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The Global Geodetic Observing System (GGOS)

GGOS is the Observing System of the International Association of Geodesy (IAG).

GGOS works with the IAG components to provide the geodetic infrastructure necessary for monitoring the Earth system and for global change research. It provides observations of the three fundamental geodetic observables and their variations, that is, the Earth's shape, the Earth's gravity field and the Earth's rotational motion.

GGOS integrates different geodetic techniques, different models, different approaches in order to ensure a long-term, precise monitoring of the geodetic observables in agreement with the Integrated Global Observing Strategy (IGOS).

GGOS provides the observational basis to maintain a stable, accurate and global reference frame and in this function is crucial for all Earth observation and many practical applications.

GGOS contributes to the emerging Global Earth Observing System of Systems (GEOSS) not only with the accurate reference frame required for many components of GEOSS but also with observations related to the global hydrological cycle, the dynamics of atmosphere and oceans, and natural hazards and disasters.

GGOS acts as the interface between the geodetic services and external users such as the Group on Earth Observation (GEO) and United Nations authorities. A major goal is to ensure the interoperability of the services and GEOSS. With this the geodetic community can provide the global geosciences community with a powerful tool consisting mainly of high quality services, standards and references, and of theoretical and observational innovations.

The GGOS Portal will provide a unique access point to all geodetic products. Thus, the Portal will emphasize Geodesy´s contribution to Earth Observation for assessing geohazards and reducing disaster. The Portal consists of the GGOS Web site and the portal itself, comprising geoportal components like a clearinghouse, a map viewer, and a metadata editor. The GGOS Portal is currently under development.

 

Surveying

Surveying method of determining accurately points and lines of direction (bearings) on the earth's surface and preparing from them maps or plans. Boundaries, areas, elevations, construction lines, and geographical or artificial features are determined by the measurement of horizontal and vertical distances and angles and by computations based on geometry and trigonometry.
1) Types and Branches of Surveying
Hydrographic surveying deals with bodies of water and coast lines, is recorded on charts, and records such features as bottom contours, channels, buoys, and shoals. Land surveying includes both geodetic surveying, used for large areas and taking into account the curvature of the earth's surface, and plane surveying, which deals with areas sufficiently small that the earth's curvature is negligible and can be disregarded. Plane surveying dates from ancient times and was highly developed in Egypt. It played an important role in American history in marking boundaries for settlements; surveying was a profession of distinction—both Washington and Jefferson worked for a time as surveyors. Branches of surveying are named according to their purpose, e.g., topographic surveying, used to determine relief, route surveying, mine surveying, construction surveying; or according to the method used, e.g., transit surveying, plane-table surveying, and photogrammetic surveying (securing data by photographs).
2) Instruments and Techniques
In surveying, measurements may be made directly, electronically, by the use of optical instruments, by computations from known lines and angles, or by combination methods. Instruments used for direct linear measurements include the Gunter's chain (known also as the surveyor's chain), which is 66 ft (20 m) long and divided into 100 links; the engineer's chain, 100 ft (30 m) long and also consisting of 100 links; the tape, usually of steel, which has largely superseded chains; and the rod. Tapes and rods made of Invar metal (an alloy of steel and nickel) are used for very precise work because of their low coefficient of thermal expansion. In many situations electronic instruments, such as the geodimeter, which uses light waves, and the tellurometer, which uses microwaves, provide a more convenient and more accurate means of determining distance than do tapes and rods.
The height of points in relation to a datum line (usually mean sea level) is measured with a leveling instrument consisting of a telescope fitted with a spirit level and usually mounted on a tripod. It is used in conjunction with a leveling rod placed at the point to be measured and sighted through the telescope. The transit is used to measure vertical and horizontal angles and may be used also for leveling; its chief elements are a telescope that can be rotated (transited) about a horizontal and about a vertical axis, spirit levels, and graduated circles supplemented by vernier scales. Known also as a transit theodolite, or transit compass, the transit is a modification of the theodolite, an instrument that, in its original form, could not be rotated in a vertical axis. A plane table consists of a drawing board fixed on a tripod and equipped with an alidade (a rule combined with a telescope); it is used for direct plotting of data on a chart and is suitable for rapid work not requiring a high degree of precision.
The stadia method of measuring distance, a rapid system useful in surveying inaccessible terrain and in checking more precise measurements, consists in observing through a telescope equipped with two horizontal cross hairs or wires (stadia hairs) the interval delimited by the hairs on a calibrated stadia rod; the interval depends on the distance between the rod and the telescope.
Surveys based on photographs are especially useful in rugged or inaccessible country and for reconnaissance surveys for construction, mapping, or military purposes. In air photographs, errors resulting from tilt of the airplane or arising from distortion of ground relief may be corrected in part by checking against control points fixed by ground surveys and by taking overlapping photographs and matching and assembling the relatively undistorted central portions into a mosaic. These are usually examined stereoscopically.



