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101-416: When applied to processing of the reflected signal in remote sensing for topography , homodyne detection lacks the ability of heterodyne detection to determine the size of a static discontinuity in elevation between two locations. (If there is a path between the two locations with smoothly changing elevation, then homodyne detection may in principle be able to track the signal phase along the path if sampling

202-495: A plane table in 1551, but it is thought that the instrument was in use earlier as his description is of a developed instrument. Gunter's chain was introduced in 1620 by English mathematician Edmund Gunter . It enabled plots of land to be accurately surveyed and plotted for legal and commercial purposes. Leonard Digges described a theodolite that measured horizontal angles in his book A geometric practice named Pantometria (1571). Joshua Habermel ( Erasmus Habermehl ) created

303-444: A GPS on large scale surveys make them popular for major infrastructure or data gathering projects. One-person robotic-guided total stations allow surveyors to measure without extra workers to aim the telescope or record data. A fast but expensive way to measure large areas is with a helicopter, using a GPS to record the location of the helicopter and a laser scanner to measure the ground. To increase precision, surveyors place beacons on

404-690: A degree or two with electronic compasses. Compasses can measure not just azimuth (i. e. degrees to magnetic north), but also altitude (degrees above the horizon), since the magnetic field curves into the Earth at different angles at different latitudes. More exact orientations require gyroscopic-aided orientation , periodically realigned by different methods including navigation from stars or known benchmarks. The quality of remote sensing data consists of its spatial, spectral, radiometric and temporal resolutions. In order to create sensor-based maps, most remote sensing systems expect to extrapolate sensor data in relation to

505-477: A fixed base station and a second roving antenna. The position of the roving antenna can be tracked. The theodolite , total station and RTK GPS survey remain the primary methods in use. Remote sensing and satellite imagery continue to improve and become cheaper, allowing more commonplace use. Prominent new technologies include three-dimensional (3D) scanning and lidar -based topographical surveys. UAV technology along with photogrammetric image processing

606-554: A great deal of data handling overhead. These data tend to be generally more useful for many applications. The regular spatial and temporal organization of Level 3 datasets makes it feasible to readily combine data from different sources. While these processing levels are particularly suitable for typical satellite data processing pipelines, other data level vocabularies have been defined and may be appropriate for more heterogeneous workflows. Satellite images provide very useful information to produce statistics on topics closely related to

707-423: A great step forward in the instrument's accuracy. William Gascoigne invented an instrument that used a telescope with an installed crosshair as a target device, in 1640. James Watt developed an optical meter for the measuring of distance in 1771; it measured the parallactic angle from which the distance to a point could be deduced. Dutch mathematician Willebrord Snellius (a.k.a. Snel van Royen) introduced

808-450: A height above sea level. As the surveying profession grew it created Cartesian coordinate systems to simplify the mathematics for surveys over small parts of the Earth. The simplest coordinate systems assume that the Earth is flat and measure from an arbitrary point, known as a 'datum' (singular form of data). The coordinate system allows easy calculation of the distances and direction between objects over small areas. Large areas distort due to

909-433: A known size. It was sometimes used before to the invention of EDM where rough ground made chain measurement impractical. Historically, horizontal angles were measured by using a compass to provide a magnetic bearing or azimuth. Later, more precise scribed discs improved angular resolution. Mounting telescopes with reticles atop the disc allowed more precise sighting (see theodolite ). Levels and calibrated circles allowed

1010-825: A large extent of geography. At the same time, the data is often complex to interpret, and bulky to store. Modern systems tend to store the data digitally, often with lossless compression . The difficulty with this approach is that the data is fragile, the format may be archaic, and the data may be easy to falsify. One of the best systems for archiving data series is as computer-generated machine-readable ultrafiche , usually in typefonts such as OCR-B , or as digitized half-tone images. Ultrafiches survive well in standard libraries, with lifetimes of several centuries. They can be created, copied, filed and retrieved by automated systems. They are about as compact as archival magnetic media, and yet can be read by human beings with minimal, standardized equipment. Generally speaking, remote sensing works on

1111-484: A legend of mapped classes that suits our purpose, taking again the example of wheat. The straightforward approach is counting the number of pixels classified as wheat and multiplying by the area of each pixel. Many authors have noticed that estimator is that it is generally biased because commission and omission errors in a confusion matrix do not compensate each other The main strength of classified satellite images or other indicators computed on satellite images

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1212-434: A loop pattern or link between two prior reference marks so the surveyor can check their measurements. Many surveys do not calculate positions on the surface of the Earth, but instead, measure the relative positions of objects. However, often the surveyed items need to be compared to outside data, such as boundary lines or previous survey's objects. The oldest way of describing a position is via latitude and longitude, and often

