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53-474: A roadstead or road is a body of water sheltered from rip currents , spring tides, or ocean swell where ships can lie reasonably safely at anchor without dragging or snatching. It can be open or natural, usually estuary -based, or may be created artificially. In maritime law, it is described as a "known general station for ships, notoriously used as such, and distinguished by the name". A roadstead can be an area of safe anchorage for ships waiting to enter

106-400: A Digital Land Surface Model in the form of a TIN . The DLSM can then be used to visualize terrain, drape remote sensing images, quantify ecological properties of a surface or extract land surface objects. The contour data or any other sampled elevation datasets are not a DLSM. A DLSM implies that elevation is available continuously at each location in the study area, i.e. that the map represents

159-418: A break in an offshore sand bar or reef, this can allow water to flow offshore more easily, and this will initiate a rip current through that gap. Water that has been pushed up near the beach flows along the shore towards the outgoing rip as "feeder currents". The excess water flows out at a right angle to the beach, in a tight current called the "neck" of the rip. The "neck" is where the flow is most rapid. When

212-492: A complete surface. Digital Land Surface Models should not be confused with Digital Surface Models, which can be surfaces of the canopy, buildings and similar objects. For example, in the case of surface models produces using the lidar technology, one can have several surfaces – starting from the top of the canopy to the actual solid earth. The difference between the two surface models can then be used to derive volumetric measures (height of trees etc.). Topographic survey information

265-519: A few meters to a kilometer or more, depending whether it is high tide or low tide. A fairly common misconception is that rip currents can pull a swimmer down, under the surface of the water. This is not true, and in reality a rip current is strongest close to the surface, as the flow near the bottom is slowed by friction. The surface of a rip current can often appear to be a relatively smooth area of water, without any breaking waves, and this deceptive appearance may cause some beach-goers to believe that it

318-405: A part of geovisualization , whether maps or GIS systems. False-color and non-visible spectra imaging can also help determine the lie of the land by delineating vegetation and other land-use information more clearly. Images can be in visible colours and in other spectrum. Photogrammetry is a measurement technique for which the co-ordinates of the points in 3D of an object are determined by

371-535: A person to recognize the presence and position of rips before entering the water, which is an important skill as studies show the majority of people are unable to identify a rip current and therefore unable to identify safe places to swim. In the United States, some beaches have signs created by the National Oceanic and Atmospheric Administration (NOAA) and United States Lifesaving Association , explaining what

424-570: A place or places, what is now largely called ' local history '. In Britain and in Europe in general, the word topography is still sometimes used in its original sense. Detailed military surveys in Britain (beginning in the late eighteenth century) were called Ordnance Surveys , and this term was used into the 20th century as generic for topographic surveys and maps. The earliest scientific surveys in France were

477-414: A port, or to form a convoy. If sufficiently sheltered and convenient, it can be used for the transshipment of goods, stores, and troops, either separately or in combination. The same applies in transfers to and from shore by lighters . In the days of sailing ships, some voyages could only easily be made with a change in wind direction, and ships would wait for a change of wind in a safe anchorage , such as

530-404: A rip current as a rapid and effortless means of transportation. [REDACTED] Media related to Rip currents at Wikimedia Commons Topography Topography is a field of geoscience and planetary science and is concerned with local detail in general, including not only relief , but also natural , artificial, and cultural features such as roads, land boundaries, and buildings. In

583-540: A rip current is and how to escape one. These signs are titled, "Rip Currents; Break the Grip of the Rip". Two of these signs are shown in the image at the top of this article. Beachgoers can get information from lifeguards, who are always watching for rip currents, and who will move their safety flags so that swimmers can avoid rips. Rip currents are a potential source of danger for people in shallow water with breaking waves, whether this

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636-403: A rip current may notice that they are moving away from the shore quite rapidly. Often, it is not possible to swim directly back to shore against a rip current, so this is not recommended. Contrary to popular misunderstanding, a rip does not pull a swimmer under the water. It carries the swimmer away from the shore in a narrow band of moving water. A rip current is like a moving treadmill , which

689-419: A rip current, a wave propagates over a sandbar with a gap in it. When this happens, most of the wave breaks on the sandbar, leading to "setup". The part of the wave that propagates over the gap does not break, and the "setdown" continues in that part. Because of this phenomenon, the mean water surface over the rest of the sandbar is higher than that which is over the gap. The result is a strong flow outward through

742-647: A rip current. In a rip current, death by drowning occurs when a person has limited water skills and panics, or when a swimmer persists in trying to swim to shore against a strong rip current, and eventually becomes exhausted and drowns. According to the NOAA rip currents caused an average of 71 deaths annually in the United States over the ten years ending in 2022 (with 69 in 2022). A 2013 Australian study found that rips killed more people in Australia than bushfires, floods, cyclones and shark attacks combined. People caught in

