Surveying and GIS
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Wikipedia-Article "Surveying"
- For the American space program, see Surveyor program
Surveying is the art and science of accurately determining the position of points and the distances between them. These points are usually, but not exclusively, associated with positions on the surface of the Earth, and are often used to establish land boundaries for ownership or governmental purposes.
In order to accomplish their objective, surveyors use elements of engineering, physics, mathematics and law.
Surveying has been an essential element in the development of the human environment since the beginning of recorded history and it is a requirement in the planning and execution of nearly every form of construction. Its most familiar modern uses are in the fields of transport, building and construction, communications, mapping, and the definition of legal boundaries for land ownership.
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Method
Historically, distances were measured using a variety of means, such as chains with links of a known length, for instance a Gunter's Chain (see Edmund Gunter), or measuring tapes made of steel or invar. In order to measure horizontal distances, these chains or tapes would be pulled taut, to reduce sagging and slack. Additionally, attempts to hold the measuring instrument level would be made. In instances of measuring up a slope, the surveyor might have to "break" the measurment- that is, raise the rear part of the tape upward, plumb from where the last measurement ended.
Horizontal angles were historically measured using compasses, which would provide a magnetic bearing, from which deflections could be measured. This type of instrument was later improved upon, through more carefully scribed discs, providing better angular resolution, as well as through mounting telescopes with reticles for more precise sighting atop the disc. Additionally, levels and calibrated circles allowing measurement of vertical angles were added, along with verniers for measurement down to a fraction of a degree- such as a turn-of-the-century Transit (surveying).
The simplest method for measuring height is with an altimeter, which is basically a barometer—using air pressure as an indication of height. But for surveying more precision is needed. Toward this end, a variety of means, such as precise levels have been developed, which are calibrated to provide a precise plane from which differentials in height between the instrument and the point in question, typically through the use of a vertical measuring rod.
The basic tool is a theodolite, set on a tripod, with which one can measure angles (horizontal and vertical), combined with triangulation. Starting from a benchmark, a position with known location and elevation, the distance and angles to the unknown point are measured. A more modern instrument is a total station, which is basically a theodolite with an electronic distance measurement device (EDM). Still more modern is the use of satellite positioning systems, such as a Global Positioning System (GPS). Though GPS systems have increased the speed of surveying, they are still only accurate to about 20 mm. It is because of this that EDMDs have not been completely phased out. Robotics allows surveyors to gather precise measurements without extra workers to look through and turn the telescope or record data. A faster way to measure (no obstacles) is with a helicopter with laser echolocation, combined with GPS to determine the height of the helicopter. To increase precision, beacons are placed on the ground (about 20 km apart). This method reaches a precision of about 5 cm.
With the triangulation method, first, one needs to know the horizontal distance to the object. If this is not known or cannot be measured directly, it is determined as explained in the triangulation article. Then the height of an object can be determined by measuring the angle between the horizontal plane and the line through that point at a known distance and the top of the object. In order to determine the height of a mountain, one should do this from sea level (the plane of reference), but here the distances can be too great and the mountain may not be visible. So it is done in steps, first determining the position of one point, then moving to that point and doing a relative measurement, and so on until the mountain top is reached.
Origins
Surveying techniques have existed throughout much of recorded history. In Ancient Egypt, when the Nile River overflowed its banks and washed out farm boundaries, boundaries were re-established through the application of simple geometry. The nearly perfect squareness and north-south orientation of the Great Pyramid of Giza, built c. 2700 BC, affirm the ancient Egyptians' command of surveying.
- The Egyptian land register (3000 BC).
- In Rome, the tax register of conquered lands(300 AD).
- In England, The Domesday Book by William the Conqueror(1086)
- covered all England
- contained names of the land owners, area, land quality, and specific information of the area's content and habitants.
- did not include maps showing exact locations
- Continental Europe's Cadastre was created in 1808
- founded by Napoleon I (Bonaparte), "A good cadastre will be my greatest achievement in my civil law", Napoleon I
- contained numbers of the parcels of land (or just land), land usage, names etc., and value of the land
- 100 million parcels of land, triangle survey, measurable survey, map scale: 1:2500 and 1:1250
- spread fast around Europe, but faced problems especially in Mediterranean countries, Balkan, and Eastern Europe due to cadastre upkeep costs and troubles.