1313-618: A multi frequency phase shift of light waves to find a distance. These instruments eliminated the need for days or weeks of chain measurement by measuring between points kilometers apart in one go. Advances in electronics allowed miniaturization of EDM. In the 1970s the first instruments combining angle and distance measurement appeared, becoming known as total stations . Manufacturers added more equipment by degrees, bringing improvements in accuracy and speed of measurement. Major advances include tilt compensators, data recorders and on-board calculation programs. The first satellite positioning system

1414-410: A plan or map, and the points at the ends of the offset lines could be joined to show the feature. Traversing is a common method of surveying smaller areas. The surveyors start from an old reference mark or known position and place a network of reference marks covering the survey area. They then measure bearings and distances between the reference marks, and to the target features. Most traverses form

1515-406: A point inside a triangle using the angles cast between the vertices at the unknown point. These could be measured more accurately than bearings of the vertices, which depended on a compass. His work established the idea of surveying a primary network of control points, and locating subsidiary points inside the primary network later. Between 1733 and 1740, Jacques Cassini and his son César undertook

1616-472: A profession. They established the basic measurements under which the Roman Empire was divided, such as a tax register of conquered lands (300 AD). Roman surveyors were known as Gromatici . In medieval Europe, beating the bounds maintained the boundaries of a village or parish. This was the practice of gathering a group of residents and walking around the parish or village to establish a communal memory of

1717-472: A reference point including distances between known points on the ground. This depends on the type of sensor used. For example, in conventional photographs, distances are accurate in the center of the image, with the distortion of measurements increasing the farther you get from the center. Another factor is that of the platen against which the film is pressed can cause severe errors when photographs are used to measure ground distances. The step in which this problem

1818-428: A reflector or prism to return the light pulses used for distance measurements. They are fully robotic, and can even e-mail point data to a remote computer and connect to satellite positioning systems , such as Global Positioning System . Real Time Kinematic GPS systems have significantly increased the speed of surveying, and they are now horizontally accurate to within 1 cm ± 1 ppm in real-time, while vertically it

1919-679: A sample with less accurate, but exhaustive, data for a covariable or proxy that is cheaper to collect. For agricultural statistics, field surveys are usually required, while photo-interpretation may better for land cover classes that can be reliably identified on aerial photographs or high resolution satellite images. Additional uncertainty can appear because of imperfect reference data (ground truth or similar). Some options are: ratio estimator , regression estimator , calibration estimators and small area estimators If we target other variables, such as crop yield or leaf area , we may need different indicators to be computed from images, such as

2020-473: A star is determined, the bearing can be transferred to a reference point on Earth. The point can then be used as a base for further observations. Survey-accurate astronomic positions were difficult to observe and calculate and so tended to be a base off which many other measurements were made. Since the advent of the GPS system, astronomic observations are rare as GPS allows adequate positions to be determined over most of

2121-495: A theodolite with a compass and tripod in 1576. Johnathon Sission was the first to incorporate a telescope on a theodolite in 1725. In the 18th century, modern techniques and instruments for surveying began to be used. Jesse Ramsden introduced the first precision theodolite in 1787. It was an instrument for measuring angles in the horizontal and vertical planes. He created his great theodolite using an accurate dividing engine of his own design. Ramsden's theodolite represented

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2222-593: A time component. Before EDM (Electronic Distance Measurement) laser devices, distances were measured using a variety of means. In pre-colonial America Natives would use the "bow shot" as a distance reference ("as far as an arrow can slung out of a bow", or "flights of a Cherokee long bow"). Europeans used chains with links of a known length such as a Gunter's chain , or measuring tapes made of steel or invar . To measure horizontal distances, these chains or tapes were pulled taut to reduce sagging and slack. The distance had to be adjusted for heat expansion. Attempts to hold

2323-474: Is a staple of contemporary land surveying. Typically, much if not all of the drafting and some of the designing for plans and plats of the surveyed property is done by the surveyor, and nearly everyone working in the area of drafting today (2021) utilizes CAD software and hardware both on PC, and more and more in newer generation data collectors in the field as well. Other computer platforms and tools commonly used today by surveyors are offered online by

2424-399: Is a term used when referring to moving the level to take an elevation shot from a different location. To "turn" the level, one must first take a reading and record the elevation of the point the rod is located on. While the rod is being kept in exactly the same location, the level is moved to a new location where the rod is still visible. A reading is taken from the new location of the level and

2525-400: Is also appearing. The main surveying instruments in use around the world are the theodolite , measuring tape , total station , 3D scanners , GPS / GNSS , level and rod . Most instruments screw onto a tripod when in use. Tape measures are often used for measurement of smaller distances. 3D scanners and various forms of aerial imagery are also used. The theodolite is an instrument for

