The Field of Surveying and the Role of the Surveyor
In its broadest sense, the term surveying encompasses all activities that measure and record information about the physical world and the environment. The term is often used interchangeably with geomatics which is the science of determining the position of points on, above or below the surface of the earth.
Humans have been undertaking surveying activities throughout recorded history. The oldest records indicate that the science began in Egypt. In 1400 BCE, Sesostris divided the land into plots so tax could be collected. The Romans also made significant developments in the field with surveying a necessary activity in their extensive building works across the empire.
The next period of major advancement was the 18th and 19th centuries. European countries needed to accurately map their land and its boundaries, often for military purposes. The UK national mapping agency, the Ordnance Survey was established at this time and used triangulation from a single baseline in the south of England to map the entire country. In the United States, the Coast Survey was established in 1807 with the remit of surveying the coastline and creating nautical charts in order to improve maritime safety.
Surveying has progressed rapidly in recent years. Increased development and the need for precise land divisions, as well as the role of mapping for military requirements have led to many improvements in instrumentation and methods.
One of the most recent advances is that of satellite surveying or Global Navigation Satellite Systems (GNSS), more commonly known as GPS. Many of us are familiar with using sat-nav systems to help us find our way to a new place, but the GPS system also has a wide range of other uses. Originally developed in 1973 by the US military, the GPS network uses 24 satellites at an orbit of 20,200 km to provide positioning and navigation services for a range of applications such as air and sea navigation, leisure applications, emergency assistance, precision timing and providing co-ordinate information when surveying.
The advances in air, space and ground based surveying techniques are in part due to the great increase in computer processing and storage capacity that we have seen over recent years. We can now collect and store vast amounts of data on the measurement of the earth and use this to build new structures, monitor natural resources and help develop new planning and policy guidelines.
There are several types of surveying:
Land Survey: The primary role of the land surveyor is to find and mark certain locations on the land. For example, they could be interested in surveying the boundary of a certain property or finding the coordinates of a specific point on the earth.
Cadastral Land Surveys: These are related to land surveys and are concerned with establishing, locating, defining or describing the legal boundaries of land parcels, often for the purpose of taxation.
Topographic Surveys: The measurement of land elevation, often with the purpose of creating contour or topographic maps.
Geodetic Surveys: Geodetic surveys locate the position of objects on the earth in relation to each other, taking into account the size, shape and gravity of the earth. These three properties vary depending where on the earth's surface you are and changes need to be taken into account if you wish to survey large areas or long lines. Geodetic surveys also provide very precise coordinates that can be used as the control values for other types of surveying.
Engineering Surveying: Often referred to as construction surveying, engineering surveying involves the geometric design of engineering project, setting out the boundaries of features such as buildings, roads and pipelines.
Deformation Surveying: These surveys are intended to ascertain whether a building or object is moving. The positions of specific points on the area of interest are determined and then re-measured after a certain amount of time.
Hydrographic Surveying: This type of surveying is concerned with the physical features of rivers, lakes and oceans. The surveys equipment is on board a moving vessel with follows pre-determined tracks to ensure the entire area is covered. The data obtained are used to create navigational charts, determine depth and measure tide currents. Hydrographic surveying is also used for underwater construction projects such as the laying of oil pipelines.
The requirements for becoming a geomatics surveyor vary from country to country. In many places, you need to obtain a license and / or become a member of a professional association. In the U.S., licensing requirements vary between states and in Canada, surveyors are registered to their province.
At present, the UK suffers from a shortage of qualified land / geomatics surveyors and many organisations have struggled to recruit over recent years.
In the UK, a graduate surveyor's starting salary usually ranges between £16,000 and £20,000. This can rise to £27,000 - £34,000 ($42,000-$54,000) once chartered status is achieved. Chartered status is gained from either the Royal Institute of Chartered Surveyors or the Chartered Institute of Civil Engineering Surveyors. A Masters degree is useful but not essential. Postgraduate qualifications also allow the opportunity to specialise in a specific area of the industry such as geodetic surveying or geographical information science. Entry to the industry with a foundation degree or Higher National Diploma is possible at lower levels such as assistant surveyor or in a related technician role.
