Army Air Forces Antisubmarine Command
The Army Air Forces Antisubmarine Command was formed in the fall of 1942 to establish a single command to control antisubmarine warfare activities of the Army Air Forces. It was formed from the resources of I Bomber Command, carrying out the antisubmarine mission in the Atlantic and Caribbean since the Attack on Pearl Harbor due to the lack of long range Naval aviation in that area; the command's units conducted ASW along the Atlantic and Gulf coasts of the United States, in the Caribbean Sea and in Europe, where it used bases in England and French Morocco. Its operations were marked by disagreements between the AAF and the Navy concerning the conduct of air ASW. In the fall of 1943, the ASW mission was transferred to the Navy and the command became a bomber training unit until it was inactivated in 1946. Within a day after the declaration of war by the United States the Army Air Forces began patrols of both the East and West coasts. Defense plans drawn up before the war began assigned the Navy responsibility for operations beyond the coastline, with Army aircraft serving in a supporting role.
Because naval aviation that could perform long range patrols was nearly non-existent along the Atlantic coast in early 1942, the burden for aerial antisubmarine patrols fell on the AAF, which had available aircraft, but whose crews had not been trained for the mission. German Navy submarines began operating in American coastal waters. By March 1942 fifty-three ships had been sunk in the North Atlantic Naval Coastal Frontier; as a result, the Commander of the North Atlantic Naval Coastal Frontier requested the Army's Eastern Defense Command to undertake offshore patrols with all available aircraft. The first patrols were performed by elements of I Bomber Command, which would be the primary AAF command involved in antisubmarine warfare in early 1942, with assistance from I Air Support Command. However, although I Bomber Command was involved in conducting ASW, it was doing so on an emergency basis, was subject to withdrawal from these duties to perform its primary bombardment function, it soon became apparent that if the AAF were to continue with the ASW mission, its units would have to be organized under a specially trained and equipped command.
In May 1942, General Arnold, Commanding General of the AAF, proposed to Admiral King, the Chief of Naval Operations, that the AAF establish a "coastal command", similar to RAF Coastal Command, operating "when necessary, under the proper Naval authority." That same month saw both a new high in sinkings by U-boats and a shift in their attacks from the Atlantic coast to the Caribbean Sea. In response, the AAF established the Gulf Task Force, with elements of Third Air Force augmenting I Bomber Command, at Miami, Florida to augment the Gulf Sea Frontier; the command situation had only worsened, with two air forces, two navy sea frontiers, two army defense commands, with differing areas of responsibility, all involved in aerial ASW with ad hoc command relations and separate administrative and operational command arrangements. In the month, the War Department requested General Arnold to reorganize I Bomber Command to fulfill the requirements of ASW air operations, either in support of, or in lieu of, naval forces to protect Allied shipping.
Disagreements between the Army and the Navy over command relationships delayed activation of the command until October 1942. The activation of the single Army Air Forces Antisubmarine Command at New York City to control all Army Air Forces units conducting antisubmarine warfare reflected the Army's desire for a single mobile force; the command drew its personnel and equipment from I Bomber Command, inactivated. In November 1942, the command's units were organized into two wings, reflecting the Navy desire that forces in a sea frontier be unified under a single command; the 25th Antisubmarine Wing, Located in New York City was responsible for patrols off the Atlantic Coast, while the 26th Antisubmarine Wing in Miami, Florida conducted operations in the Caribbean and the Gulf of Mexico. By late 1942, German strategy had shifted to one of defending against planned Allied offensives in Europe and North Africa by striking at their lines of communication, rather than striking merchant shipping wherever it was most vulnerable.