2626-412: Is an alternate method of determining the position of objects, and was often used to measure imprecise features such as riverbanks. The surveyor would mark and measure two known positions on the ground roughly parallel to the feature, and mark out a baseline between them. At regular intervals, a distance was measured at right angles from the first line to the feature. The measurements could then be plotted on

2727-458: Is balanced homodyne detection (BHD) using a positive–intrinsic–negative ( PIN ) diode. Remote sensing This is an accepted version of this page Remote sensing is the acquisition of information about an object or phenomenon without making physical contact with the object, in contrast to in situ or on-site observation . The term is applied especially to acquiring information about Earth and other planets . Remote sensing

2828-505: Is because divergent conditions further away from the base reduce accuracy. Surveying instruments have characteristics that make them suitable for certain uses. Theodolites and levels are often used by constructors rather than surveyors in first world countries. The constructor can perform simple survey tasks using a relatively cheap instrument. Total stations are workhorses for many professional surveyors because they are versatile and reliable in all conditions. The productivity improvements from

2929-585: Is currently about half of that to within 2 cm ± 2 ppm. GPS surveying differs from other GPS uses in the equipment and methods used. Static GPS uses two receivers placed in position for a considerable length of time. The long span of time lets the receiver compare measurements as the satellites orbit. The changes as the satellites orbit also provide the measurement network with well conditioned geometry. This produces an accurate baseline that can be over 20 km long. RTK surveying uses one static antenna and one roving antenna. The static antenna tracks changes in

3030-400: Is dense enough). Homodyne detection is more readily applicable to velocity sensing. In optical interferometry , homodyne signifies that the reference radiation (i.e. the local oscillator ) is derived from the same source as the signal before the modulating process . For example, in a laser scattering measurement, the laser beam is split into two parts. One is the local oscillator and

3131-604: Is impossible to directly measure temperatures in the upper atmosphere, it is possible to measure the spectral emissions from a known chemical species (such as carbon dioxide) in that region. The frequency of the emissions may then be related via thermodynamics to the temperature in that region. To facilitate the discussion of data processing in practice, several processing "levels" were first defined in 1986 by NASA as part of its Earth Observing System and steadily adopted since then, both internally at NASA (e. g., ) and elsewhere (e. g., ); these definitions are: A Level 1 data record

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3232-418: Is providing cheap information on the whole target area or most of it. This information usually has a good correlation with the target variable (ground truth) that is usually expensive to observe in an unbiased and accurate way. Therefore it can be observed on a probabilistic sample selected on an area sampling frame . Traditional survey methodology provides different methods to combine accurate information on

3333-450: Is relevant to highlight that probabilistic sampling is not critical for the selection of training pixels for image classification, but it is necessary for accuracy assessment of the classified images and area estimation. Additional care is recommended to ensure that training and validation datasets are not spatially correlated. We suppose now that we have classified images or a land cover map produced by visual photo-interpretation, with

3434-432: Is resolved is called georeferencing and involves computer-aided matching of points in the image (typically 30 or more points per image) which is extrapolated with the use of an established benchmark, "warping" the image to produce accurate spatial data. As of the early 1990s, most satellite images are sold fully georeferenced. In addition, images may need to be radiometrically and atmospherically corrected. Interpretation

3535-489: Is that of examined areas or objects that reflect or emit radiation that stand out from surrounding areas. For a summary of major remote sensing satellite systems see the overview table. To coordinate a series of large-scale observations, most sensing systems depend on the following: platform location and the orientation of the sensor. High-end instruments now often use positional information from satellite navigation systems . The rotation and orientation are often provided within

3636-402: Is that of increasingly smaller sensor pods such as those used by law enforcement and the military, in both manned and unmanned platforms. The advantage of this approach is that this requires minimal modification to a given airframe. Later imaging technologies would include infrared, conventional, Doppler and synthetic aperture radar. The development of artificial satellites in the latter half of

3737-440: Is the critical process of making sense of the data. The first application was that of aerial photographic collection which used the following process; spatial measurement through the use of a light table in both conventional single or stereographic coverage, added skills such as the use of photogrammetry, the use of photomosaics, repeat coverage, Making use of objects' known dimensions in order to detect modifications. Image Analysis

3838-560: Is the most fundamental (i. e., highest reversible level) data record that has significant scientific utility, and is the foundation upon which all subsequent data sets are produced. Level 2 is the first level that is directly usable for most scientific applications; its value is much greater than the lower levels. Level 2 data sets tend to be less voluminous than Level 1 data because they have been reduced temporally, spatially, or spectrally. Level 3 data sets are generally smaller than lower level data sets and thus can be dealt with without incurring

