The Elements of Geodesy

Characteristic elements of gravity. Features of the national reference system. Identifying features of the global positioning system. Assessment of developing applications to the next level: CORS and GIS. The elements of geodesy: the vertical datum.

Рубрика Геология, гидрология и геодезия
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Язык английский
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Министерство образования и науки Российской Федерации

Государственное образовательное учреждение

Высшего профессионального образования

«Тульский государственный университет»

Институт гуманитарных и социальных наук

Кафедра Иностранных языков

The Elements of Geodesy

Выполнил студент гр. 321641

Володин Владимир Александрович

Проверил ассистент каф. Ин.яз.

Лазеба Ирина Владимировна

Тула 2014

Contents

gravity positioning geodesy reference

The Elements of Geodesy: Gravity

The National Spatial Reference System

Do you know where you are? - The Global Positioning System

Taking it to the next level: CORS and GIS

The Elements of Geodesy: The Vertical Datum

The Elements of Geodesy: Gravity

Gravity is the force that pulls all objects in the universe toward each other. On Earth, gravity pulls all objects "downward" toward the center of the planet. According to Sir Isaac Newton's Universal Law of Gravitation, the gravitational attraction between two bodies is stronger when the masses of the objects are greater and closer together. This rule applies to the Earth's gravitational field as well. Because the Earth rotates and its mass and density vaiy at different locations on the planet, gravity also varies.

One reason that geodesists measure variations in the Earth's gravity is because gravity plays a major role in determining mean sea level.

Geodesists calculate the elevation of locations on the Earth's surface based on the mean sea level. So knowing how gravity changes sea level helps geodesists make more accurate measurements. In general, in areas of the planet where gravitational forces are stronger, the mean sea level will be higher. In areas where the Earth's gravitational forces are weaker, the mean sea level will be lower.

To measure the Earth's gravity field, geodesists use instruments in space and on land. In space, satellites gather data on gravitational changes as they pass over points on the Earth's surface. On land, devices called gravimeters measure the Earth's gravitational pull on a suspended mass. With this data, geodesists can create detailed maps of gravitational fields and adjust elevations on existing maps. Gravity principally affects the vertical datum because it changes the elevation of the land surface (Geodesy for the Layman, 1984).

The National Spatial Reference System

All of the elements of geodesy are joined together in the National Spatial Reference System (NSRS). For almost 200 years, the National Geodetic Survey (NGS) and its predecessors have been using geodesy to map the U.S. shoreline, determine land boundaries, and improve transportation and navigation safety. NGS evolved from the Survey of the Coast, an agency established by Thomas Jefferson in 1807. The creation of the United States' first civilian scientific agency was prompted by the increasing importance of waterborne commerce to the fledgling country. As the nation grew westward, NGS's mission began to include surveys of the North American interior.

With numerous surveys being conducted simultaneously across the growing nation, the surveyors needed to establish a common set of reference points. This would insure that surveyors' maps and charts, which often covered hundreds of miles, would align with each other and not overlap. The common set of reference points they used were the benchmarks from the horizontal and vertical datums. Today, the complete set of vertical and horizontal benchmarks for the United States is known as the NSRS. This defined group of reference points acts as the foundation for innumerable activities requiring accurate geodetic information.

Think of it this way: When construction workers begin to build, they have to be sure that the area where they are building is free from dangerous power lines. The construction team will have to find out where the power lines are and make sure they are not building on top of them. To ensure success, the team needs to know the coordinates of the building site and of the local power lines. The NSRS provides a framework for identifying these coordinates. The team can then compare the two sets of coordinates and make sure they do not overlap.

To identify the benchmarks in the NSRS, NGS has traditionally placed markers, or permanent monuments, where the coordinates have been determined. These markers are brass or bronze disks (metals that sustain weathering) and are set in concrete or bedrock. Each marker is about 9 centimeter's wide and has information about NGS printed on its surface.

With the advent of the Global Positioning System (GPS), NGS began to use different kinds of markers. These are made from long steel rods, driven to refusal (pushed into the ground until they won't go any farther.) The top of each rod is then covered with a metal plate. This method ensures that the mark won't move and that people can't destroy or remove it. After tying these marks into a specific horizontal or vertical datum, the mark can be included in the NSRS database. Once the coordinates of the mark are entered into this database, they are available for anyone to use.

Do you know where you are? - The Global Positioning System

Using the Global Positioning System (GPS), every point on Earth can be given its own unique address -- its latitude, longitude, and height. The U.S. Department of Defense developed GPS satellites as a strategic system in 1978. But now, anyone can gather data from them. For instance, many new cars have a GPS receiver built into them. These receivers help drivers know exactly where they are, and can help them from getting lost.

GPS is a constellation of satellites that orbit approximately 11,000 miles above the Earth and transmit radio wave signals to receivers across the planet. By determining the time that it takes for a GPS satellite signal to reach your receiver, you can calculate your distance to the satellite and figure out your exact location on the Earth. Sound easy? In fact it is a very complicated process. For the GPS system to work, you need to have incredibly precise clocks on the satellites and receivers, and you must be able to access and interpret the signals from several orbiting satellites simultaneously. Fortunately, the receivers take care of all the calculations.

