What Is a Geographic Information System? In 1988 the Federal Interagency Coordinating Committee defined the term Geographic Information System in the following manner: “a system of computer hardware, software, and procedures designed to support the capture, management, manipulation, analysis, and display of spatially referenced data for solving complex planning and management problems.” In essence, such […]
What Is a Geographic Information System?
In 1988 the Federal Interagency Coordinating Committee defined the term Geographic Information System in the following manner: “a system of computer hardware, software, and procedures designed to support the capture, management, manipulation, analysis, and display of spatially referenced data for solving complex planning and management problems.” In essence, such a system is an electronic spreadsheet coupled with powerful graphic-manipulation and display capabilities.
The three most important elements of a typical Geographic Information System can be summarized as follows:
1. Cartographic capability
2. Data management capability
3. Analytical capability
The cartographic capabilities built into a Geographic Information System permit the computer – amply aided by skilled human operators – to produce accurate maps and engineering drawings in a convenient pictorial format. Once the digital maps have been constructed and annotated, the computer is used to manipulate the finished product in various specific ways to produce layered maps bristling with colorful attribute symbols.
The data management capabilities enable the GIS operators to store and manipulate map-related information in convenient graphic and non-graphic formats. The storage and manipulation of the non-graphic information is often called “attribute processing”. Operators who are trained to handle the attribute processing can select the desired map data to produce colorful reports laced with a rich mixture of graphics, tabular information, and pictorial attributes. The analytical capabilities associated with today’s GIS software permit the trained operators to process and interpret spatial, tabular, and graphical data in a variety of useful ways. They can, for instance, measured the distance between two points or determine the areas of the various shapes pictured on the screen. The analytical capabilities also help the operators plan, design, and manage such important resources as roads, buildings, bridges, and waterways with maximum practical efficiency.
Reaping The Practical Benefits of GIS Technology All around the world, government professionals, utility engineers, and efficiencyminded entrepreneurs have been quietly investing tens of millions of dollars in attempting to perfect a wide variety of Geographic Information Systems. The GIS routines they have been financing are capable of storing, manipulating, and analyzing complicated electronic maps to increase the efficiency of various largescale operations including city planning, resource management, emergency vehicle dispatch, and water distribution.
Regional and state governments, for example, use GIS to develop country maps, devise the most efficient deployments for public buses, repair roads, collect taxes, chart the spread of contagious diseases, and nail down new election districts.
GIS technology is also being used in some of the most economically underdeveloped countries in the world. As you will learn at a later blog, technicians in Gambia, a tiny country on the West Coast of Africa, have been using GIS processing techniques coupled with inexpensive Navstar GPS receivers to monitor illegal fishing activities in their country’s territorial waters. Jack Dangermond, President of Environmental Systems Research; is convinced that Geographic Information Systems will rapidly spread to other Third-World countries whose citizens will experience immediate benefits. “GIS technology, because of its low-cost, high reliability, user-friendliness and wide usefulness, will be adopted by many users outside the highly developed technological societies,” he asserts. “This offers tremendous promise for improving the future for billions of people on planet Earth.”
Of course, Geographic Information Systems will be broadly adopted by users around the world only if sponsors can foresee measurable economic benefits. Fortunately, for several decades, such benefits have been reported in industry literature and by many users. In 1968, for instance, the Texas Electronic Service Company introduced a grid-based load-management system for its massive electrical transformers. Using rather primitive GIS techniques, company technicians easily found and documented $1 billion in savings over a four-year period.
Similarly, when the Denver Water Department implemented a GIS-based system for its engineering and planning functions, professional technicians on their staff pinpointed immediate savings in time, energy, and labor. Before automation, drafters typically spent two months turning out drawings for each set of 100 cross-sectional maps. After automation, those same products were typically completed in less than two days
POSITIONING MAJOR LANDMARKS In 1988 a team of surveyors used the signals from the Navstar satellites to reestablish the locations of 250,000 landmarks sprinkled across the United States. According to one early press report, their space-age measurements caused the research team to “move the Washington Monument 94.5 feet to the northwest ” And during that […]
POSITIONING MAJOR LANDMARKS
In 1988 a team of surveyors used the signals from the Navstar satellites to reestablish the locations of 250,000 landmarks sprinkled across the United States. According to one early press report, their space-age measurements caused the research team to “move the Washington Monument 94.5 feet to the northwest ” And during that same surveying campaign, they moved the Empire State Building 120.5 feet to the northeast, and they repositioned Chicago’s Sears Tower 90.1 feet to the northwest.