As a result, the Kriegsmarine concentrated its U-boats in the eastern Atlantic. To meet this challenge, the command moved two squadrons to England in November, where they operated with RAF Coastal Command; the two units were attached to the 1st Antisubmarine Group, which became the 2037th Antisubmarine Wing. The decision to deploy the squadrons was made and preparations for their reception and the maintenance of their aircraft were nonexistent. Lack of hangar space at RAF St Eval forced them to perform major maintenance outside during the winter. A deployed squadron was responsible for the first confirmed sinking of an enemy sub by one of the command's aircraft. Although they did not sink a large number of U-boats, their harassing tactics required German subs to adopt evasive tactics and reduced the number and effectiveness of attacks on convoys crossing the Atlantic. In February 1943, The squadrons participated in Operation Gondola, whose goal was to attack every U-boat traversing the Bay of Biscay at least once.
The Navy believed that more antisubmarine forces were required to protect convoys in the North Atlantic, where attacks were becoming more concentrated. In March, the 19th Antisubmarine Squadron relocated to Gander Airport in Newfoundland, soon joined by two other squadrons; the command formed two groups in England and Morocco. In June 1943, the 480th Antisubmarine Group was formed at Craw Field, French Morocco to conduct pat
According to the International Civil Aviation Organization, a runway is a "defined rectangular area on a land aerodrome prepared for the landing and takeoff of aircraft". Runways may be a natural surface. In January 1919, aviation pioneer Orville Wright underlined the need for "distinctly marked and prepared landing places, the preparing of the surface of reasonably flat ground an expensive undertaking there would be a continuous expense for the upkeep." Runways are named by a number between 01 and 36, the magnetic azimuth of the runway's heading in decadegrees. This heading differs from true north by the local magnetic declination. A runway numbered 09 points east, runway 18 is south, runway 27 points west and runway 36 points to the north; when taking off from or landing on runway 09, a plane is heading around 90°. A runway can be used in both directions, is named for each direction separately: e.g. "runway 15" in one direction is "runway 33" when used in the other. The two numbers differ by 18.
For clarity in radio communications, each digit in the runway name is pronounced individually: runway one-five, runway three-three, etc.. A leading zero, for example in "runway zero-six" or "runway zero-one-left", is included for all ICAO and some U. S. military airports. However, most U. S. civil aviation airports drop the leading zero. This includes some military airfields such as Cairns Army Airfield; this American anomaly may lead to inconsistencies in conversations between American pilots and controllers in other countries. It is common in a country such as Canada for a controller to clear an incoming American aircraft to, for example, runway 04, the pilot read back the clearance as runway 4. In flight simulation programs those of American origin might apply U. S. usage to airports around the world. For example, runway 05 at Halifax will appear on the program as the single digit 5 rather than 05. If there is more than one runway pointing in the same direction, each runway is identified by appending left and right to the number to identify its position — for example, runways one-five-left, one-five-center, one-five-right.
Runway zero-three-left becomes runway two-one-right. In some countries, regulations mandate that where parallel runways are too close to each other, only one may be used at a time under certain conditions. At large airports with four or more parallel runways some runway identifiers are shifted by 1 to avoid the ambiguity that would result with more than three parallel runways. For example, in Los Angeles, this system results in runways 6L, 6R, 7L, 7R though all four runways are parallel at 69°. At Dallas/Fort Worth International Airport, there are five parallel runways, named 17L, 17C, 17R, 18L, 18R, all oriented at a heading of 175.4°. An airport with only three parallel runways may use different runway identifiers, such as when a third parallel runway was opened at Phoenix Sky Harbor International Airport in 2000 to the south of existing 8R/26L — rather than confusingly becoming the "new" 8R/26L it was instead designated 7R/25L, with the former 8R/26L becoming 7L/25R and 8L/26R becoming 8/26.