3939-424: Is the recently developed automated computer-aided application that is in increasing use. Object-Based Image Analysis (OBIA) is a sub-discipline of GIScience devoted to partitioning remote sensing (RS) imagery into meaningful image-objects, and assessing their characteristics through spatial, spectral and temporal scale. Old data from remote sensing is often valuable because it may provide the only long-term data for

4040-406: Is the technique, profession, art, and science of determining the terrestrial two-dimensional or three-dimensional positions of points and the distances and angles between them. These points are usually on the surface of the Earth, and they are often used to establish maps and boundaries for ownership , locations, such as the designated positions of structural components for construction or

4141-410: Is used in numerous fields, including geophysics , geography , land surveying and most Earth science disciplines (e.g. exploration geophysics , hydrology , ecology , meteorology , oceanography , glaciology , geology ). It also has military, intelligence, commercial, economic, planning, and humanitarian applications, among others. In current usage, the term remote sensing generally refers to

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4242-432: Is with an altimeter  using air pressure to find the height. When more precise measurements are needed, means like precise levels (also known as differential leveling) are used. When precise leveling, a series of measurements between two points are taken using an instrument and a measuring rod. Differences in height between the measurements are added and subtracted in a series to get the net difference in elevation between

4343-598: The Amazon Basin , glacial features in Arctic and Antarctic regions, and depth sounding of coastal and ocean depths. Military collection during the Cold War made use of stand-off collection of data about dangerous border areas. Remote sensing also replaces costly and slow data collection on the ground, ensuring in the process that areas or objects are not disturbed. Orbital platforms collect and transmit data from different parts of

4444-1013: The EGU or Digital Earth encourage the development of learning modules and learning portals . Examples include: FIS – Remote Sensing in School Lessons , Geospektiv , Ychange , or Spatial Discovery, to promote media and method qualifications as well as independent learning. Remote sensing data are processed and analyzed with computer software, known as a remote sensing application . A large number of proprietary and open source applications exist to process remote sensing data. There are applications of gamma rays to mineral exploration through remote sensing. In 1972 more than two million dollars were spent on remote sensing applications with gamma rays to mineral exploration. Gamma rays are used to search for deposits of uranium. By observing radioactivity from potassium, porphyry copper deposits can be located. A high ratio of uranium to thorium has been found to be related to

4545-547: The European Commission . Forest area and deforestation estimation have also been a frequent target of remote sensing projects, the same as land cover and land use Ground truth or reference data to train and validate image classification require a field survey if we are targetting annual crops or individual forest species, but may be substituted by photointerpretation if we look at wider classes that can be reliably identified on aerial photos or satellite images. It

4646-695: The Great Pyramid of Giza , built c.  2700 BC , affirm the Egyptians' command of surveying. The groma instrument may have originated in Mesopotamia (early 1st millennium BC). The prehistoric monument at Stonehenge ( c.  2500 BC ) was set out by prehistoric surveyors using peg and rope geometry. The mathematician Liu Hui described ways of measuring distant objects in his work Haidao Suanjing or The Sea Island Mathematical Manual , published in 263 AD. The Romans recognized land surveying as

4747-571: The Magellan spacecraft provided detailed topographic maps of Venus , while instruments aboard SOHO allowed studies to be performed on the Sun and the solar wind , just to name a few examples. Recent developments include, beginning in the 1960s and 1970s, the development of image processing of satellite imagery . The use of the term "remote sensing" began in the early 1960s when Evelyn Pruitt realized that advances in science meant that aerial photography

4848-527: The MetOp spacecraft of EUMETSAT are all operated at altitudes of about 800 km (500 mi). The Proba-1 , Proba-2 and SMOS spacecraft of European Space Agency are observing the Earth from an altitude of about 700 km (430 mi). The Earth observation satellites of UAE, DubaiSat-1 & DubaiSat-2 are also placed in Low Earth orbits (LEO) orbits and providing satellite imagery of various parts of

4949-533: The NDVI , a good proxy to chlorophyll activity. The modern discipline of remote sensing arose with the development of flight. The balloonist G. Tournachon (alias Nadar ) made photographs of Paris from his balloon in 1858. Messenger pigeons, kites, rockets and unmanned balloons were also used for early images. With the exception of balloons, these first, individual images were not particularly useful for map making or for scientific purposes. Systematic aerial photography

5050-499: The Principal Triangulation of Britain . The first Ramsden theodolite was built for this survey. The survey was finally completed in 1853. The Great Trigonometric Survey of India began in 1801. The Indian survey had an enormous scientific impact. It was responsible for one of the first accurate measurements of a section of an arc of longitude, and for measurements of the geodesic anomaly. It named and mapped Mount Everest and