In reality, of course, the Navstar satellites do not give anyone the power to move large, imposing structures, but the precise signals they broadcast do provide our geodetic experts with amazingly accurate and convenient position-fixing capabilities that have been quietly revolutionizing today’s surveying profession. Someday soon the deed to your house may be specified in GPS coordinates. Surveying with a GPS receiver entails a number of critical advantages over classical ground-based methods for pinpointing the locations of widely scattered landmarks on the Earth’s undulating surface. For one thing, intervisibility between benchmarks is not required. Navstar receivers positioned at surveyors benchmarks often have access to the signals from the GPS satellites sailing overhead even though they may not be within sight of one another. This can be especially important in tree-shrouded areas, such is the dense rain forests of Indonesia and Brazil. In such cluttered conditions, conventional surveying teams sometimes spend hours E erecting big, portable towers at each site to achieve the required intervisibility high above the forest canopy. When it is time to move on, they tear the towers down one by one and lug their girders to different locations, and then build them back up again. GPS surveying is advantageous because it is essentially weather-independent, and because it permits convenient and accurate day-night operations. With carrier-aided navigation techniques, site-to-site positioning errors as small as a quarter of an inch can sometimes be achieved.
The signals from the space-based Transit Navigation System have been used for many years to aid specialized terrestrial surveying operations. Unfortunately, Transit surveying suffers from a number of practical limitations as compared with similar operations using the GPS. A Transit satellite, for instance, climbs up above the horizon, on average, only every hour or so compared with the continuous GPS satellite observations. Moreover, achieving and accuracy of a foot or so requires approximately 48 hours of intermittent access to the signals from the Transit satellites. By contrast, the GPS provides inch-level accuracies with the satellite observation interval lasting, at most, only about 1 hour.
DETERMINING THE SHAPE OF PLANET EARTH
For thousands of years scientists have tried to determine the size and shape of planet Earth. During those centuries, shapes resembling tabletops, magnifying glasses, turkey eggs, and Bartlett pears have all, at one time or another, been chosen to model its conjectured shape. The ancient Babylonians, for instance, were convinced that the earth was essentially flat, probably due to erroneous everyday observations. But by 900 BC, they had changed their minds and decided it was shaped like a convex disc. This will belief probably arose when some observant mariner noticed that, whenever a sailboat approaches the horizon, it’s hull drops out of view while the sail was still clearly visible. By 1000 BC Egyptian and Greek scientists had concluded that the earth was a big, round ball. In that era, in fact , Erastothenes managed to make a surprisingly accurate estimate of the actual circumference of the spherical earth. He realized that such an estimate was possible when he happened to notice that it noontime on a particular day, the sun’s rays plunged directly down a well and Aswan, but at that same time due north at Alexandria it’s rays came down at a more shallow angle.
Once he had measured the peak elevation angle of the solar disk at Alexandria on the appropriate day (see Figure 1), Erastothenes estimated the distance from Aswan to Alexandria – probably by noting the travel times of sailing boats or camel caravans. He then a value weighted a simple ratio to get an estimate for the circumference of planet Earth. Translating measurement units across centuries is not an easy thing to do, but our best guess indicates that his estimate for the earth’s radius was too large by around 15 percent. Twenty-five centuries later, Christopher Columbus underestimated the Earth’s radius by 25 percent. He wanted to believe that he inhabited a smaller planet so the Orient would not be prohibitively far away from Europe, sailing west. In 1687, England’s intellectual giant, Sir Isaac Newton, displayed his powerful insights when he reasoned that his home planet, Earth must have a slight midriff bulge. Its shape, he reasoned is governed by hydrostatic equilibrium, as it spinning mass creates enough centrifugal force to sling a big curving girdle of water upward against the pull of gravity. Newton’s mathematical calculations showed that this enormous water-girdle must be around 17 miles high. But were the landmasses affected in the same way as that bulge of water in the seas? Newton understood that if the earth was rigid enough, the landmasses would not be reshaped by the centrifugal forces but he reasoned that, since there were no mountains 17 miles high, the landmasses must be similarly affected, otherwise, no islands would peak up through the water in the vicinity of the equator.
GPS CALIBRATIONS AT THE TURTMANN TEST RANGE
Surveying demonstrations carried out at the Turtmann Test Range in the Swiss Alps have demonstrated that, when a GPS receiver is operated in the carrieraided (interferometry) mode, it can provide positioning inaccuracies comparable to those obtained from the finest available laser-ranging techniques. Figure 2 summarizes the positioning accuracies that the Swiss surveying team was able to achieve in the Turtmann test campaign. In this clever bird’s-eye-view depiction of the range, the various baseline lengths or all accurately proportioned. The short vectors are proportional to the surveying errors in the horizontal plane, but they have been magnified 100,000 times, compared with the dimensions of the baseline lengths.