Geodetic Surveying
The art of Surveying the earth surface considering its shape and size is called Geodetic Surveying . Geodetic Surveying is suitable for finding out the area of any region on the earth surface, the length and directions of the border lines, contour lines and location of basic points.
It is assumed that the shape of earth is spheroid. The convention held by the International Geodetic and Geophysical Union in 1924 assumed 41, 852, 960 ft as the earth's diameter at the equator and at the poles the diameter is 41, 711, 940 ft. Computation of the equatorial diameter was based on the fact that due to gravitational attraction the earth was flattened exactly by 1/297. Thus, measurement of distances are taken along curved surfaces and not along straight lines. Therefore for Geodetic Surveying, earth's both the diameters are considered. The latitudes and longitudes are determined considering the spheroidical shape of the earth. The points which are used to find out the shape, size and coordinates of the earth surface is called Geodetic Datum in Geodetic Surveying . Hundreds of such points are marked for carrying out Geodetic Survey.
Geodetic Surveying: Finding exact location of an object
Triangulation: As the name indicates, a triangle is incorporated to find out the location of the point in respect of latitude and longitude. The measurements of the sides of the triangle and the angles in the triangle which is drawn with respect to the particular point is found out. With the help of these measurements, longitude and latitude of the triangulation point is calculated.
Bench mark: In Geodetic Surveying, benchmarks are also used for determining the height or elevation of a point. The surveyor gives a permanent mark in the area which shows the benchmark for ages.
GPS based control station: The GPS or Global Positioning System based control station capture the radio signal given by the satellite. This signal is then processed and analyzed to find out the latitudes and logitudes of the given point.
Main instrument forGeodetic Surveying:
Theodolite: It is the basic surveying unit used for Geodetic Surveying. Theodolite consists of a telescope which is placed on a swivel and it can be rotated both horizontally and vertically. Triangulation point are determined by the theodolite in Geodetic Surveying. Two circles-one vertical and another horizontal, are used to read out the readings. But in the modern theodolite the reading is done electronically. Geodetic Surveying can be done by geographers, engineers and surveyors specialised in related disciplines.
Use of Geodetic Surveying:
Engineering purposes: The engineers uses Geodetic Surveying for finding out the exact location of the concerned point or area. Latitudes and longitudes are needed for any engineering constructions.
Construction purposes: The builders used Geodetic Surveying for finding out the direction of the buildings or their exact location for vaastu shastra.
Land Surveying and assessment: The vertical elevation and the horzontal attributes , the latitude and longitudes of the area surveyed are found out through Geodetic Surveying .
Geodetic Surveying is thus considered as an important method of Surveying.
Part II
Geodetic Datums
Geodetic datums define the size and shape of the earth and the origin and orientation of the coordinate systems used to map the earth. Hundreds of different datums have been used to frame position descriptions since the first estimates of the earth's size were made by Aristotle. Datums have evolved from those describing a spherical earth to ellipsoidal models derived from years of satellite measurements.
Modern geodetic datums range from flat-earth models used for plane surveying to complex models used for international applications which completely describe the size, shape, orientation, gravity field, and angular velocity of the earth. While cartography, surveying, navigation, and astronomy all make use of geodetic datums, the science of geodesy is the central discipline for the topic.
Referencing geodetic coordinates to the wrong datum can result in position errors of hundreds of meters. Different nations and agencies use different datums as the basis for coordinate systems used to identify positions in geographic information systems, precise positioning systems, and navigation systems. The diversity of datums in use today and the technological advancements that have made possible global positioning measurements with sub-meter accuracies requires careful datum selection and careful conversion between coordinates in different datums.
The Figure of the Earth
Geodetic datums and the coordinate reference systems based on them were developed to describe geographic positions for surveying, mapping, and navigation. Through a long history, the "figure of the earth" was refined from flat-earth models to spherical models of sufficient accuracy to allow global exploration, navigation and mapping. True geodetic datums were employed only after the late 1700s when measurements showed that the earth was ellipsoidal in shape.