5151-601: The Torrens system in South Australia in 1858. Torrens intended to simplify land transactions and provide reliable titles via a centralized register of land. The Torrens system was adopted in several other nations of the English-speaking world. Surveying became increasingly important with the arrival of railroads in the 1800s. Surveying was necessary so that railroads could plan technologically and financially viable routes. At

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5252-785: The U.S. Federal Government and other governments' survey agencies, such as the National Geodetic Survey and the CORS network, to get automated corrections and conversions for collected GPS data, and the data coordinate systems themselves. Surveyors determine the position of objects by measuring angles and distances. The factors that can affect the accuracy of their observations are also measured. They then use this data to create vectors, bearings, coordinates, elevations, areas, volumes, plans and maps. Measurements are often split into horizontal and vertical components to simplify calculation. GPS and astronomic measurements also need measurement of

5353-615: The electromagnetic spectrum , which in conjunction with larger scale aerial or ground-based sensing and analysis, provides researchers with enough information to monitor trends such as El Niño and other natural long and short term phenomena. Other uses include different areas of the earth sciences such as natural resource management , agricultural fields such as land usage and conservation, greenhouse gas monitoring , oil spill detection and monitoring, and national security and overhead, ground-based and stand-off collection on border areas. The basis for multispectral collection and analysis

5454-613: The 20th century allowed remote sensing to progress to a global scale as of the end of the Cold War. Instrumentation aboard various Earth observing and weather satellites such as Landsat , the Nimbus and more recent missions such as RADARSAT and UARS provided global measurements of various data for civil, research, and military purposes. Space probes to other planets have also provided the opportunity to conduct remote sensing studies in extraterrestrial environments, synthetic aperture radar aboard

5555-509: The Earth. To get global coverage with a low orbit, a polar orbit is used. A low orbit will have an orbital period of roughly 100 minutes and the Earth will rotate around its polar axis about 25° between successive orbits. The ground track moves towards the west 25° each orbit, allowing a different section of the globe to be scanned with each orbit. Most are in Sun-synchronous orbits . Surveying Surveying or land surveying

5656-474: The German students use the services of Google Earth ; in 2006 alone the software was downloaded 100 million times. But studies have shown that only a fraction of them know more about the data they are working with. There exists a huge knowledge gap between the application and the understanding of satellite images. Remote sensing only plays a tangential role in schools, regardless of the political claims to strengthen

5757-438: The basis for dividing the western territories into sections to allow the sale of land. The PLSS divided states into township grids which were further divided into sections and fractions of sections. Napoleon Bonaparte founded continental Europe 's first cadastre in 1808. This gathered data on the number of parcels of land, their value, land usage, and names. This system soon spread around Europe. Robert Torrens introduced

5858-457: The bearing from every vertex in a figure, a surveyor can measure around the figure. The final observation will be between the two points first observed, except with a 180° difference. This is called a close . If the first and last bearings are different, this shows the error in the survey, called the angular misclose . The surveyor can use this information to prove that the work meets the expected standards. The simplest method for measuring height

5959-462: The beginning of the century, surveyors had improved the older chains and ropes, but they still faced the problem of accurate measurement of long distances. Trevor Lloyd Wadley developed the Tellurometer during the 1950s. It measures long distances using two microwave transmitter/receivers. During the late 1950s Geodimeter introduced electronic distance measurement (EDM) equipment. EDM units use

6060-516: The boundaries. Young boys were included to ensure the memory lasted as long as possible. In England, William the Conqueror commissioned the Domesday Book in 1086. It recorded the names of all the land owners, the area of land they owned, the quality of the land, and specific information of the area's content and inhabitants. It did not include maps showing exact locations. Abel Foullon described

6161-405: The development of the human environment since the beginning of recorded history . It is used in the planning and execution of most forms of construction . It is also used in transportation, communications, mapping, and the definition of legal boundaries for land ownership. It is an important tool for research in many other scientific disciplines. The International Federation of Surveyors defines

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6262-495: The discovery of the Earth's Van Allen radiation belts . The TIROS-1 spacecraft, launched on April 1, 1960, as part of NASA's Television Infrared Observation Satellite (TIROS) program, sent back the first television footage of weather patterns to be taken from space. In 2008, more than 150 Earth observation satellites were in orbit, recording data with both passive and active sensors and acquiring more than 10 terabits of data daily. By 2021, that total had grown to over 950, with

6363-562: The early days of surveying, this was the primary method of determining accurate positions of objects for topographic maps of large areas. A surveyor first needs to know the horizontal distance between two of the objects, known as the baseline . Then the heights, distances and angular position of other objects can be derived, as long as they are visible from one of the original objects. High-accuracy transits or theodolites were used, and angle measurements were repeated for increased accuracy. See also Triangulation in three dimensions . Offsetting