795-458: A rip, or in some cases make use of the flow. Rip currents often look somewhat like a road or river running straight out to sea. They are easiest to notice and identify when the zone of breaking waves is viewed from a high vantage point. The following are some visual characteristics that can be used to identify a rip: These characteristics are helpful in learning to recognize and understand the nature of rip currents. Learning these signs can enable

848-460: A strong rip can simply relax, either floating or treading water , and allow the current to carry them until it dissipates completely once it is beyond the surf line. Then the person can signal for help, or swim back through the surf, doing so diagonally, away from the rip and towards the shore. It is necessary for coastal swimmers to understand the danger of rip currents, to learn how to recognize them, and how to deal with them. And when possible, it

901-593: A valuable set of information for large-scale analysis. The original American topographic surveys (or the British "Ordnance" surveys) involved not only recording of relief, but identification of landmark features and vegetative land cover. Remote sensing is a general term for geodata collection at a distance from the subject area. Besides their role in photogrammetry, aerial and satellite imagery can be used to identify and delineate terrain features and more general land-cover features. Certainly they have become more and more

954-423: Is a misnomer, in areas of significant tidal range, rip currents may only occur at certain stages of the tide, when the water is shallow enough to cause the waves to break over a sand bar, but deep enough for the broken wave to flow over the bar. In parts of the world with a big difference between high tide and low tide, and where the shoreline shelves gently, the distance between a bar and the shoreline may vary from

1007-433: Is a suitable place to enter the water. A more detailed and technical description of rip currents requires understanding the concept of radiation stress . Radiation stress is the force (or momentum flux) that is exerted on the water column by the presence of the wave. When a wave reaches shallow water and shoals , it increases in height prior to breaking. During this increase in height, radiation stress increases, because of

1060-456: Is concerned with underlying structures and processes to the surface, rather than with identifiable surface features. The digital elevation model (DEM) is a raster -based digital dataset of the topography ( hypsometry and/or bathymetry ) of all or part of the Earth (or a telluric planet ). The pixels of the dataset are each assigned an elevation value, and a header portion of the dataset defines

1113-428: Is essential for the planning and construction of any major civil engineering , public works , or reclamation projects. There are a variety of approaches to studying topography. Which method(s) to use depends on the scale and size of the area under study, its accessibility, and the quality of existing surveys. Surveying helps determine accurately the terrestrial or three-dimensional space position of points and

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1166-438: Is historically based upon the notes of surveyors. They may derive naming and cultural information from other local sources (for example, boundary delineation may be derived from local cadastral mapping). While of historical interest, these field notes inherently include errors and contradictions that later stages in map production resolve. As with field notes, remote sensing data (aerial and satellite photography, for example),

1219-404: Is in seas, oceans or large lakes. Rip currents are the proximate cause of 80% of rescues carried out by beach lifeguards . Rip currents typically flow at about 0.5 m/s (1.6 ft/s). They can be as fast as 2.5 m/s (8.2 ft/s), which is faster than any human can swim. Most rip currents are fairly narrow, and even the widest rip currents are not very wide. Swimmers can usually exit

1272-521: Is necessary that people enter the water only in areas where lifeguards are on duty. In a planned trial in a large rip current at Muriwai Beach in New Zealand, an Australian researcher from the School of Biological, Earth and Environmental Sciences, UNSW Sydney found that "just swim to the side" would not work as the rip current was too wide to see its sides, and said that, despite a rescue boat being near, he

1325-543: Is often considered to include the graphic representation of the landform on a map by a variety of cartographic relief depiction techniques, including contour lines , hypsometric tints , and relief shading . The term topography originated in ancient Greece and continued in ancient Rome , as the detailed description of a place. The word comes from the Greek τόπος ( topos , "place") and -γραφία ( -graphia , "writing"). In classical literature this refers to writing about

1378-525: Is raw and uninterpreted. It may contain holes (due to cloud cover for example) or inconsistencies (due to the timing of specific image captures). Most modern topographic mapping includes a large component of remotely sensed data in its compilation process. In its contemporary definition, topographic mapping shows relief. In the United States, USGS topographic maps show relief using contour lines . The USGS calls maps based on topographic surveys, but without contours, "planimetric maps." These maps show not only

1431-644: Is strong longshore variability in wave breaking. This variability may be caused by such features as sandbars, by piers and jetties , and even by crossing wave trains . They are often located in places where there is a gap in a reef , or low area on a sandbar . Rip currents, once they have formed, may deepen the channel through a sandbar. Rip currents are usually quite narrow, but they tend to be more common, wider, and faster, when and where breaking waves are large and powerful. Local underwater topography makes some beaches more likely to have rip currents. A few beaches are notorious in this respect. Although rip tide