Geometric Earth Models
Early ideas of the figure of the earth resulted in descriptions of the earth as an oyster (The Babylonians before 3000 B.C.), a rectangular box, a circular disk, a cylindrical column, a spherical ball, and a very round pear (Columbus in the last years of his life).
Flat earth models are still used for plane surveying, over distances short enough so that earth curvature is insignificant (less than 10 kms).
Spherical earth models represent the shape of the earth with a sphere of a specified radius. Spherical earth models are often used for short range navigation (VOR-DME) and for global distance approximations. Spherical models fail to model the actual shape of the earth. The slight flattening of the earth at the poles results in about a twenty kilometer difference at the poles between an average spherical radius and the measured polar radius of the earth.
Ellipsoidal earth models are required for accurate range and bearing calculations over long distances. Loran-C, and GPS navigation receivers use ellipsoidal earth models to compute position and waypoint information. Ellipsoidal models define an ellipsoid with an equatorial radius and a polar radius. The best of these models can represent the shape of the earth over the smoothed, averaged sea-surface to within about one-hundred meters.
Earth Surfaces
The earth has a highly irregular and constantly changing surface. Models of the surface of the earth are used in navigation, surveying, and mapping. Topographic and sea-level models attempt to model the physical variations of the surface, while gravity models and geoids are used to represent local variations in gravity that change the local definition of a level surface.
The topographical surface of the earth is the actual surface of the land and sea at some moment in time. Aircraft navigators have a special interest in maintaining a positive height vector above this surface.
Sea level is the average (methods and temporal spans vary) surface of the oceans. Tidal forces and gravity differences from location to location cause even this smoothed surface to vary over the globe by hundreds of meters.
Gravity models attempt to describe in detail the variations in the gravity field. The importance of this effort is related to the idea of leveling. Plane and geodetic surveying uses the idea of a plane perpendicular to the gravity surface of the earth, the direction perpendicular to a plumb bob pointing toward the center of mass of the earth. Local variations in gravity, caused by variations in the earth's core and surface materials, cause this gravity surface to be irregular.
Geoid models attempt to represent the surface of the entire earth over both land and ocean as though the surface resulted from gravity alone. Bomford described this surface as the surface that would exist if the sea was admitted under the land portion of the earth by small frictionless channels.
Global Coordinate Systems
Coordinate systems to specify locations on the surface of the earth have been used for centuries. In western geodesy the equator, the tropics of Cancer and Capricorn, and then lines of latitude and longitude were used to locate positions on the earth. Eastern cartographers like Phei Hsiu used other rectangular grid systems as early as 270 A. D.
Various units of length and angular distance have been used over history. The meter is related to both linear and angular distance, having been defined in the late 18th century as one ten-millionth of the distance from the pole to the equator.
Latitude, Longitude, and Height
The most commonly used coordinate system today is the latitude, longitude, and height system.
The Prime Meridian and the Equator are the reference planes used to define latitude and longitude.
The geodetic latitude (there are many other defined latitudes) of a point is the angle from the equatorial plane to the vertical direction of a line normal to the reference ellipsoid.
The geodetic longitude of a point is the angle between a reference plane and a plane passing through the point, both planes being perpendicular to the equatorial plane.
The geodetic height at a point is the distance from the reference ellipsoid to the point in a direction normal to the ellipsoid.
Earth Centered, Earth Fixed X, Y, and Z
Earth Centered, Earth Fixed Cartesian coordinates are also used to define three dimensional positions.Earth centered, earth-fixed, X, Y, and Z, Cartesian coordinates (XYZ) define three dimensional positions with respect to the center of mass of the reference ellipsoid. The Z-axis points toward the North Pole. The X-axis is defined by the intersection of the plane define by the prime meridian and the equatorial plane. The Y-axis completes a right handed orthogonal system by a plane 90° east of the X-axis and its intersection with the equator.