Runway designations may change over time because Earth's magnetic lines drift on the surface and the magnetic direction changes. Depending on the airport location and how much drift occurs, it may be necessary to change the runway designation; as runways are designated with headings rounded to the nearest 10°, this affects some runways sooner than others. For example, if the magnetic heading of a runway is 233°, it is designated Runway 23. If the magnetic heading changes downwards by 5 degrees to 228°, the runway remains Runway 23. If on the other hand the original magnetic heading was 226°, the heading decreased by only 2 degrees to 224°, the runway becomes Runway 22; because magnetic drift itself is slow, runway designation changes are uncommon, not welcomed, as they require an accompanying change in aeronautical charts and descriptive documents. When runway designations do change at major airports, it is changed at night as taxiway signs need to be changed and the huge numbers at each end of the runway need to be repainted to the new runway designators.
In July 2009 for example, London Stansted Airport in the United Kingdom changed its runway designations from 05/23 to 04/22 during the night. For fixed-wing aircraft it is advantageous to perform takeoffs and landings into the wind to reduce takeoff or landing roll and reduce the ground speed needed to attain flying speed. Larger airports have several runways in different directions, so that one can be selected, most nearly aligned with the wind. Airports with one runway are constructed to be aligned with the prevailing wind. Compiling a wind rose is in fact one of the preliminary steps taken in constructing airport runways. Note that wind direction is given as the direction the wind is coming from: a plane taking off from runway 09 faces east, into an "east wind" blowing from 090°. Runway dimensions vary from as small as 245 m long and 8 m wide in s
The airport apron or apron referred to as the tarmac, is the area of an airport where aircraft are parked, unloaded or loaded, refueled, or boarded. Although the use of the apron is covered by regulations, such as lighting on vehicles, it is more accessible to users than the runway or taxiway. However, the apron is not open to the general public and a permit may be required to gain access. By extension, the term "apron" is used to identify the air traffic control position responsible for coordinating movement on this surface at busier airports; the use of the apron may be controlled by the apron management service to provide coordination between the users. The apron is designated by the ICAO as not being part of the maneuvering area. All vehicles and people using the apron are referred to as apron traffic; the US military refers to the apron area as the flight line. Many people in the general public and news media refer to the apron at airports as the tarmac though most of these areas are paved with concrete, not tarmac referred to as PQ concrete.
In the United States, the word ramp is an older term for an area where pre-flight activities were done. Passenger gates are the main feature of a terminal ramp; the word apron is the ICAO and FAA terminology, so the word ramp is not used with this meaning outside the US, Canada and the Philippines. Pavement Classification Number Hardstand Media related to Airport aprons at Wikimedia Commons Challenges to airport ramp and runway debris control
A glider or sailplane is a type of glider aircraft used in the leisure activity and sport of gliding. This unpowered aircraft uses occurring currents of rising air in the atmosphere to remain airborne. Gliders are aerodynamically streamlined and are capable of gaining altitude and remaining airborne, maintaining forward motion. Gliders benefit from producing the least drag for any given amount of lift, this is best achieved with long, thin wings, a faired narrow cockpit and a slender fuselage. Aircraft with these features are able to soar - climb efficiently in rising air produced by thermals or hills. In still air, gliders can glide long distances at high speed with a minimum loss of height in between. Gliders have either skids or undercarriage. In contrast hang gliders and paragliders use the pilot's feet for the start of the launch and for the landing; these latter types are described in separate articles, though their differences from gliders are covered below. Gliders are launched by winch or aerotow, though other methods: auto tow and bungee, are used.
Some gliders do not soar and are engineless aircraft towed by another aircraft to a desired destination and cast off for landing. Military gliders are single-use only, are abandoned after landing, having served their purpose. Motor gliders are gliders with engines which can be used for extending a flight and in some cases, for take-off; some high-performance motor gliders may have an engine-driven retractable propeller which can be used to sustain flight. Other motor gliders have enough thrust to launch themselves before the engine is retracted and are known as "self-launching" gliders. Another type is the self-launching "touring motor glider", where the pilot can switch the engine on and off in flight without retracting their propellers. Sir George Cayley's gliders achieved brief wing-borne hops from around 1849. In the 1890s, Otto Lilienthal built gliders using weight shift for control. In the early 1900s, the Wright Brothers built gliders using movable surfaces for control. In 1903, they added an engine.