A cadastre loses its value if register and maps are not constantly updated.
Large scale surveys are a necessary pre-requisite to map-making. In the late 1780s, a team from the Ordnance Survey of Great Britain, originally under General William Roy began the Principal Triangulation of Britain using the specially built Ramsden theodolite.
Types of surveys
- ALTA/ACSM survey: a surveying standard jointly proposed by the American Land Title Association and the American Congress on Surveying and Mapping that incorporates elements of the boundary survey, mortgage survey, and topographic survey. ALTA/ACSM surveys, frequently shortened to ALTA surveys, are often required for real estate transactions.
- Boundary survey: the actual physical extent of property ownership, typically witnessed by monuments or markers, such as (typically iron rods, pipes or concrete monuments in the ground, but also tacks or blazes in trees, piled stone corners or other types of monuments) are measured, and a map, or plat, is drawn from the data.
- Deformation survey: a survey to determine if a structure or object is changing shape or moving. The three-dimensional positions of specific points on an object are determined, a period of time is allowed to pass, these positions are then re-measured and calculated, and a comparison between the two sets of positions is made.
- Draw lot: one lot from a plat is drawn, with any easements and setbacks that may be on it.
- Foundation survey: the position of the house is measured before it is finished being built.
- Mortgage survey: a simple survey that generally determines land boundaries and building locations. Mortgage surveys are required by title companies and lending institutions when they provide financing to show that there are no structures encroaching on the property and that the position of structures is generally within zoning and building code requirements. Mortgage surveys are not sufficiently accurate for use in building new structures.
- Physical survey: the finished house and driveway are measured, and all markers on the boundary are indicated. This is recorded when the lot is sold.
- Plot plan: a proposal for a house or other building and driveway or parking lot are added to a draw lot.
- Subdivision plat: a plot or map based on a survey of a parcel of land. Lines are drawn inside it, indicating the location of roads and lots. Plats are usually discussed back and forth between the developer and the surveyor until they are agreed upon, at which point pins are driven into the ground to mark the lot corners and curve ends, and the plat is recorded in the cadastre (USA, elsewhere) or land registry (UK).
- Topographic survey: a survey that measures the elevation of points on a particular piece of land, and presents them as contours on a plot.
- Hydrographic survey: a survey conducted with the purpose of mapping the seabed for navigation, engineering, or resource management purposes. Products of such surveys are nautical charts. See hydrography.
- Construction surveying (otherwise 'lay-out' or 'setting-out'): the process of establishing and marking the position and detailed layout of new structures such as roads or buildings for subsequent construction. In this sense, surveying may be regarded as a sub-discipline of civil engineering.
Surveying as a career
The basic principles of surveying have changed little over the ages, but the tools used by surveyors have evolved tremendously. Engineering, especially civil engineering, depends heavily on the surveyor. Whenever there are roads, dams, retaining walls, bridges or residential areas to be built, surveyors are involved. They determine the boundaries of private property and the boundaries of various lines of political divisions. They also provide advice and data for geographical information systems (GIS), computer databases that contain data on land features and boundaries.
Surveyors must have a thorough knowledge of algebra, basic calculus, geometry, and trigonometry. They must also know the laws that deal with surveys, property, and contracts. In addition, they must be able to use delicate instruments with accuracy and precision.
In most states of the U.S., surveying is recognized as a distinct profession apart from engineering. Licensing requirements vary by state, however these requirements generally all have a component of education, experience and examinations. In the past, experience gained through an apprenticeship, together with passing a series of state-administered examinations, was required to attain licensure. Nowadays, many states require a Bachelor of Science in Surveying, or a Bachelor of Science in Civil Engineering with additional coursework in surveying, in addition to experience and examination requirements. Typically the process for registration follows two phases- first, upon graduation, the candidate may be eligible to sit for the Fundamentals of Land Surveying exam, to be certified upon passing and meeting all other requrements as a Surveyor In Training (SIT). Upon being certified as an SIT, the candidate then needs to gain additional experience until he or she becomes eligible for the second phase, which typically consists of the Principles and Practice of Land Surveying exam along with a state-specific examination.