Geodetic Datums
Datum types include horizontal, vertical and complete datums. Hundreds of geodetic datums are in use around the world. The Global Positioning system is based on the World Geodetic System 1984 (WGS-84). Parameters for simple XYZ conversion between many datums and WGS-84 are published by the Defense mapping Agency. Coordinate values resulting from interpreting latitude, longitude, and height values based on one datum as though they were based in another datum can cause position errors in three dimensions of up to one kilometer. Datum conversions are accomplished by various methods. Complete datum conversion is based on seven parameter transformations that include three translation parameters, three rotation parameters and a scale parameter. Simple three parameter conversion between latitude, longitude, and height in different datums can be accomplished by conversion through Earth-Centered, Earth Fixed XYZ Cartesian coordinates in one reference datum and three origin offsets that approximate differences in rotation, translation and scale. The Standard Molodensky formulas can be used to convert latitude, longitude, and ellipsoid height in one datum to another datum if the Delta XYZ constants for that conversion are available and ECEF XYZ coordinates are not required.
Earth Rotation Studies
The more information we gather from space, the subtler the questions we can ask about the Earth. For instance, take what might seem a simple question—how fast is the Earth turning?
Einstein is supposed to have said that a theory should be as simple as possible, but no simpler. In the case of Earth's rotation, describing its motion as "once every 24 hours" is too simple. Even the answer "23 hours 56 minutes 4.091 seconds" is just a starting point. That number is an average, and every day is a slightly different length by a few microseconds.
How the Earth varies from the average on any given day is not random. There is information—what scientists call a signal—in that variation. And length-of-day, or LOD, has been carefully measured for many decades as one small part of the field of geodesy (the International Association of Geodesy or IAG is the mother ship of this field of study).
The International Earth Rotation Service (IERS) keeps close tabs on this quantity as well as the actual position of the Earth as the world turns. Laser ranging—lidar—and long-baseline interferometry allow the positions of satellites and spots on the Earth's surface to be determined to within about a centimeter. More recently the amazing GPS or Global Positioning System has made these studies almost effortless. With high-quality data, compiled over decades, variations in LOD can usually be traced to particular causes. The simplest possible theory of LOD is complicated.
The length of the day varies when any mass on or in the Earth moves, affecting the state of its angular momentum. Take weather in the atmosphere, for instance. The seasonal changes in the trade winds and monsoons have a well-known effect on the length-of-day over the course of the year. The IERS calculates the angular momentum of the whole atmosphere every six hours, allowing the signal of large-scale weather systems to be detected.
The tides of the ocean have the long-term effect of slowing the Earth down and speeding up the Moon (which thus moves away from Earth a few centimeters per year). They also have short-term effects that are being modeled more accurately all the time. Changes in ocean currents change the length-of-day. Our computer models of ocean circulation are getting good enough, thanks to centimeter-precise measurements of the sea surface, that we can analyze this signal too. The National Earth Orientation Service has a page explaining this stuff in clear detail. (These are also the people who announce leap seconds.)
Other factors affecting the LOD data include rises and subsidences of the land surface, the buildup of glaciers, large earthquakes, large-scale pumping of groundwater and construction of reservoirs, and the shape of the ocean's surface in response to air masses above it.
Each of these can be estimated and their signals extracted from the raw data, untangling the many mixed threads of information in the LOD record. One by one, the sources of variation can be determined and subtracted out, leaving another level to be analyzed.
The last level of variation, a slow drift on the decade scale, seems to be related to the motion of liquid iron in the Earth's core. This layer allows the solid inner core to rotate freely with respect to the outer mantle and crust. Thus every twist and torque exerted by the atmosphere, oceans, Moon, Sun, other planets and the rest of the universe stirs that inner iron ocean, affecting the great dynamo that drives the Earth's magnetic field.
Length-of-day data, then, carries profound information. And without the space program we'd be almost blind to it. Not bad for asking one simple question.
Future of Paper Maps
In a world driven by digital communication, information is no longer shared primarily through paper and postage. Books and letters are frequently generated and transmitted through the computer, as are maps. With the rise of Geographic Information Systems (GIS) and Global Positioning Systems (GPS), the use of traditional paper maps is on a certain decline.