Map Reading

Maps are the basic tools of geography. They enable us to depict spatial phenomenon on paper. There are conventions used in cartography which allow a map to be read efficiently and quickly.

A good map will have a legend or key which will show the user what different symbols mean. For instance, a square with a flag on top usually represents a school and roads are represented by a variety of widths and combinations of lines. Often a dashed line represents a border. Note, however, that map symbols used in the United States are often used for different things in other countries. The symbol for a secondary highway on a USGS Topographic map is equivalent to a railroad in Switzerland. Make sure to read the legend and you'll understand the symbols.

Without a north arrow, it is difficult to determine the orientation of a map. With a north arrow (pointing in the correct direction), a user can determine direction. Some maps, such as topographic maps, will point to "true north" (the north pole) and to magnetic north (where your compass points, to northern Canada). Usually, you won't see something quite as detailed as a compass rose but a map does need to provide orientation.

A neatline is the border of a map. It helps to define the edge of the map area and obviously keeps things looking "neat."

Since the map is a flat representation of the curved surface of the earth, all maps are inherently inaccurate.

A map's title provides important clues about the cartographer's intentions and goals. You can hope to expect entirely different information on a map titled "Unemployment in Jefferson County" versus "Topography of Mount St. Helens."

Color appears so often on maps that we often take it for granted that mountains are brown and rivers are blue. Just as there are many types of color maps, there are also many different color schemes used by cartographers. The map user should look to the legend for an explanation of colors on a map.

Our expectations of colors on a map lead to some problems when it is used for elevation. Elevation is often represented as a sequence of dark greens (low elevation or even below sea level) to browns (hills) to white or gray (highest elevation). Since many people associate green with a fertile region, many map users will see lower elevations, which may be deserts, and assume those areas are filled with lush vegetation. Also, people may see the reds and browns of mountains and assume that they are barren, Grand Canyon-type landscapes of desolation but the mountains may be forested and covered in brush.

Additionally, as water always appears bright blue on a map, the user is often inclined to visualize any water on a map as pristine and clear blue - even though it might be entirely different color due to pollution.

Topographic Maps

In the late 17th century, French finance minister Jean Baptiste Colbert hired surveyor, astronomer, and physician Jean Dominique Cassini for an ambitious project, the topographic mapping of France.

After a century of work by Cassini, his son, grandson, and great-grandson, France was the proud owner of a complete set of topographic maps -- the first country to have produced such a prize.

Since the 1600s, topographic mapping has become an integral part of a country's cartography. These maps (called topo maps for short) remain among the most valuable maps for government and the public alike. In the United States, the U.S. Geological Survey (USGS) is responsible for topographic mapping.

There are over 54,000 quadrangles (map sheets) that cover every inch of the United States. The USGS' primary scale for mapping topographic maps is 1:24,000. This means that one inch on the map equals 24,000 inches on the ground, the equivalent of 2000 feet. These quadrangles are called 7.5 minute quadrangles because they show an area that is 7.5 minutes of longitude wide by 7.5 minutes of latitude high. These paper sheets are approximately 29 inches high and 22 inches wide.