Registered surveyors usually denote themselves with the letters P.S. (professional surveyor), L.S. (land surveyor), or P.L.S. (professional land surveyor), or P.S.M. (professional surveyor and mapper) following their names, depending upon the dictates of their particular state of registration.
Typically a licensed land surveyor is required to seal all plans with a seal, the format of which is dictated by their state jurisdiction, which shows their name and registration number. In many states, land surveyors are also required to place caps bearing their registration number on property corners which they have set.
See also
- Architecture
- Benchmark
- Chain
- Dumpy level
- Geodesy
- Geographic information system
- Geoid
- Geomatics
- Geomatics Engineering
- Global Positioning System
- Jacob's staff
- Point of Beginning
- Real estate
- Royal Institution of Chartered Surveyors (RICS)
- Spirit leveling
- Tacheometry
- Technical drawing
- Theodolite
- Topography
Famous surveyors
- Benjamin Banneker
- Len Beadell
- Daniel Boone
- Admiral John Bossler
- William Austin Burt
- Captain James Cook (navigator)
- Walt Disney (subdivision maps)
- Andrew Ellicott
- John Ericsson
- Sir George Everest
- Peter Fidler
- Sir John Forrest
- Malcolm Fraser
- Captain John C. Fremont
- Carl Friedrich Gauss
- Edmund Gunter
- Thomas Jefferson
- Joseph Jenckes (1656-1740), Governor of Rhode Island
- Harry Johnston
- Lewis and Clark
- Abraham Lincoln
- Liu Hui
- Colonel William Light
- Sir Alexander Mackenzie
- Charles Mason and Jeremiah Dixon
- Metius
- Major Sir Thomas Mitchell
- Peter Pond
- John Septimus Roe
- General William Roy
- Willem Schermerhorn
- David Thompson
- Henry David Thoreau
- Charles Tyers
- Captain George Vancouver
- George Washington
- Per Ivar Gjengedal
External links
- Organizations
- Technology
Wikipedia-Article "GIS"
- GIS redirects here. For other meanings, see GIS (disambiguation).
A geographic information system (GIS) is a system for creating and managing spatial data and associated attributes. In the strictest sense, it is a computer system capable of integrating, storing, editing, analyzing, and displaying geographically-referenced information. In a more generic sense, GIS is a "smart map" tool that allows users to create interactive queries (user created searches), analyze the spatial information, and edit data.
Geographic information systems technology can be used for scientific investigations, resource management, asset management, development planning, cartography and route planning. For example, a GIS might allow emergency planners to easily calculate emergency response times in the event of a natural disaster, or a GIS might be used to find wetlands that need protection from pollution.
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History of development
35,000 years ago, on the walls of caves near Lascaux, France, Cro-Magnon hunters drew pictures of the animals they hunted. Associated with the animal drawings are track lines and tallies thought to depict migration routes. These early records followed the two-element structure of modern geographic information systems: a graphic file linked to an attribute database.
In the 18th century, modern surveying techniques for topographic mapping were implemented, along with early versions of thematic mapping, e.g. for scientific or census data.
A notable example of this is John Snow's 1854 map depicting a cholera outbreak in London, which provided analysis to narrow the source of the cholera to a contaminated pump, stemming the outbreak.Images of John Snow's maps
The early 20th century saw the development of "photo lithography" where maps were separated into layers. Computer hardware development spurred by nuclear weapon research would lead to general purpose computer "mapping" applications by the early 1960s.
The year 1967 saw the development of the world's first true operational GIS in Ottawa, Ontario by the federal Department of Energy, Mines and Resources. Developed by Roger Tomlinson, it was called "Canadian GIS" (CGIS) and was used to store, analyse and manipulate data collected for the Canada Land Inventory (CLI)—an initiative to determine the land capability for rural Canada by mapping information about soils, agriculture, recreation, wildlife, waterfowl, forestry, and land use at a scale of 1:250,000. A rating classification factor was also added to permit analysis.