Paper maps have been created and used since the development of basic geographic principles. The foundation of geographic analysis was established by Claudius Ptolemy during the second century CE in his Tetrabiblos. He created numerous world maps, regional maps of varying scale, and fathered the concept of our modern-day atlas. Through its highly topographic nature, Ptolemy’s work transcended time, and greatly influenced Renaissance scholars’ perception of the Earth. His cartography dominated European mapmaking between the 15th and 16th centuries.
By the late 16th century, cosmographer and topographer Gerhard Mercator introduced the Mercator map. The first globe was presented in 1541, and in 1569 the first Mercator world map was published. Using a conformal projection, it represented the Earth as accurately as possible for its time. Meanwhile, land surveying was pioneered in India’s Akbar Empire. A procedure for gathering information on area and land use was developed, in which statistics and land revenue figures were mapped on paper.
The years following the Renaissance Era witnessed groundbreaking cartographic achievement. In 1675, the establishment of the Royal Observatory at Greenwich, England marked the prime meridian at Greenwich, our current longitudinal standard. In 1687, Sir Isaac Newton’s Principia Mathematica on gravitation supported the decrease of latitudinal distance when moving away from the equator, and suggested the slight flattening of Earth at the poles. Similar advances made world maps astonishingly accurate.
Aerial photography made its debut during the mid-1800s, in which land surveying was done from the sky. Aerial photography set the stage for remote sensing and advanced cartographic technique. These basic principles laid the foundation for cartography, modern day paper maps, and digital mapmaking.
Throughout the 1800s and 1900s, the paper map was the layman’s navigational tool of choice. It was accurate and reliable. During the latter half of the 20th century, the progression of paper maps came to a slow. At the same time, advances in technology sparked a human reliance on all things digital, notably data processing and communication.
During the 1960s, mapping software development began with Howard Fisher. Under Fisher, the Harvard Laboratory for Computer Graphics and Spatial Analysis was established. From there, GIS and automated mapping systems grew, and associated databases evolved. In 1968, the Environmental Science Research Institute (ESRI) was founded as a private consulting group. Their research on cartographic software tools and data structure revolutionized modern mapping, and they continue to set precedent in the GIS industry.
In 1970, instruments like Skylab enabled the collection of information about Earth on a fixed schedule. Data were constantly measured and updated, one of the primary advantages of GIS and GPS. The Landsat Program was established during this time, a series of satellite missions managed by the National Aeronautics and Space Administration (NASA) and the United States Geological Survey (USGS). Landsat obtained high resolution data at a global scale. Ever since, we’ve had an improved understanding of Earth’s dynamic surface, and man’s environmental impact.
Space based navigation and positioning systems were designed during the 1970s as well. The U.S. Department of Defense utilized GPS primarily for military purposes. Available for civilian use in the 1980s, GPS provide signals for the tracking of movement anywhere on the planet. GPS systems are not affected by topography or weather, making them reliable tools for navigation. Today, the IE Market Research Corporation expects a 51.3% global market increase for GPS products by 2014.
As a result of public reliance on digital navigation systems, traditional cartography jobs are being downsized, and in many cases eliminated. For example, the California State Automobile Association (CSAA) produced its last paper map of highways in 2008. Since 1909, the had created their own maps and distributed them free to members. A near century later, CSAA had eliminated their cartography team and produce maps only through the AAA national headquarters in Florida. For organizations like the CSAA, mapmaking is now seen as an unnecessary expense. Although the CSAA is no longer investing in traditional cartography, they realize the importance of providing paper maps, and will continue to do so. According to their spokesperson Jenny Mack, “free maps are one of our most popular member benefits”.
A downside to the outsourcing of cartographic skill is the lack of regional knowledge. In the case of the CSAA, their original cartographic team personally surveyed local roads and intersections. The accuracy of survey and cartography from thousands of miles away is questionable. In fact, studies show that paper maps are more accurate than GPS navigation systems. In an experiment done at the University of Tokyo, participants traveled on foot using either a paper map or GPS device. Those using the GPS paused frequently, traveled greater distances, and took longer to get to their destination. Paper map users were more successful.