I n one early test series, the one sigma deviations between the GPS measurements and the earlier glacier-ranging calibrations turned out to be
Sigma X = 0.2 inches
Sigma Y = 0.15 inches
Sigma Z = 0.17 inches.
In an earlier test involving only for base stations with three unknown baseline lengths of 382.2 feet, 1644.4 feet, and 333 feet, the average surveying errors were:
Sigma X = 0.2 inches
Sigma Y = 0.35 inches
Sigma Z = 0.35 inches.
Both sets of measurements were estimated using static surveying techniques in which the GPS receiver sits at each site for about a half-hour to record several hundred pseudo-range measurements. All of the measurements from the various sites are then processed simultaneously to achieve the desired results.
INTRODUCTION Professional surveyors measure, map, and analyze relatively large portions of the Earth’s surface. Armed with precision instruments, they define and record accurate land contours and property boundaries. And they pinpoint the locations of natural landmarks and man-made structures. Surveying has, for centuries, been an essential element of civilized human existence. But it’s practical, everyday […]
Professional surveyors measure, map, and analyze relatively large portions of the Earth’s surface. Armed with precision instruments, they define and record accurate land contours and property boundaries. And they pinpoint the locations of natural landmarks and man-made structures. Surveying has, for centuries, been an essential element of civilized human existence. But it’s practical, everyday im-portance is often overlooked.
Accurate surveying measurements, and the maps that result, make individual property ownership possible. And property ownership, in turn, fosters fruitful human interactions, accentuates the steady accumulation of wealth, and en-hances social prosperity.
“Property is that which is necessary for all civil societies,” observed the famous Scottish philosopher David Hume. America’s 12th president, Abraham Lincoln, echoed a similar sentiment when he concluded that: Property is the fruit of labor . . . It is a positive good in the world. Journalist Leo Rosen was not inclined to contradict President Lincoln’s enthusiastic endorsement. “Property is a sacred trust,” he once concluded, “expressly granted by God, the Bible, and the Recorder’s Office.”
Compelling evidence that property boundaries were being established by sur-veyors as early as 1400 BC has been found among stone carvings found on the broad floodplains and the fertile valleys of ancient Egypt. During the Roman occupation of that prosperous and fertile kingdom, Roman technicians studied, absorbed, and copied the techniques the Egyptians had perfected while they were constructing the great pyramid at Giza.with its nearly perfect proportions and its surprisingly precise north-south alignment.
Around 15 BC, Roman engineers made at least one innovative contribution to the art and science of surveying when they mounted a large, thin wheel in barrel-fashioned on the bottom of a sturdy cart.
When their clever mechanism was pushed along the ground, it automatically dropped a single pebble into a small container with each 360-degree revolution. The number of pebbles rattling around in the container provided a direct measure of the distance traveled by the device. When perfected, it became the world’s first crude, but reliable, odometer!
Roman surveyors refined the methods and mechanisms pioneered by the Egyptians and used their techniques in surveying more than 40,000 miles of Roman roads and in laying out hundreds of miles of aqueducts funneling water to their thirsty cities.
SURVEYING INVENTIONS THAT SPROUTED UP DURING THE RENAISSANCE
In 1620 the famous English mathematician Edmund Gunter develop the earliest surveying chain. It was widely used by surveyors until the steel tape carne into existence 400 years later.
The vernier, a precise auxiliary scale that permitted more accurate readings of dis-tances and angles, was invented in 1631. It was followed by the micrometer micro-scope in 1638 and telescope sights in 1669. The spirit level followed around 1700.A spirit level relies on a small bubble floating in a liquid-filled glass cylinder that is precisely centered when the device is perpendicular the local direction of gravity. .
By the 1920s photogrammetry–the science of constructing accurate maps from aerial photographs–came into general use. And, 50 years later, in the 1970s, orbiting satellites began to serve as dedicated reference points for measuring millions of attitude angles and distances. These measurements allowed contem-porary experts to construct ground-level maps with unprecedented levels of accuracy and convenience. By the 1990s spaceborne centimeter-level surveying had become convenient, practical, and considerably less expensive, too.
Surveying methodologies can be divided into two broad categories: plane surveying – which typically involves distances shorter than 12 miles, and geodetic surveying–which spans areas so large the curvature of the earth must come into play.
Plane surveying assumes that the earth is flat in a small local area. Under this con-dition, relatively simple computational algorithms from Euclidean geometry and plane trigonometry can be employed in processing the measurements the surveyor makes. The region being surveyed is typically divided into a small chain of triangles or quadrangles.