1484-422: Is to something else). Topography has been applied to different science fields. In neuroscience , the neuroimaging discipline uses techniques such as EEG topography for brain mapping . In ophthalmology , corneal topography is used as a technique for mapping the surface curvature of the cornea . In tissue engineering , atomic force microscopy is used to map nanotopography . In human anatomy , topography

1537-618: The Cassini maps after the family who produced them over four generations. The term "topographic surveys" appears to be American in origin. The earliest detailed surveys in the United States were made by the "Topographical Bureau of the Army", formed during the War of 1812 , which became the Corps of Topographical Engineers in 1838. After the work of national mapping was assumed by the U.S. Geological Survey in 1878,

1590-466: The Downs or Yarmouth Roads . Rip current A rip current (or just rip ) is a specific type of water current that can occur near beaches where waves break. A rip is a strong, localized, and narrow current of water that moves directly away from the shore by cutting through the lines of breaking waves, like a river flowing out to sea. The force of the current in a rip is strongest and fastest next to

1643-412: The United States they cause an average of 71 deaths by drowning per year as of 2022 . A rip current is not the same thing as undertow , although some people use that term incorrectly when they are talking about a rip current. Contrary to popular belief, neither rip nor undertow can pull a person down and hold them under the water. A rip simply carries floating objects, including people, out to just beyond

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1696-565: The United States, topography often means specifically relief , even though the USGS topographic maps record not just elevation contours, but also roads, populated places, structures, land boundaries, and so on. Topography in a narrow sense involves the recording of relief or terrain , the three-dimensional quality of the surface, and the identification of specific landforms ; this is also known as geomorphometry . In modern usage, this involves generation of elevation data in digital form ( DEM ). It

1749-669: The area of coverage, the units each pixel covers, and the units of elevation (and the zero-point). DEMs may be derived from existing paper maps and survey data, or they may be generated from new satellite or other remotely sensed radar or sonar data. A geographic information system (GIS) can recognize and analyze the spatial relationships that exist within digitally stored spatial data. These topological relationships allow complex spatial modelling and analysis to be performed. Topological relationships between geometric entities traditionally include adjacency (what adjoins what), containment (what encloses what), and proximity (how close something

1802-455: The basis for much derived topographic work. Digital Elevation Models, for example, have often been created not from new remote sensing data but from existing paper topographic maps. Many government and private publishers use the artwork (especially the contour lines) from existing topographic map sheets as the basis for their own specialized or updated topographic maps. Topographic mapping should not be confused with geologic mapping . The latter

1855-454: The contours, but also any significant streams or other bodies of water, forest cover , built-up areas or individual buildings (depending on scale), and other features and points of interest. While not officially "topographic" maps, the national surveys of other nations share many of the same features, and so they are often called "topographic maps." Existing topographic survey maps, because of their comprehensive and encyclopedic coverage, form

1908-509: The direct survey still provides the basic control points and framework for all topographic work, whether manual or GIS -based. In areas where there has been an extensive direct survey and mapping program (most of Europe and the Continental U.S., for example), the compiled data forms the basis of basic digital elevation datasets such as USGS DEM data. This data must often be "cleaned" to eliminate discrepancies between surveys, but it still forms

1961-476: The distances and angles between them using leveling instruments such as theodolites , dumpy levels and clinometers . GPS and other global navigation satellite systems (GNSS) are also used. Work on one of the first topographic maps was begun in France by Giovanni Domenico Cassini , the great Italian astronomer. Even though remote sensing has greatly sped up the process of gathering information, and has allowed greater accuracy control over long distances,

2014-407: The force exerted by the weight of the water that has been pushed upwards. To balance this, the local mean surface level drops. This is known as the setdown . When the wave breaks and starts reducing in height, the radiation stress decreases as the amount of water that is elevated decreases. When this happens, the mean surface level increases — this is known as the setup . In the formation of

2067-526: The gap. This strong flow is the rip current. The vorticity and inertia of rip currents have been studied. From a model of the vorticity of a rip current done at Scripps Institute of Oceanography, it was found that as a fast rip current extends away from shallow water, the vorticity of the current increases, and the width of the current decreases. This model acknowledges that friction plays a role and waves are irregular in nature. From data from Sector-Scanning Doppler Sonar at Scripps Institute of Oceanography, it