After World War I gliders were first built for sporting purposes in Germany. Germany's strong links to gliding were to a large degree due to post-WWI regulations forbidding the construction and flight of motorised planes in Germany, so the country's aircraft enthusiasts turned to gliders and were encouraged by the German government at flying sites suited to gliding flight like the Wasserkuppe; the sporting use of gliders evolved in the 1930s and is now their main application. As their performance improved, gliders began to be used for cross-country flying and now fly hundreds or thousands of kilometres in a day if the weather is suitable. Early gliders had the pilot sat on a small seat located just ahead of the wing; these were known as "primary gliders" and they were launched from the tops of hills, though they are capable of short hops across the ground while being towed behind a vehicle. To enable gliders to soar more than primary gliders, the designs minimized drag. Gliders now have smooth, narrow fuselages and long, narrow wings with a high aspect ratio and winglets.
The early gliders were made of wood with metal fastenings and control cables. Fuselages made of fabric-covered steel tube were married to wood and fabric wings for lightness and strength. New materials such as carbon-fiber, fiber glass and Kevlar have since been used with computer-aided design to increase performance; the first glider to use glass-fiber extensively was the Akaflieg Stuttgart FS-24 Phönix which first flew in 1957. This material is still used because of its high strength to weight ratio and its ability to give a smooth exterior finish to reduce drag. Drag has been minimized by more aerodynamic shapes and retractable undercarriages. Flaps are fitted to the trailing edges of the wings on some gliders to minimize the drag from the tailplane at all speeds. With each generation of materials and with the improvements in aerodynamics, the performance of gliders has increased. One measure of performance is the glide ratio. A ratio of 30:1 means that in smooth air a glider can travel forward 30 meters while losing only 1 meter of altitude.
Comparing some typical gliders that might be found in the fleet of a gliding club – the Grunau Baby from the 1930s had a glide ratio of just 17:1, the glass-fiber Libelle of the 1960s increased that to 39:1, modern flapped 18 meter gliders such as the ASG29 have a glide ratio of over 50:1. The largest open-class glider, the eta, has a span of 30.9 meters and has a glide ratio over 70:1. Compare this to the Gimli Glider, a Boeing 767 which ran out of fuel mid-flight and was found to have a glide ratio of 12:1, or to the Space Shuttle with a glide ratio of 4.5:1. Due to the critical role that aerodynamic efficiency plays in the performance of a glider, gliders have aerodynamic features found in other aircraft; the wings of a modern racing glider have a specially designed low-drag laminar flow airfoil. After the wings' surfaces have been shaped by a mold to great accuracy, they are highly polished. Vertical winglets at the ends of the wings are computer-designed to decrease drag and improve handling performance.
Special aerodynamic seals are used at the ailerons and elevator to prevent the flow of air through control surface gaps. Turbulator devices in the form of a zig-zag tape or multiple blow holes positioned in a span-wise line along the wing are used to trip laminar flow air into turbulent flow at a desired location on the wing; this flow control prevents the formation of laminar flow bubbles and ensures t
The hectare is an SI accepted metric system unit of area equal to a square with 100-metre sides, or 10,000 m2, is used in the measurement of land. There are 100 hectares in one square kilometre. An acre is about 0.405 hectare and one hectare contains about 2.47 acres. In 1795, when the metric system was introduced, the "are" was defined as 100 square metres and the hectare was thus 100 "ares" or 1⁄100 km2; when the metric system was further rationalised in 1960, resulting in the International System of Units, the are was not included as a recognised unit. The hectare, remains as a non-SI unit accepted for use with the SI units, mentioned in Section 4.1 of the SI Brochure as a unit whose use is "expected to continue indefinitely". The name was coined from the Latin ārea; the metric system of measurement was first given a legal basis in 1795 by the French Revolutionary government. The law of 18 Germinal, Year III defined five units of measure: The metre for length The are for area The stère for volume of stacked firewood The litre for volumes of liquid The gram for massIn 1960, when the metric system was updated as the International System of Units, the are did not receive international recognition.