CGIS was the world's first "system" and was an improvement over "mapping" applications as it provided capabilities for overlay, measurement, digitizing/scanning, supported a national coordinate system that spanned the continent, coded lines as "arcs" having a true embedded topology, and it stored the attribute and locational information in separate files. Its developer, geographer Roger Tomlinson, has become known as the "father of GIS."
CGIS lasted into the 1990s and built the largest digital land resource data base in Canada. It was developed as a mainframe based system in support of federal and provincial resource planning and management. Its strength was continent-wide analysis of complex data sets. The CGIS was never available in a commercial form. Its initial development and success stimulated various commercial mapping applications being sold by vendors such as Intergraph. The development of micro-computer hardware spurred vendors such as ESRI, MapInfo and CARIS to successfully incorporate many of the CGIS features, combining the first generation approach to separation of spatial and attribute information with a second generation approach to organizing attribute data into database structures. The 1980s and 1990s industry growth were spurred on by the growing use of GIS on Unix workstations and the personal computer. By the end of the 20th century, the rapid growth in various systems had been consolidated and standardized on relatively few platforms and users were beginning to export the concept of viewing GIS data over the Internet, requiring data format and transfer standards.
Techniques used in GIS
Relating information from different sources
If you could relate information about the rainfall of your state to aerial photographs of your county, you might be able to tell which wetlands dry up at certain times of the year. A GIS, which can use information from many different sources in many different forms, can help with such analyses. The primary requirement for the source data consists of knowing the locations for the variables. Location may be annotated by x,y, and z coordinates of longitude, latitude, and elevation, or by other geocode systems like ZIP Codes or by highway mile markers. Any variable that can be located spatially can be fed into a GIS. Several computer databases that can be directly entered into a GIS are being produced by government agencies and non-government organizations. Different kinds of data in map form can be entered into a GIS.
A GIS can also convert existing digital information, which may not yet be in map form, into forms it can recognize and use. For example, digital satellite images generated through remote sensing can be analyzed to produce a map-like layer of digital information about vegetative covers. Another fairly developed resource for naming GIS objects is the Getty Thesaurus of Geographic Names (GTGN), which is a structured vocabulary containing around 1,000,000 names and other information about places[1].
Likewise, census or hydrologic tabular data can be converted to map-like form, serving as layers of thematic information in a GIS.
Data representation
GIS data represents real world objects (roads, land use, elevation) with digital data. Real world objects can be divided into two abstractions: discrete objects (a house) and continuous fields (rain fall amount or elevation). There are two broad methods used to store data in a GIS for both abstractions: Raster and Vector.
Raster data type consists of rows and columns of cells where in each cell is stored a single value. Most often, raster data are images (raster images), but besides just color, the value recorded for each cell may be a discrete value, such as land use, a continuous value, such as rainfall, or a null value if no data is available. While a raster cell stores a single value, it can be extended by using raster bands to represent RGB (red, green, blue) colors, colormaps (a mapping between a thematic code and RGB value), or an extended attribute table with one row for each unique cell value. The resolution of the raster dataset is its cell width in ground units. For example, in a LIDAR raster image, each cell is a pixel that represents an area of 3 meters by 3 meters. Usually cells represent square areas of the ground, but other shapes can also be used.
Vector data type uses geometries such as points, lines (series of point coordinates), or polygons, also called areas (shapes bounded by lines), to represent objects. Examples include property boundaries for a housing subdivision represented as polygons and well locations represented as points. Vector features can be made to respect spatial integrity through the application of topology rules such as 'polygons must not overlap'. Vector data can also be used to represent continuously varying phenomena. Contour lines and triangulated irregular networks (TIN) are used to represent elevation or other continuously changing values. TINs record values at point locations, which are connected by lines to form an irregular mesh of triangles. The face of the triangles represent the terrain surface.
There are advantages and disadvantages to using a raster or vector data model to represent reality. Raster datasets record a value for all points in the area covered which may require more storage space than representing data in a vector format that can store data only where needed. Raster data also allows easy implementation of overlay operations, which are more difficult with vector data. Vector data can be displayed as vector graphics used on traditional maps, whereas raster data will appear as an image that may have a blocky appearance for object boundaries.