6464-625: The farmer who plants his fields in a remote corner of the country knows its value." The development of remote sensing technology reached a climax during the Cold War with the use of modified combat aircraft such as the P-51 , P-38 , RB-66 and the F-4C , or specifically designed collection platforms such as the U2/TR-1 , SR-71 , A-5 and the OV-1 series both in overhead and stand-off collection. A more recent development

6565-417: The fields of media and methods apart from the mere visual interpretation of satellite images. Many teachers have great interest in the subject "remote sensing", being motivated to integrate this topic into teaching, provided that the curriculum is considered. In many cases, this encouragement fails because of confusing information. In order to integrate remote sensing in a sustainable manner organizations like

6666-581: The first commercial satellite (IKONOS) collecting very high resolution imagery was launched. Remote Sensing has a growing relevance in the modern information society. It represents a key technology as part of the aerospace industry and bears increasing economic relevance – new sensors e.g. TerraSAR-X and RapidEye are developed constantly and the demand for skilled labour is increasing steadily. Furthermore, remote sensing exceedingly influences everyday life, ranging from weather forecasts to reports on climate change or natural disasters . As an example, 80% of

6767-533: The first prototype satellites of the Global Positioning System (GPS) in 1978. GPS used a larger constellation of satellites and improved signal transmission, thus improving accuracy. Early GPS observations required several hours of observations by a static receiver to reach survey accuracy requirements. Later improvements to both satellites and receivers allowed for Real Time Kinematic (RTK) surveying. RTK surveys provide high-accuracy measurements by using

6868-467: The first triangulation of France. They included a re-surveying of the meridian arc , leading to the publication in 1745 of the first map of France constructed on rigorous principles. By this time triangulation methods were well established for local map-making. It was only towards the end of the 18th century that detailed triangulation network surveys mapped whole countries. In 1784, a team from General William Roy 's Ordnance Survey of Great Britain began

6969-509: The function of surveying as follows: A surveyor is a professional person with the academic qualifications and technical expertise to conduct one, or more, of the following activities; Surveying has occurred since humans built the first large structures. In ancient Egypt , a rope stretcher would use simple geometry to re-establish boundaries after the annual floods of the Nile River . The almost perfect squareness and north–south orientation of

7070-748: The ground (about 20 km (12 mi) apart). This method reaches precisions between 5–40 cm (depending on flight height). Surveyors use ancillary equipment such as tripods and instrument stands; staves and beacons used for sighting purposes; PPE ; vegetation clearing equipment; digging implements for finding survey markers buried over time; hammers for placements of markers in various surfaces and structures; and portable radios for communication over long lines of sight. Land surveyors, construction professionals, geomatics engineers and civil engineers using total station , GPS , 3D scanners, and other collector data use land surveying software to increase efficiency, accuracy, and productivity. Land Surveying Software

7171-509: The ground to large beacons that can be seen from long distances. The surveyors can set up their instruments in this position and measure to nearby objects. Sometimes a tall, distinctive feature such as a steeple or radio aerial has its position calculated as a reference point that angles can be measured against. Triangulation is a method of horizontal location favoured in the days before EDM and GPS measurement. It can determine distances, elevations and directions between distant objects. Since

7272-400: The height difference is used to find the new elevation of the level gun, which is why this method is referred to as differential levelling . This is repeated until the series of measurements is completed. The level must be horizontal to get a valid measurement. Because of this, if the horizontal crosshair of the instrument is lower than the base of the rod, the surveyor will not be able to sight

7373-401: The intensity of the laser.. The generated current signal from the photodetector is often too weak to measure. It is therefore converted into a voltage using a Transimpedance amplifier . In radio technology, the distinction is not the source of the local oscillator, but the frequency used. In heterodyne detection, the local oscillator is frequency-shifted, while in homodyne detection it has

7474-461: The largest number of satellites operated by US-based company Planet Labs . Most Earth observation satellites carry instruments that should be operated at a relatively low altitude. Most orbit at altitudes above 500 to 600 kilometers (310 to 370 mi). Lower orbits have significant air-drag , which makes frequent orbit reboost maneuvers necessary. The Earth observation satellites ERS-1, ERS-2 and Envisat of European Space Agency as well as

7575-459: The launch of the first artificial satellite, Sputnik 1 , by the Soviet Union on October 4, 1957. Sputnik 1 sent back radio signals, which scientists used to study the ionosphere . The United States Army Ballistic Missile Agency launched the first American satellite, Explorer 1 , for NASA's Jet Propulsion Laboratory on January 31, 1958. The information sent back from its radiation detector led to

7676-464: The measurement of angles. It uses two separate circles , protractors or alidades to measure angles in the horizontal and the vertical plane. A telescope mounted on trunnions is aligned vertically with the target object. The whole upper section rotates for horizontal alignment. The vertical circle measures the angle that the telescope makes against the vertical, known as the zenith angle. The horizontal circle uses an upper and lower plate. When beginning