The International Committee for Weights and Measures makes no mention of the are in the current definition of the SI, but classifies the hectare as a "Non-SI unit accepted for use with the International System of Units". In 1972, the European Economic Community passed directive 71/354/EEC, which catalogued the units of measure that might be used within the Community; the units that were catalogued replicated the recommendations of the CGPM, supplemented by a few other units including the are whose use was limited to the measurement of land. The names centiare, deciare and hectare are derived by adding the standard metric prefixes to the original base unit of area, the are; the centiare is one square metre. The deciare is ten square metres; the are is a unit of area, used for measuring land area. It was defined by older forms of the metric system, but is now outside the modern International System of Units, it is still used in colloquial speech to measure real estate, in particular in Indonesia, in various European countries.
In Russian and other languages of the former Soviet Union, the are is called sotka. It is used to describe the size of suburban dacha or allotment garden plots or small city parks where the hectare would be too large; the decare is derived from deca and are, is equal to 10 ares or 1000 square metres. It is used in Norway and in the former Ottoman areas of the Middle East and the Balkans as a measure of land area. Instead of the name "decare", the names of traditional land measures are used, redefined as one decare: Stremma in Greece Dunam, donum, or dönüm in Israel, Jordan, Lebanon and Turkey Mål is sometimes used for decare in Norway, from the old measure of about the same area; the hectare, although not a unit of SI, is the only named unit of area, accepted for use within the SI. In practice the hectare is derived from the SI, being equivalent to a square hectometre, it is used throughout the world for the measurement of large areas of land, it is the legal unit of measure in domains concerned with land ownership and management, including law, agriculture and town planning throughout the European Union.
The United Kingdom, United States, to some extent Canada use the acre instead. Some countries that underwent a general conversion from traditional measurements to metric measurements required a resurvey when units of measure in legal descriptions relating to land were converted to metric units. Others, such as South Africa, published conversion factors which were to be used "when preparing consolidation diagrams by compilation". In many countries, metrication clarified existing measures in terms of metric units; the following legacy units of area have been redefined as being equal to one hectare: Jerib in Iran Djerib in Turkey Gong Qing in Hong Kong / mainland China Manzana in Argentina Bunder in The Netherlands The most used units are in bold. One hectare is equivalent to: 1 square hectometre 15 mǔ or 0.15 qǐng 10 dunam or dönüm 10 stremmata 6.25 rai ≈ 1.008 chō ≈ 2.381 feddan Conversion of units Hecto- Hectometre Order of magnitude Official SI website: Table 6. Non-SI units accepted for use with the International System of Units
Palm Beach County, Florida
Palm Beach County is a county in the state of Florida, directly north of Broward County. As of the 2010 census, the population was 1,320,134, making it the third-most populous county in Florida; the largest city and county seat is West Palm Beach. Named after one of its oldest settlements, Palm Beach, the county was established in 1909, after being split from Dade County; the county's modern-day boundaries were established in 1963. Palm Beach County is one of the three counties in South Florida that make up the Miami metropolitan area, home to an estimated 6,158,824 people in 2017; the area had been increasing in population since the late 19th century, with the incorporation of West Palm Beach in 1894 and after Henry Flagler extended the Florida East Coast Railway and built the Royal Poinciana Hotel, The Breakers, Whitehall. In 1928, the Okeechobee hurricane caused thousands of deaths. More the county acquired national attention during the 2000 presidential election, when a controversial recount occurred.