2120-439: The measurements made in two photographic images (or more) taken starting from different positions, usually from different passes of an aerial photography flight. In this technique, the common points are identified on each image . A line of sight (or ray ) can be built from the camera location to the point on the object. It is the intersection of its rays ( triangulation ) which determines the relative three-dimensional position of

2173-480: The most applications in environmental sciences , land surface is represented and modelled using gridded models. In civil engineering and entertainment businesses, the most representations of land surface employ some variant of TIN models. In geostatistics , land surface is commonly modelled as a combination of the two signals – the smooth (spatially correlated) and the rough (noise) signal. In practice, surveyors first sample heights in an area, then use these to produce

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2226-427: The patterns identified to be producing rip currents. The location of rip currents can be difficult to predict. Some tend to recur always in the same places, but others can appear and disappear suddenly at various locations along the beach. The appearance and disappearance of rip currents is dependent upon the bottom topography and the direction from which the surf and swells are coming. Rip currents occur wherever there

2279-525: The point. Known control points can be used to give these relative positions absolute values. More sophisticated algorithms can exploit other information on the scene known a priori (for example, symmetries in certain cases allowing the rebuilding of three-dimensional co-ordinates starting from one only position of the camera). Satellite RADAR mapping is one of the major techniques of generating Digital Elevation Models (see below). Similar techniques are applied in bathymetric surveys using sonar to determine

2332-475: The position of any feature or more generally any point in terms of both a horizontal coordinate system such as latitude, longitude, and altitude . Identifying (naming) features, and recognizing typical landform patterns are also part of the field. A topographic study may be made for a variety of reasons: military planning and geological exploration have been primary motivators to start survey programs, but detailed information about terrain and surface features

2385-399: The rip easily by swimming at a right angle to the flow, parallel to the beach. Swimmers who are unaware of this fact may exhaust themselves trying unsuccessfully to swim directly against the flow. The flow of the current fades out completely at the head of the rip, outside the zone of the breaking waves, so there is a definite limit to how far the swimmer will be taken out to sea by the flow of

2438-413: The surface of the water. Rip currents can be hazardous to people in the water. Swimmers who are caught in a rip current and who do not understand what is happening, or who may not have the necessary water skills, may panic, or they may exhaust themselves by trying to swim directly against the flow of water. Because of these factors, rip currents are the leading cause of rescues by lifeguards at beaches. In

2491-456: The swimmer can get out of quite easily by swimming at a right angle, across the current, i.e. parallel to the shore in either direction. Rip currents are usually not very wide, so getting out of one only takes a few strokes. Once out of the rip current, getting back to shore is not difficult, since waves are breaking, and floating objects, including swimmers, will be pushed by the waves towards the shore. As an alternative, people who are caught in

2544-399: The term topographical remained as a general term for detailed surveys and mapping programs, and has been adopted by most other nations as standard. In the 20th century, the term topography started to be used to describe surface description in other fields where mapping in a broader sense is used, particularly in medical fields such as neurology . An objective of topography is to determine

2597-417: The terrain of the ocean floor. In recent years, LIDAR ( LI ght D etection A nd R anging), a remote sensing technique that uses a laser instead of radio waves, has increasingly been employed for complex mapping needs such as charting canopies and monitoring glaciers. Terrain is commonly modelled either using vector ( triangulated irregular network or TIN) or gridded ( raster image ) mathematical models. In

2650-499: The water in the rip current reaches outside of the lines of breaking waves, the flow disperses sideways, loses power, and dissipates in what is known as the "head" of the rip. Rip currents can form by the coasts of oceans, seas, and large lakes, whenever there are waves of sufficient energy. Rip currents often occur on a gradually shelving shore, where breaking waves approach the shore parallel to it, or where underwater topography encourages outflow at one specific area. Baïnes are one of

2703-419: The zone of the breaking waves, at which point the current dissipates and releases everything it is carrying. A rip current forms because wind and breaking waves push surface water towards the land. This causes a slight rise in the water level along the shore. This excess water will tend to flow back to the open water via the route of least resistance. When there is a local area which is slightly deeper, such as

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2756-515: Was found that rip currents in La Jolla, California, lasted several minutes, that they reoccurred one to four times per hour, and that they created a wedge with a 45° arch and a radius of 200–400 meters. Rip currents have a characteristic appearance, and, with some experience, they can be visually identified from the shore before entering the water. This is helpful to lifeguards, swimmers, surfers, boaters, divers and other water users, who may need to avoid

2809-400: Was unable to relax and not panic. The current took him 300 metres along the beach in a channel feeding the rip current, and then 400 metres offshore at "speeds approaching those of swimming world records". Experienced and knowledgeable water users, including surfers, body boarders, divers , surf lifesavers and kayakers, when they wish to get out beyond the breaking waves, will sometimes use

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