When the simpler triangles.are employed, the three interior angles for each tri-angle must sum to 180 degrees and the common side being shared by a pair of the adjacent triangles must be constrained to have the same length in both of the relevant trigonometric calculations. Specialized numerical adjustments force the computations to produce mutually consistent results.
The approach that relies on quadrangles involves four sides, eight angles, and two diagonals. All shared dimensions are forced to end up with mutually consistent results.
GEODETIC SURVEYING ON A MUCH LARGER. SCALE
Geodetic surveying must be applied when the areas being surveyed are so ex-tensive the Earth’s curvature has an appreciable effect on the surveyor’s measurements. In this case spherical trigonometry is required despite the fact that it involves greater complexity and more intricate visualization for those in-terpreting the results.
In 1687 Sir Isaac Newton demonstrated that the earth exhibits a pronounced bulge at the equator. Its first-order spherical shape is distorted by the centrifugal forces induced by its daily rotation.The shape it assumes can be approximated as a oblate spheroid with an equatorial diameter approximately 27 miles longer than its polar diameter.
Huge numbers of measurements affecting the Earth’s non-spherical shape have been incorporated into a variety of mathematical reconstructions of the Earth’s equatorial bulge. These approximations are called datums when they are being used in connection with geodetic surveying.
Leveling measurements establishing a fictitious local sea level are often used in constructing the precise oblate spheroids used in modeling and analyzing surveying operations. One of the earliest and most popular of these models is the Clark ellipsoid of 1866. For more than a century it has been employed as an engineering model defining the shape and gravitational characteristics of our home planet.
Surfaces determined by leveling measurements approximate the average long-term sea level of our home planet. Such surfaces are distorted slightly because, at high northern and southern latitudes, the outer edge of the oblate spheroid is in closer proximity to the Earth’s center where most of its gravity is concentrated.
MODERN ACCOMPLISHMENTS IN AERIAL PHOTOGRAPHY
Military commanders have always struggled to capture and hold the “high ground” because an elevated vantage point often provides an unobstructed view of enemy activities on the ground below. During the American Civil War (circa 1860) hot-air balloons carried reconnaissance experts up among the clouds where they could observe enemy troop deployments and equipment placements.
During World War I and World War II, substantial resources were expended by the various combatants in attempting to survey the sprawling battlefields scattered across continent-wide dimensions. And, when peace ascended over though the smoke-powder battlegrounds, the accuracy and convenience of military surveying and mapmaking operations were appreciably accentuated by aerial observations.
Later in Kentucky (the author’s home state) tobacco acreages were measured, estimated, and controlled by precise government-sponsored surveys of this type. Indeed, this allotment system is still1 .. today, controlled by that same highly efficient approach to terrestrial surveying.
SURVEYING GOD’S GREEN EARTH WITH ORBITING SATELLITES
Orbiting satellites became relatively inaccurate surveying tools shortly after the Rus-sians launched their first Sputnik into outer space in October of 1957. The earliest American satellites used in this manner were the two 100-foot Echo Balloons clearly visible from the surface of the earth. These aluminum-coated mylar balloons allowed crude, but convenient, mapping of otherwise inaccessible regions of the Earth. This could be accomplished by bouncing a sequence of brief radar pulses off the skin of the balloon and timing the bent-pipe signal travel-times between a known location on earth and the one that was yet to be determined.
Camera-equipped satellites have also found widespread applications in surveying and mapmaking enterprises. Shortly after the first Sputnik reached orbit, President Eisenhower presented the ambassador of Brazil with an accurate map of his forest-shrouded country. NASA’s imaging experts had kludged it together by combining dozens of satellite images into a countrywide composite.
Later the six Transit Navigation Satellites and the two dozen or so satellites in the GPS constellation made surveying considerably more accurate, convenient, and cost-effective. GPS-derived sub-centimeter accuracies soon became possible using the precise timing measurements made available by the GPS satellites and their international competitors. Positioning errors were dramatically reduced compared with most conventional surveying techniques. In part, this became possible because ground-based and space-based hardware units and new software modules were soon providing accurate and reliable positioning corrections.
Professional surveyors measure, map, and analyze relatively large portions of the Earth’s surface. Armed with precision instruments, they define and record accurate land contours and property boundaries. And they pinpoint the spatial locations of natural landmarks and man-made structures. Surveying has, for many centuries, been an essential element of civilized human existence. But it’s practical, everyday importance is sometimes overlooked.
Hopefully, this brief article will help bring the fundamental importance of pre-cision surveying back into sharp focus.
Seal Beach, California