While digital maps are helpful in getting from "Point A" to "Point B," they lack topographic details and cultural landmarks, among other details. Paper maps show “the big picture”, whereas navigation systems only show direct routes and immediate surroundings. These shortages can lead to geographic illiteracy and dissipate our sense of direction.
Electronic navigation systems are advantageous, especially when driving. However, these advantages are limited, and the best navigational tool to use depends on the situation. Paper maps are simple and informative, yet advanced navigational tools such as Google Maps and GPS are useful as well. Henry Poirot, president of the International Map Trade Association says there is a niche for both digital and paper maps. Paper maps are often used as backup for drivers. He says, “The more people use GPS, the more they realize the importance of the paper product”.
Are paper maps in danger of becoming obsolete? Just as e-mail and e-books are convenient and reliable, we have yet to see the death of libraries, bookstores, and the postal service. In reality, this is highly unlikely. These ventures are losing profit to alternatives, but they simply cannot be replaced. GIS and GPS have made data acquisition and road navigation more convenient, but they do not equate unfolding a map and learning from it. In fact, they would not exist without the contributions of historic scholars. Paper maps and traditional cartography have been rivaled by technology, but they will never be matched.
Map Colors
Cartographers utilize color on a map to represent certain features. Color use is often consistent across different types of maps by different cartographers or publishers. Map colors are (or should be, for a professional looking map) always consistent on a single map.
Many colors used on maps have a relationship to the object or feature on the ground. For example, blue is almost always the color chosen for fresh water or ocean (bust blue may not just represent water).
Political maps, which show more human created features (especially boundaries), usually use more map colors than physical maps, which represent the landscape often without regard for human modification.
Political maps will often use four or more colors to represent different countries or internal divisions of countries (such as states). Political maps will also use such colors as blue for water and black and/or red for cities, roads, and railways. Political maps will also often use black to show boundaries, differing the type of dashes and/or dots used in the line to represent the type of boundary - international, state, or county or other political subdivision.
Physical maps commonly use color most dramatically to show changes in elevation. A palette of greens is often used to display common elevations. Dark green usually represents low-lying land with lighter shades of green used for higher elevations. In the higher elevations, physical maps will often use a palette of light brown to dark brown to show higher elevations. Such maps will commonly use reds or white or purples to represent the highest elevations on the map.
With such a map that uses shades of greens, browns, and the like, it is very important to remember that the color does not represent the ground cover. For example, just because the Mojave Desert is shown in green due to the low elevation, it doesn't mean that the desert is lush with green crops. Likewise, the peaks of mountains shown in white does not indicate that the mountains are capped in ice and snow all year long.
On physical maps, blues are used for water, with darker blues used for the deepest water and lighter blues used for more shallow water. For elevations below sea level, a green-grey or red or blue-grey or some other color is used.
Road maps and other general use maps are often a jumble of color. They use map colors in a variety of ways:
· Blue - lakes, rivers, streams, oceans, reservoirs, highways, local borders
· Red - major highways, roads, urban areas, airports, special interest sites, military sites, place names, buildings, borders
· Yellow - built-up or urban areas
· Green - parks, golf courses, reservations, forest, orchards, highways
· Brown - deserts, historical sites, national parks, military reservations or bases, contour (elevation) lines
· Black - roads, railroads, highways, bridges, place names, buildings, borders
· Purple - highways, (also used on U.S.G.S. topographic maps to represent features added to the map since the original survey)
As you can see, different maps can use colors in a variety of ways. It is important to look at the map key or map legend for the map you are using to become familiar with the color scheme, lest you decide to turn right at an aqueduct.
Special maps called choropleth maps use map color to represent statistical data. The color schemes used by choropleth maps is different from general maps in that the color represents data for a given area. Typically, a choropleth maps will color each county, state, or country a color based on the data for that area. For example, a common choropleth map in the United States shows a state-by-state breakdown of which states voted Republican (red states) and which states voted Democrat (blue states).
