for the first 1000 meters, pressure drops 113 hPa and for the second 1000 meters, pressure drops 100 hPa. So each 10 meter change in elevation will result about 1 hPa change in air pressure. That means that the level of water in your tube will be 1 cm higher or lower.
For example, if at your town the height of water column is 850 cm, the air pressure is 850 hPa and the elevation is about 1500 meters above sea level.

To continue our research on modern technologies in measuring the elevation, first, we did a search on the phrase “calculating elevation,” which seemed like it might narrow things down more than “sea level.” One of the first results landed us smack dab in the middle of the National Center for Geographic Information and Analysis’s Core Curriculum for Technical Programs, where John Schaeffer describes using particular “datum planes” when determining elevation via the Global Positioning System (GPS). It seems that a great deal depends on whether you’re using a geoid model or an ellipsoid model.

Confused? Don’t worry. So were we. Here’s the short course: Sometimes elevation above sea level is determined by the geographic feature’s height above “mean sea level”-basically the average height of all of the oceans. While this may be pretty good for determining the relative height of all geographic features worldwide, it’s really not all that accurate, since yes, as you mentioned, sea level isn’t the same all over.

For a more accurate determination of local elevations, scientists are forced into creating mathematical models of the sea’s curvature. That’s what Schaeffer is talking about in his lecture on datum planes.

Of course, that got us wondering if anyone has thought to map the ocean’s topography–the sea’s elevation above (or below) sea level, if you will. Sure enough, after a few quality clicks we spied TOPEX/Poseidon, a joint U.S.-French effort to “measure global sea level.” By exploring the site, we learned that the TOPEX satellite can determine the height of the ocean to an accuracy of five centimeters.

Materials and Equipment:

List of material depends on your final experiment method. Following is a sample:

• 40 feet clear narrow PVC tube #2 or #3.
• Bucket of water.
• A string that can help you to lift the closed end of the tube or measure the elevation height.
• Measuring tape.
• A well polished short nail or plastic rod that can close (air tight) the end of the tube.

Results of Experiment (Observation):

Experiments are often done in series. A series of experiments can be done by changing one variable a different amount each time. A series of experiments is made up of separate experimental “runs.” During each run you make a measurement of how much the variable affected the system under study. For each run, a different amount of change in the variable is used. This produces a different amount of response in the system. You measure this response, or record data, in a table for this purpose. This is considered “raw data” since it has not been processed or interpreted yet. When raw data gets processed mathematically, for example, it becomes results.

Calculations:

Described in the experiment section.

Summary of Results:

Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.

It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.

Conclusion:

Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.

Related Questions & Answers:

What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.

Possible Errors:

If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.

If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious.

Atmospheric conditions also affect the air pressure. Our results will not be accurate if we are on the verge of a weather change.

References:

List of References

http://cfa-www.harvard.edu/space_geodesy/ATLAS/gme.html