Topographic maps use a wide variety of symbols to represent human and physical features. Among the most striking are the topo maps' display of the topography or terrain of the area. Contour lines are used to represent elevation by connecting points of equal elevation. These imaginary lines do a nice job of representing the terrain. As with all isolines, when contour lines lie close together, they represent a steep slope; lines far apart represent a gradual slope. Each quadrangle uses a contour interval (the distance in elevation between contour lines) appropriate for that area. While flat areas may be mapped with a five-foot contour interval, rugged terrain may have a 25-foot or more contour interval. Through the use of contour lines, an experienced topographic map reader can easily visualize the direction of stream flow and the shape of the terrain.

Most topographic maps are produced at a large enough scale to show individual buildings and all streets in cities. In urbanized areas, larger and specific important buildings are represented in black though the urbanized area surrounding them is represented with a red shading. Some topographic maps also include features in purple. These quadrangles have been revised solely through aerial photographs and not by the typical field checking that is involved with the production of a topographic map. These revisions are shown in purple on the map and can represent newly urbanized areas, new roads, and even new lakes.

Topographic maps also use standardized cartographic conventions to represent additional features such as the color blue for water and green for forests.

Several different coordinate systems are shown on topographic maps. In addition to latitude and longitude, the base coordinates for the map, these maps show UTM grids, township and range, and others.

 

Great Circles

A great circle is defined as any circle drawn on a globe (or other sphere) with a center that includes the center of the globe. Thus, a great circle divides the globe into two equal halves. Since they must follow the circumference of the Earth to divide it, great circles are about 40,000 kilometers (24,854 miles) in length along meridians. At the equator though, a great circle is a little bit longer as the Earth is not a perfect sphere.

In addition, great circles represent the shortest distance between two points anywhere on the Earth's surface. Because of this, great circles have been important in navigation for hundreds of years but their presence was discovered by ancient mathematicians.

Great circles are easily identified on a globe based on the lines of latitude and longitude. Each line of longitude, or meridian, is the same length and represents half of a great circle. This is because each meridian has a corresponding line on the opposite side of the Earth. When combined, they cut the globe into equal halves, representing a great circle. For example, the Prime Meridian at 0° is half of a great circle. On the opposite side of the globe is the International Date Line at 180°. It too represents half of a great circle. When the two are combined, they create a full great circle which cuts the Earth into equal halves.

The only line of latitude, or parallel, characterized as a great circle is the equator because it passes through the exact center of the Earth and divides it in half. Lines of latitude north and south of the equator are not great circles because their length decreases as they move toward the poles and they do not pass through Earth's center. As such, these parallels are considered small circles.

The most famous use of great circles in geography is for navigation because they represent the shortest distance between two points on a sphere. Due to the earth's rotation, sailors and pilots using great circle routes must constantly adjust their route as the heading changes over long distances. The only places on Earth where the heading does not change is on the equator or when traveling due north or south.

Because of these adjustments, great circle routes are broken up into shorter lines called Rhumb lines which show the constant compass direction needed for the route being traveled. The Rhumb lines also cross all meridians at the same angle, making them useful for breaking up great circles in navigation.

To determine great circle routes for navigation or other knowledge, the gnomic map projection is often used. This is the projection of choice because on these maps the arc of a great circle is depicted as a straight line. These straight lines are then often plotted on a map with the Mercator projection for use in navigation because it follows true compass directions and is therefore useful in such a setting.

It is important to note though that when long distance routes following great circles are drawn on Mercator maps, they look curved and longer than straight lines along the same routes. In reality though, the longer looking, curved line is actually shorter because it is on the great circle route.

Today, great circle routes are still used in long distance travel because they are the most efficient way to move across the globe. They are most commonly used by ships and aircraft where wind and water currents are not a significant factor though because currents like the jet stream are often more efficient for long distance travel than following the great circle. For example in the northern hemisphere, planes traveling west normally follow a great circle route that moves into the Arctic to avoid having to travel in the jet stream when going the opposite direction as its flow. When traveling east however, it is more efficient for these planes to use the jet stream as opposed to the great circle route.

Whatever their use though, great circle routes have been an important part of navigation and geography for hundreds of years and knowledge of them is essential for long distance travel across the globe.


Date: 2015-12-24; view: 116


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