7777-409: The measurement of vertical angles. Verniers allowed measurement to a fraction of a degree, such as with a turn-of-the-century transit . The plane table provided a graphical method of recording and measuring angles, which reduced the amount of mathematics required. In 1829 Francis Ronalds invented a reflecting instrument for recording angles graphically by modifying the octant . By observing

7878-424: The measuring instrument level would also be made. When measuring up a slope, the surveyor might have to "break" (break chain) the measurement- use an increment less than the total length of the chain. Perambulators , or measuring wheels, were used to measure longer distances but not to a high level of accuracy. Tacheometry is the science of measuring distances by measuring the angle between two ends of an object with

7979-411: The modern systematic use of triangulation . In 1615 he surveyed the distance from Alkmaar to Breda , approximately 72 miles (116 km). He underestimated this distance by 3.5%. The survey was a chain of quadrangles containing 33 triangles in all. Snell showed how planar formulae could be corrected to allow for the curvature of the Earth . He also showed how to resect , or calculate, the position of

8080-642: The other Himalayan peaks. Surveying became a professional occupation in high demand at the turn of the 19th century with the onset of the Industrial Revolution . The profession developed more accurate instruments to aid its work. Industrial infrastructure projects used surveyors to lay out canals , roads and rail. In the US, the Land Ordinance of 1785 created the Public Land Survey System . It formed

8181-522: The other hand, emits energy in order to scan objects and areas whereupon a sensor then detects and measures the radiation that is reflected or backscattered from the target. RADAR and LiDAR are examples of active remote sensing where the time delay between emission and return is measured, establishing the location, speed and direction of an object. Remote sensing makes it possible to collect data of dangerous or inaccessible areas. Remote sensing applications include monitoring deforestation in areas such as

8282-463: The other is sent to the system to be probed. The scattered light is then mixed with the local oscillator on the detector. This arrangement has the advantage of being insensitive to fluctuations in the frequency of the laser. Usually the scattered beam will be weak, in which case the (nearly) steady component of the detector output is a good measure of the instantaneous local oscillator intensity and therefore can be used to compensate for any fluctuations in

8383-562: The possibility of providing a room temperature quantum sensor with continuous-variable quantum information . However, challenges include reducing noise, increasing bandwidth and improving the integration of electronic and photonic components. Recently, these challenges have been overcome to demonstrate a free-space-coupled room temperature quantum sensor with large-scale integrated photonics and electronics. An encrypted secure communication system can be based on quantum key distribution (QKD). An efficient receiver scheme for implementing QKD

8484-848: The presence of hydrothermal copper deposits. Radiation patterns have also been known to occur above oil and gas fields, but some of these patterns were thought to be due to surface soils instead of oil and gas. An Earth observation satellite or Earth remote sensing satellite is a satellite used or designed for Earth observation (EO) from orbit , including spy satellites and similar ones intended for non-military uses such as environmental monitoring , meteorology , cartography and others. The most common type are Earth imaging satellites, that take satellite images , analogous to aerial photographs ; some EO satellites may perform remote sensing without forming pictures, such as in GNSS radio occultation . The first occurrence of satellite remote sensing can be dated to

8585-412: The principle of the inverse problem : while the object or phenomenon of interest (the state ) may not be directly measured, there exists some other variable that can be detected and measured (the observation ) which may be related to the object of interest through a calculation. The common analogy given to describe this is trying to determine the type of animal from its footprints. For example, while it

8686-462: The processing of signals in some applications of magnetic resonance imaging , homodyne detection can offer advantages over magnitude detection. The homodyne technique can suppress excessive noise and undesired quadrature components (90° out-of-phase), and provide stable access to information that may be encoded into the phase or polarity of images. Homodyne detection was one of the key techniques in demonstrating quantum entanglement . This has led to

8787-500: The reflection of sunlight is detected by the sensor). Remote sensing can be divided into two types of methods: Passive remote sensing and Active remote sensing. Passive sensors gather radiation that is emitted or reflected by the object or surrounding areas. Reflected sunlight is the most common source of radiation measured by passive sensors. Examples of passive remote sensors include film photography , infrared , charge-coupled devices , and radiometers . Active collection, on

8888-474: The rod and get a reading. The rod can usually be raised up to 25 feet (7.6 m) high, allowing the level to be set much higher than the base of the rod. The primary way of determining one's position on the Earth's surface when no known positions are nearby is by astronomic observations. Observations to the Sun, Moon and stars could all be made using navigational techniques. Once the instrument's position and bearing to