Let's tackle the distance calculation first. GPS satellites have very precise clocks that tell time to within 40 nanoseconds or 40 billionths (0.000000040) of a second. There are also clocks in the GPS receivers. Radio wave signals from the satellites travel at 186,000 miles per second. To find the distance from a satellite to a receiver, use the following equation: (186,000 mi/sec) x (signal travel time in seconds) = Distance of the satellite to the receiver in miles.

Knowing the distance of your GPS receiver to a single satellite is useful, but it will not provide you with enough information to determine your exact position on the Earth. For that, you need to simultaneously access the signals from four satellites. By calculating its distance from three satellites simultaneously, a GPS receiver can determine its general position with respect to latitude, longitude, and elevation.

You may wonder why the GPS receiver needs a fourth satellite. Essentially, it provides for even greater precision. To accurately calculate the distance from the GPS receiver to each of the three orbiting satellites, you must know the precise radio signal transmission and reception times. To do this, the clocks in the satellites and the clocks in the receivers must be perfectly synchronized. A mismatch of as little as one millionth of a second between the clock in the satellite and the clock in the receiver could translate into a positioning error of as much as 900 feet (Herring).

Taking it to the next level: CORS and GIS

In a field of study that is thousands of years old, GPS represents a quantum leap in geodesy. As advanced as GPS technology is, most commercially available GPS receivers are only accurate within several meters. Considering that the Earth is almost 25,000 miles in circumference, the difference of a few meters may not seem important. This level of accuracy may be adequate for a hiker in the woods or someone driving a car. But there are many scientific, military, and engineering activities that require much higher levels of positioning accuracy - often to within a few centimeters or less!

To provide measurements at this level of accuracy, NGS developed the Continuously Operating Reference Stations (CORS) network.

CORS is a network of hundreds of stationary, permanently operating GPS receivers throughout the United States. Working 24 hours a day, seven days a week, CORS

stations continuously receive GPS radio signals and integrate their positional data into the National Spatial Reference System. This data is then distributed over the Internet. After logging onto the CORS Web site, users can determine the accuracy of their coordinates to the centimeter. This system has been especially useful in assessing the integrity of buildings and bridges in areas that are geologically active or have been impacted by natural disasters such as hurricanes or floods.

Another powerful tool that has evolved along with GPS technology is the Geographic Information System (GIS). A GIS is comprised of three parts: spatial information, special software, and a computer. These components work together to provide a digital platform for viewing and processing layers of spatial information.

A GIS assembles information from a several of sources, including ground surveys, existing maps, aerial photos, and satellite imagery. In a GIS, specific information about a place, such as the locations of utility lines, roads, streams, buildings, and even trees and animal populations, is layered over a set of geodetic data. Using special software, regional planners and scientists can examine the layers individually or in various combinations to improve traffic flow, merge construction with utility systems, develop around environmentally sensitive areas, and protect the public from potential natural disasters.

Because a GIS stores data digitally, information can be quickly and economically updated, easily reproduced, and made widely available. In fact, because of its power and speed, GIS technology is doing most of the cartographic (mapmaking) work that, in the past, was laboriously done by hand on paper charts and maps.

The Elements of Geodesy: The Vertical Datum

The vertical datum is a collection of specific points on the Earth with known heights either above or below mean sea level. Near coastal areas, mean sea level is determined with a tide gauge. In areas far away from the shore, mean sea level is determined by the shape of the geoid.

Similar to the survey markers used to identify known positions in the horizontal datum, round brass plates mark positions in the vertical datum. The traditional method for setting these vertical benchmarks is called differential leveling. This method uses a known elevation at one location to determine the elevation at another location. As with horizontal datums, the advanced technology of GPS has almost completely replaced this classical technique of vertical measurement.

In 1929, the National Geodetic Survey (NGS) compiled all of the existing vertical benchmarks and created the National Geodetic Vertical Datum of 1929 (NGVD 29). Since then, movements of the Earth's crust have changed the elevations of many benchmarks. In 1988, NGVD 29 was adjusted to remove inaccuracies and to correct distortions. The new datum, called the North American Vertical Datum of 1988 (NAVD 88), is the most commonly used vertical datum in the United States today.

One of the main uses of the vertical datum is to measure rates of subsidence, or land sinking. In Louisiana, for example, large areas of land are rapidly sinking. This is the result of development, coastal erosion, and high population levels. In many areas, the only way to escape an incoming hurricane is to follow specific hurricane evacuation routes. If state and local officials do not have accurate elevation information about these routes, residents trying to leave during an emergency might get trapped in fast-rising water.

By referencing the vertical datum, officials can determine the true elevation and position of the hurricane evacuation routes, as well as how much they have sunk over past decades. For example, in Plaquemines Parish, Louisiana, the main hurricane evacuation route, Highway 23, and the surrounding levees are subsiding by one-quarter to one-half inch per year.

Список используемой литературы

http://oceanservice.noaa.gov/education/kits/geodesy/geo03_figure.html

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