As of 2004, Palm Beach County is Florida's wealthiest county, with a per capita personal income of $44,518. It leads the state in agricultural productivity. Around 10,200 years ago, Native Americans began migrating into Florida. An estimated 20,000 Native Americans lived in South Florida, their population diminished by the 18th century, due to warfare and diseases from Europe. In 1513, Juan Ponce de León, who led a European expedition to Florida earlier that year, became the first non-Native American to reach Palm Beach County, after landing in the modern-day Jupiter area. Among the first non-Native American residents were African Americans, many of whom were former slaves or immediate descendants of former slaves. Runaway African slaves started coming to what was Spanish Florida in the late 17th century and they found refuge among the Seminoles. During the Seminole Wars, these African-American slaves fought with the Seminoles against White settlers and bounty hunters. Portions of the Second Seminole War occurred in Palm Beach County, including the Battle of Jupiter Inlet in 1838.
The oldest surviving structure, the Jupiter Lighthouse, was built in 1860, after receiving authorization to the land from President Franklin Pierce in 1854. During the American Civil War, Florida was a member of the Confederate States of America. Two Confederate adherents removed the lighting mechanism from the lighthouse. One of the men who removed the light, Augustus O. Lang, was the first White settler in Palm Beach County, he built a palmetto shack along the eastern shore of Lake Worth in 1863 after abandoning the cause of the Confederacy. After the Civil War ended, the Jupiter Lighthouse was relit in 1866. Thirteen years a National Weather Service office was established at the lighthouse complex. However, the office was moved to Miami in 1911 after that city's population began to grow. In October 1873, a hurricane caused a shipwreck between the New River; the crew nearly died due to starvation because of the desolation of the area. In response, five Houses of Refuge were built along the east coast of Florida from the Fort Pierce Inlet southward to Biscayne Bay.
Orange Grove House of Refuge No. 3 was built near Delray Beach in 1876. Henry Flagler, instrumental in the county's development in the late 19th century and early 20th century, first visited in 1892, he subsequently purchased land on both sides of Lake Worth. Other investors followed suit, causing a small boom and bringing in existing businesses and resulting in the establishment of many new businesses; the Royal Poinciana Hotel, constructed by Flagler to accommodate wealthy tourists, opened for business in February 1894. About a month the Florida East Coast Railway, owned by Flagler, reached West Palm Beach. On November 5, 1894, Palm Beach County's oldest city, West Palm Beach, was incorporated. In 1896, another hotel built by Flagler was opened, the Palm Beach Inn renamed The Breakers, he constructed his own winter home beginning in 1900. Flagler died there after falling down a flight of marble stairs; the Florida Legislature voted to establish Palm Beach County in 1909, carving it out of what was the northern portion of Dade County and including all of Lake Okeechobee.
The southernmost part of Palm Beach County was separated to create the northern portion of Broward County in 1915, the northwestern portion became part of Okeechobee County in 1917, southern Martin County was created from northernmost Palm Beach County in 1925. The boundaries remained the same until 1963, when about three-quarters of Lake Okeechobee was removed from Palm Beach County and divided among Glades, Hendry and Okeechobee Counties; this was the final change to the county's boundaries. Early on September 17, 1928, the Okeechobee hurricane made landfall near West Palm Beach as a category-4 storm and crossed Lake Okeechobee shortly thereafter. Coastal cities were devastated West Palm Beach, where more than 1,711 homes were destroyed. Further inland, wind-driven storm surge in Lake Okeechobee inundated adjacent communities Belle Glade and South Bay. Hundreds of square miles were flooded, including some areas with up to 20 feet of water. Numerous houses were damaged after crashing into other obstacles.
At least 2,500 deaths occurred. Damage in South Florida totaled $25 million. In response to the storm, the Herbert Hoover Dike was constructed to prevent a similar disaster; as a result of this hurrican
Mean sea level is an average level of the surface of one or more of Earth's oceans from which heights such as elevation may be measured. MSL is a type of vertical datum – a standardised geodetic datum –, used, for example, as a chart datum in cartography and marine navigation, or, in aviation, as the standard sea level at which atmospheric pressure is measured to calibrate altitude and aircraft flight levels. A common and straightforward mean sea-level standard is the midpoint between a mean low and mean high tide at a particular location. Sea levels can be affected by many factors and are known to have varied over geological time scales; however 20th century and current millennium sea level rise is caused by global warming, careful measurement of variations in MSL can offer insights into ongoing climate change. The term above sea level refers to above mean sea level. Precise determination of a "mean sea level" is difficult to achieve because of the many factors that affect sea level. Instantaneous sea level varies quite a lot on several scales of space.
