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The North American Vertical Datum of 1988 (NAVD 88) is the vertical datum for orthometric heights established for vertical control surveying in the United States of America based upon the General Adjustment of the North American Datum of 1988.
A vertical datum or height datum is a reference surface for vertical positions, such as the elevations of Earth features including terrain, bathymetry, water level, and man-made structures. Vertical datums are either: tidal, based on sea levels; gravimetric, based on a geoid; or geodetic, based on the same ellipsoid models of the Earth used for computing horizontal datums.
The orthometric height of a point is the distance H along a plumb line from the point to a reference height. When the reference height is a geoid model, orthometric height is for practical purposes "height above sea level".
Surveying or land surveying is the technique, profession, art and science of determining the terrestrial or three-dimensional positions of points and the distances and angles between them. A land surveying professional is called a land surveyor. 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 building corners or the surface location of subsurface features, or other purposes required by government or civil law, such as property sales.
NAVD 88 was established in 1991 by the minimum-constraint adjustment of geodetic leveling observations in Canada, the United States, and Mexico. It held fixed the height of the primary tide gauge benchmark (surveying), referenced to the International Great Lakes Datum of 1985 local mean sea level (MSL) height value, at Rimouski, Quebec, Canada. Additional tidal bench mark elevations were not used due to the demonstrated variations in sea surface topography, i.e., that MSL is not the same equipotential surface at all tidal bench marks.
Geodesy, is the Earth science of accurately measuring and understanding Earth's geometric shape, orientation in space, and gravitational field. The field also incorporates studies of how these properties change over time and equivalent measurements for other planets. Geodynamical phenomena include crustal motion, tides, and polar motion, which can be studied by designing global and national control networks, applying space and terrestrial techniques, and relying on datums and coordinate systems.
Canada is a country in the northern part of North America. Its ten provinces and three territories extend from the Atlantic to the Pacific and northward into the Arctic Ocean, covering 9.98 million square kilometres, making it the world's second-largest country by total area. Canada's southern border with the United States, stretching some 8,891 kilometres (5,525 mi), is the world's longest bi-national land border. Its capital is Ottawa, and its three largest metropolitan areas are Toronto, Montreal, and Vancouver. As a whole, Canada is sparsely populated, the majority of its land area being dominated by forest and tundra. Consequently, its population is highly urbanized, with over 80 percent of its inhabitants concentrated in large and medium-sized cities, with 70% of citizens residing within 100 kilometres (62 mi) of the southern border. Canada's climate varies widely across its vast area, ranging from arctic weather in the north, to hot summers in the southern regions, with four distinct seasons.
The United States of America (USA), commonly known as the United States or America, is a country comprising 50 states, a federal district, five major self-governing territories, and various possessions. At 3.8 million square miles, the United States is the world's third or fourth largest country by total area and is slightly smaller than the entire continent of Europe's 3.9 million square miles. With a population of over 327 million people, the U.S. is the third most populous country. The capital is Washington, D.C., and the largest city by population is New York City. Forty-eight states and the capital's federal district are contiguous in North America between Canada and Mexico. The State of Alaska is in the northwest corner of North America, bordered by Canada to the east and across the Bering Strait from Russia to the west. The State of Hawaii is an archipelago in the mid-Pacific Ocean. The U.S. territories are scattered about the Pacific Ocean and the Caribbean Sea, stretching across nine official time zones. The extremely diverse geography, climate, and wildlife of the United States make it one of the world's 17 megadiverse countries.
The definition of NAVD 88 uses the Helmert orthometric height, which calculates the location of the geoid (which approximates MSL) from modeled local gravity. The NAVD 88 model is based on then-available measurements, and remains fixed despite later improved geoid models.
The geoid is the shape that the ocean surface would take under the influence of the gravity and rotation of Earth alone, if other influences such as winds and tides were absent. This surface is extended through the continents. According to Gauss, who first described it, it is the "mathematical figure of the Earth", a smooth but irregular surface whose shape results from the uneven distribution of mass within and on the surface of Earth. It can be known only through extensive gravitational measurements and calculations. Despite being an important concept for almost 200 years in the history of geodesy and geophysics, it has been defined to high precision only since advances in satellite geodesy in the late 20th century.
NAVD 88 replaced the National Geodetic Vertical Datum of 1929 (NGVD 29), previously known as the Sea Level Datum of 1929. The elevation difference between points in a local area will show negligible change from one datum to the other, even though the elevation of both does change. NGVD 29 used a simple model of gravity based on latitude to calculate the geoid and did not take into account other variations. Thus, the elevation difference for points across the country does change between datums.
Altitude or height is defined based on the context in which it is used. As a general definition, altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The reference datum also often varies according to the context. Although the term altitude is commonly used to mean the height above sea level of a location, in geography the term elevation is often preferred for this usage.
The Sea Level Datum of 1929 was the vertical datum established for vertical control surveying in the United States of America by the General Adjustment of 1929. The datum was used to measure elevation (altitude) above, and depression (depth) below, mean sea level (MSL).
Topography is the study of the shape and features of land surfaces. The topography of an area could refer to the surface shapes and features themselves, or a description.
The National Geodetic Survey (NGS), formerly the United States Survey of the Coast (1807–1836), United States Coast Survey (1836–1878), and United States Coast and Geodetic Survey (USC&GS) (1878–1970), is a United States federal agency that defines and manages a national coordinate system, providing the foundation for transportation and communication; mapping and charting; and a large number of applications of science and engineering. Since its foundation in its present form in 1970, it has been part of the National Oceanic and Atmospheric Administration (NOAA), of the United States Department of Commerce.
Mean sea level (MSL) 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 – that is 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, consequently, aircraft flight levels. A common and relatively straightforward mean sea-level standard is the midpoint between a mean low and mean high tide at a particular location.
In the mountaineering parlance of the Western United States, a fourteener is a mountain peak with an elevation of at least 14,000 feet. There are 96 fourteeners in the United States, all west of the Mississippi River. Colorado has the most (53) of any single state; Alaska is in second place with 29. Many peak baggers try to climb all fourteeners in the contiguous United States, one particular state, or another region.
The elevation of a geographic location is its height above or below a fixed reference point, most commonly a reference geoid, a mathematical model of the Earth's sea level as an equipotential gravitational surface . The term elevation is mainly used when referring to points on the Earth's surface, while altitude or geopotential height is used for points above the surface, such as an aircraft in flight or a spacecraft in orbit, and depth is used for points below the surface.
The term benchmark, or bench mark, originates from the chiseled horizontal marks that surveyors made in stone structures, into which an angle-iron could be placed to form a "bench" for a leveling rod, thus ensuring that a leveling rod could be accurately repositioned in the same place in the future. These marks were usually indicated with a chiseled arrow below the horizontal line.
Normalhöhennull or NHN is a vertical datum used in Germany.
VERTCON is a computer program that computes the modeled difference in orthometric height between the North American Vertical Datum of 1988 and the National Geodetic Vertical Datum of 1929 for a location in the contiguous United States. The parameters required are the latitude and longitude of the location.