8989-546: The same frequency as the radiation to be detected. See direct conversion receiver . Lock-in amplifiers are homodyne detectors integrated into measurement equipment or packaged as stand-alone laboratory equipment for sensitive detection and highly selective filtering of weak or noisy signals. Homodyne/lock-in detection has been one of the most commonly used signal processing methods across a wide range of experimental disciplines for decades. Homodyne and heterodyne techniques are commonly used in thermoreflectance techniques. In

9090-422: The satellite positions and atmospheric conditions. The surveyor uses the roving antenna to measure the points needed for the survey. The two antennas use a radio link that allows the static antenna to send corrections to the roving antenna. The roving antenna then applies those corrections to the GPS signals it is receiving to calculate its own position. RTK surveying covers smaller distances than static methods. This

9191-423: The support for teaching on the subject. A lot of the computer software explicitly developed for school lessons has not yet been implemented due to its complexity. Thereby, the subject is either not at all integrated into the curriculum or does not pass the step of an interpretation of analogue images. In fact, the subject of remote sensing requires a consolidation of physics and mathematics as well as competences in

9292-678: The surface location of subsurface features, or other purposes required by government or civil law, such as property sales. A professional in land surveying is called a land surveyor . Surveyors work with elements of geodesy , geometry , trigonometry , regression analysis , physics , engineering, metrology , programming languages , and the law. They use equipment, such as total stations , robotic total stations, theodolites , GNSS receivers, retroreflectors , 3D scanners , lidar sensors, radios, inclinometer , handheld tablets, optical and digital levels , subsurface locators, drones, GIS , and surveying software. Surveying has been an element in

9393-423: The surface of the Earth. Few survey positions are derived from the first principles. Instead, most surveys points are measured relative to previously measured points. This forms a reference or control network where each point can be used by a surveyor to determine their own position when beginning a new survey. Survey points are usually marked on the earth's surface by objects ranging from small nails driven into

9494-406: The survey, the surveyor points the instrument in a known direction (bearing), and clamps the lower plate in place. The instrument can then rotate to measure the bearing to other objects. If no bearing is known or direct angle measurement is wanted, the instrument can be set to zero during the initial sight. It will then read the angle between the initial object, the theodolite itself, and the item that

9595-536: The telescope aligns with. The gyrotheodolite is a form of theodolite that uses a gyroscope to orient itself in the absence of reference marks. It is used in underground applications. The total station is a development of the theodolite with an electronic distance measurement device (EDM). A total station can be used for leveling when set to the horizontal plane. Since their introduction, total stations have shifted from optical-mechanical to fully electronic devices. Modern top-of-the-line total stations no longer need

9696-615: The territory, such as agriculture, forestry or land cover in general. The first large project to apply Landsata 1 images for statistics was LACIE (Large Area Crop Inventory Experiment), run by NASA, NOAA and the USDA in 1974–77. Many other application projects on crop area estimation have followed, including the Italian AGRIT project and the MARS project of the Joint Research Centre (JRC) of

9797-459: The two endpoints. With the Global Positioning System (GPS), elevation can be measured with satellite receivers. Usually, GPS is somewhat less accurate than traditional precise leveling, but may be similar over long distances. When using an optical level, the endpoint may be out of the effective range of the instrument. There may be obstructions or large changes of elevation between the endpoints. In these situations, extra setups are needed. Turning

9898-422: The use of satellite - or aircraft-based sensor technologies to detect and classify objects on Earth. It includes the surface and the atmosphere and oceans , based on propagated signals (e.g. electromagnetic radiation ). It may be split into "active" remote sensing (when a signal is emitted by a satellite or aircraft to the object and its reflection is detected by the sensor) and "passive" remote sensing (when

9999-579: Was developed for military surveillance and reconnaissance purposes beginning in World War I . After WWI, remote sensing technology was quickly adapted to civilian applications. This is demonstrated by the first line of a 1941 textbook titled "Aerophotography and Aerosurverying," which stated the following: "There is no longer any need to preach for aerial photography-not in the United States- for so widespread has become its use and so great its value that even

10100-548: Was no longer an adequate term to describe the data streams being generated by new technologies. With assistance from her fellow staff member at the Office of Naval Research, Walter Bailey, she coined the term "remote sensing". Several research groups in Silicon Valley including NASA Ames Research Center , GTE , and ESL Inc. developed Fourier transform techniques leading to the first notable enhancement of imagery data. In 1999

10201-497: Was the US Navy TRANSIT system . The first successful launch took place in 1960. The system's main purpose was to provide position information to Polaris missile submarines. Surveyors found they could use field receivers to determine the location of a point. Sparse satellite cover and large equipment made observations laborious and inaccurate. The main use was establishing benchmarks in remote locations. The US Air Force launched

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