This is because the sea is in constant motion, affected by the tides, atmospheric pressure, local gravitational differences, salinity and so forth. The easiest way this may be calculated is by selecting a location and calculating the mean sea level at that point and use it as a datum. For example, a period of 19 years of hourly level observations may be averaged and used to determine the mean sea level at some measurement point. Still-water level or still-water sea level is the level of the sea with motions such as wind waves averaged out. MSL implies the SWL further averaged over a period of time such that changes due to, e.g. the tides have zero mean. Global MSL refers to a spatial average over the entire ocean. One measures the values of MSL in respect to the land. In the UK, the Ordnance Datum is the mean sea level measured at Newlyn in Cornwall between 1915 and 1921. Prior to 1921, the vertical datum was MSL at the Victoria Liverpool. Since the times of the Russian Empire, in Russia and other former its parts, now independent states, the sea level is measured from the zero level of Kronstadt Sea-Gauge.
In Hong Kong, "mPD" is a surveying term meaning "metres above Principal Datum" and refers to height of 1.230m below the average sea level. In France, the Marégraphe in Marseilles measures continuously the sea level since 1883 and offers the longest collapsed data about the sea level, it is used for main part of Africa as official sea level. As for Spain, the reference to measure heights below or above sea level is placed in Alicante. Elsewhere in Europe vertical elevation references are made to the Amsterdam Peil elevation, which dates back to the 1690s. Satellite altimeters have been making precise measurements of sea level since the launch of TOPEX/Poseidon in 1992. A joint mission of NASA and CNES, TOPEX/Poseidon was followed by Jason-1 in 2001 and the Ocean Surface Topography Mission on the Jason-2 satellite in 2008. Height above mean sea level is the elevation or altitude of an object, relative to the average sea level datum, it is used in aviation, where some heights are recorded and reported with respect to mean sea level, in the atmospheric sciences, land surveying.
An alternative is to base height measurements on an ellipsoid of the entire Earth, what systems such as GPS do. In aviation, the ellipsoid known as World Geodetic System 84 is used to define heights; the alternative is to use a geoid-based vertical datum such as NAVD88. When referring to geographic features such as mountains on a topographic map, variations in elevation are shown by contour lines; the elevation of a mountain denotes the highest point or summit and is illustrated as a small circle on a topographic map with the AMSL height shown in metres, feet or both. In the rare case that a location is below sea level, the elevation AMSL is negative. For one such case, see Amsterdam Airport Schiphol. To extend this definition far from the sea means comparing the local height of the mean sea surface with a "level" reference surface, or geodetic datum, called the geoid. In a state of rest or absence of external forces, the mean sea level would coincide with this geoid surface, being an equipotential surface of the Earth's gravitational field.
In reality, due to currents, air pressure variations and salinity variations, etc. this does not occur, not as a long-term average. The location-dependent, but persistent in time, separation between mean sea level and the geoid is referred to as ocean surface topography, it varies globally in a range of ± 2 m. Adjustments were made to sea-level measurements to take into account the effects of the 235 lunar month Metonic cycle and the 223-month eclipse cycle on the tides. Several terms are used to describe the changing relationships between sea level and dry land; when the term "relative" is used, it means change relative to a fixed point in the sediment pile. The term "eustatic" refers to global changes in sea level relative to a fixed point, such as the centre of the earth, for example as a result of melting ice-caps; the term "steric" refers to global changes in sea level due to thermal expansion and salinity variations. The term "isostatic" refers to changes in