Choropleth maps can also be used to show population, educational attainment, ethnicity, density, life expectancy, prevalence of a certain disease, and so much more. When mapping certain percentages, cartographers who design choropleth mas will often use different shades of the same color, which produces a very nice visual effect. For example, a map of county-by-county per capita income could use a range of green from light green for lowest per-capita income to dark green for highest per-capita income.
Topographic Maps
Topographic maps are frequently paired with handheld GPS devices, sports & fitness GPS devices, and smartphone applications.
"Topo" maps provide highly detailed information about the natural and man-made aspects of the terrain, but are best known for their series of contour lines that show elevation changes, and colors signifying varying land types and bodies of water. Topographic maps in their paper form have been in use for many years, and are a mainstay of outdoorspeople and those who must understand landscape details for business purposes.
Topographic maps are increasingly stored, transmitted, and used in digital format. For example, Garmin and DeLorme offer dozens of topo mapsets that may be purchased on DVD, SD card, or via direct download.
Topographic maps come in different scales, and the differences are important. For example, the common "24K" topo map is in the scale of 1:24,000 (1 inch = 2,000 feet) and shows great detail. The 24K map is also known as a "7.5 minute" map, because it covers 7.5 minutes of latitude and longitude. Another common format, the "100K" topo map, is in the scale of 1:100,000 (1 centimeter = 1 kilometer) and shows less detail, but covers a wider area than the 24K topo.
Map Projections
It is impossible to accurately represent the spherical surface of the earth on a flat piece of paper. While a globe can represent the planet accurately, a globe large enough to display most features of the earth at a usable scale would be too large to be useful, so we use maps. Also imagine peeling an orange and pressing the orange peel flat on a table - the peel would crack and break as it was flattened because it can't easily transform from a sphere to a plane. The same is true for the surface of the earth and that's why we use map projections.
The term map projection can be thought of literally as a projection. If we were to place a light bulb inside a translucent globe and project the image onto a wall - we'd have a map projection. However, instead of projecting a light, cartographers use mathematical formulas to create projections.
Depending on the purpose of a map, the cartographer will attempt to eliminate distortion in one or several aspects of the map. Remember that not all aspects can be accurate so the map maker must choose which distortions are less important than the others. The map maker may also choose to allow a little distortion in all four of these aspects to produce the right type of map.
· Conformality - the shapes of places are accurate
· Distance - measured distances are accurate
· Area/Equivalence - the areas represented on the map are proportional to their area on the earth
· Direction - angles of direction are portrayed accurately
A very famous projection is the Mercator Map.
Geradus Mercator invented his famous projection in 1569 as an aid to navigators. On his map, lines of latitude and longitude intersect at right angles and thus the direction of travel - the rhumb line - is consistent. The distortion of the Mercator Map increases as you move north and south from the equator. On Mercator's map Antarctica appears to be a huge continent that wraps around the earth and Greenland appears to be just as large as South America although Greenland is merely one-eighth the size of South America. Mercator never intended his map to be used for purposes other than navigation although it became one of the most popular world map projections.
During the 20th century, the National Geographic Society, various atlases, and classroom wall cartographers switched to the rounded Robinson Projection. The Robinson Projection is a projection that purposely makes various aspects of the map sightly distorted to produce an attractive world map. Indeed, in 1989, seven North American professional geographic organizations (including the American Cartographic Association, National Council for Geographic Education, Association of American Geographers, and the National Geographic Society) adopted a resolution that called for a ban on all rectangular coordinate maps due to their distorion of the planet. .
References:
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23. http://www.ngi.gov.za/index.php/Geodesy-GPS/history-of-geodetic-surveying-in-sa.html
24. Thomas G. Manning, U.S. Coast Survey vs. Naval Hydrographie Office A 19th-Century Rivalry in Science and Politics (Tuscaloosa & London: University of Alabama Press, 1988).
25."The Coast and Geodetic Survey." American Eras. 1997. Encyclopedia.com. 28 Mar. 2012<http://www.encyclopedia.com>.
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