Additional non-spatial data can also be stored besides the spatial data represented by the coordinates of a vector geometry or the position of a raster cell. In vector data, the additional data are attributes of the object. For example, a forest inventory polygon may also have an identifier value and information about tree species. In raster data the cell value can store attribute information, but it can also be used as an identifier that can relate to records in another table.
Data capture
Data capture—entering information into the system—consumes much of the time of GIS practitioners. There are a variety of methods used to enter data into a GIS where it is stored in a digital format.
Existing data printed on paper or mylar maps can be digitized or scanned to produce digital data. A digitizer produces vector data as an operator traces points, lines, and polygon boundaries from a map. Scanning a map results in raster data that could be further processed to produce vector data.
Survey data can be directly entered into a GIS from digital data collection systems on survey instruments. Positions from a global positioning system (GPS), another survey tool, can also be directly entered into a GIS.
Remotely sensed data also plays an important role in data collection and consist of sensors attached to a platform. Sensors include cameras, digital scanners and LIDAR, while platforms usually consist of aircraft and satellites.
The majority of digital data currently comes from photo interpretation of aerial photographs. Soft copy workstations are used to digitize features directly from stereo pairs of digital photographs. These systems allow data to be captured in 2 and 3 dimensions, with elevations measured directly from a stereo pair using principles of photogrammetry. Currently, analog aerial photos are scanned before being entered into a soft copy system, but as high quality digital cameras become cheaper this step will be skipped.
Satellite remote sensing provides another important source of spatial data. Here satellites use different sensor packages to passively measure the reflectance from parts of the electromagnetic spectrum or radio waves that were sent out from an active sensor such as radar. Remote sensing collects raster data that can be further processed to identify objects and classes of interest, such as land cover.
When data is captured, the user should consider if the data should be captured with either a relative accuracy or absolute accuracy, since this could not only influence how information will be interpreted but also the cost of data capture.
In addition to collecting and entering spatial data, attribute data is also entered into a GIS. For vector data this includes additional information about the objects represented in the system.
After entering data into a GIS, it usually requires editing, to remove errors, or further processing. For vector data it must be made "topologically correct" before it can be used for some advanced analysis. For example, in a road network, lines must connect with nodes at an intersection. Errors such as undershoots and overshoots must also be removed. For scanned maps, blemishes on the source map may need to be removed from the resulting raster. For example, a fleck of dirt might connect two lines that should not be connected.
Data manipulation
Data restructuring can be performed by a GIS to convert data into different formats. For example, a GIS may be used to convert a satellite image map to a vector structure by generating lines around all cells with the same classification, while determining the cell spatial relationships, such as adjacency or inclusion.
Since digital data are collected and stored in various ways, the two data sources may not be entirely compatible. So a GIS must be able to convert geographic data from one structure to another.
Projections, coordinate systems and registration
A property ownership map and a soils map might show data at different scales. Map information in a GIS must be manipulated so that it registers, or fits, with information gathered from other maps. Before the digital data can be analyzed, they may have to undergo other manipulations—projection and coordinate conversions, for example—that integrate them into a GIS.
The earth can be represented by various models, each of which may provide a different set of coordinates (e.g., latitude, longitude, elevation) for any given point on the earth's surface. The simplest model is to assume the earth is a perfect sphere. As more measurements of the earth have accumulated, the models of the earth have become more sophisticated and more accurate. In fact, there are models that apply to different areas of the earth to provide increased accuracy (e.g., North American Datum, 1983 - NAD83 - works well in North America, but not in Europe). See Datum for more information.
Projection is a fundamental component of map making. A projection is a mathematical means of transferring information from a model of the Earth, which represents a three-dimensional curved surface, to a two-dimensional medium—paper or a computer screen. Different projections are used for different types of maps because each projection particularly suits certain uses. For example, a projection that accurately represents the shapes of the continents will distort their relative sizes. See Map projection for more information.
Since much of the information in a GIS comes from existing maps, a GIS uses the processing power of the computer to transform digital inform