The degree of conformance between the estimated or measured position, time, and/or velocity of a GPS receiver and its true time, position, and/or velocity as compared with a constant standard. Radionavigation system accuracy is usually presented as a statistical measure of system error and is characterized as follows:
1/3600th of a degree (1 second) of latitude or longitude.
Pertaining to the science of astronomy. Astronomy is the science of the heavenly bodies (fixed stars, planets, satellites, and comets) their nature, distribution, magnitudes, motions, distances, periods of revolution, eclipses, etc.
Azimuth is the angle of horizontal deviation, measured clockwise, of a bearing from a standard direction.
The measurement of depths of water in oceans, seas, and lakes. Also, the information derived from such measurements.
Pertaining to cartography, the art or practice of making charts or maps.
In a circular normal distribution, the radius of the circle containing 50 percent of the individual measurements being made, or the radius of the circle within which there is a 50 percent probability of being located.
Imaginary line on the ground, all points of which are at the same elevation above or below a specified datum.
CPTs are features of known ground location that can be accurately located on imagery.
In surveying, a reference system for computing or correlating the results of surveys.
There are two principal types of datums: vertical and horizontal. A vertical datum is a level surface to which heights are referred. Several other types of datums exist.
There are two ways to define a datum. The first is to use a primary reference point and an azimuth line. This defines a theoretical datum. The datum is then "realized" by a series of accurately surveyed points starting at the primary point. This is the realization of the datum. For datums that cover a small area, the two definitions are usually quite close. For large area datums such as the North American Datums, they can differ significantly away from the primary reference point.
The formal or theoretical horizontal datum, used as a reference for position, is defined by: the latitude and longitude of an initial point, the direction of a line between this point and a specified second point, and two dimensions which define the spheroid. In the United States, the initial point for the horizontal datum is located at Meades Ranch in Kansas.
A set of elevation data on an evenly spaced grid. These are available from several sources including NIMA and the USGS. The U.S. Geological Survey produces five primary types of digital elevation model data.
The DMA was established in 1972, when mapping, charting, and geodesy functions of the Defense Community were combined into this joint Department of Defense agency. The mission of the DMA have been taken over by the National Imagery Mapping Agency, NIMA.
A DTM is a land surface represented in digital form by an elevation grid or lists of three-dimensional coordinates.
Cartesian coordinate system where the X direction is the intersection of the prime meridian (Greenwich) with the equator. The vectors rotate with the earth. Z is the direction of the spin axis.
In geodesy, a mathematical figure formed by revolving an ellipse about its minor axis. It is often used interchangeably with spheroid. Two quantities define an ellipsoid, the length of the semimajor axis, a, and the flattening, f = (a - b)/a, where b is the length of the semiminor axis. Prolate and triaxial ellipsoids are always described as such.
The measure of vertical distance above the ellipsoid. Not the same as elevation above sea level. GPS receivers output position fix height in the WGS-84 datum.
Of or determined by geodesy; that part of applied mathematics which deals with the determination of the magnitude and figure either of the whole Earth or of a large portion of its surface. Also refers to the exact location points on the Earth's surface.
When used to modify a location value ( such as geodetic latitude or geodetic height ) it implies points taken with respect to the ellipsoid.
The accuracy with which geographic position and elevation of features on the Earth's surface are mapped. This accuracy incorporates information in which the size and shape of the Earth has been taken into account.
The particular equipotential surface that coincides with mean sea level and that may be imagined to extend through the continents. This surface is everywhere perpendicular to the force of gravity.
The height above the geoid is often called elevation above mean sea level.
The GPS is a worldwide satellite navigation system that is funded and supervised by the U.S. Department of Defense. GPS satellites transmit specially coded signals. These signals are processed by a GPS receiver that computes extremely accurate measurements, including 3-dimensional position, velocity, and time on a continuous basis. [Woods Hole Field Center]
Relating to weight measurement. A GRAVIMETER is an instrument used for determining the specific gravity of bodies, solid or liquid.
GCPs are physical points on the ground whose positions are known with respect to some horizontal coordinate system and/or vertical datum. When mutually identifiable on the ground and on a map or photographic image, ground control points can be used to establish the exact spatial position and orientation of the image to the ground. Ground control points may be either horizontal control points, vertical control points, or both.
A High Accuracy Reference Network (HARN) and a High Precision Geodetic Network (HPGN)
were two designations used for a statewide geodetic network upgrade. The generic acronym
HARN is now used for both HARN and HPGN and was adopted to remove the confusion arising
from the use of two acronyms.
A HARN is a statewide or regional upgrade in accuracy of NAD
83 coordinates using Global Positioning System (GPS) observations. HARNs were observed to
support the use of GPS by Federal, state, and local surveyors, geodesists, and many other
applications.
The HARN is a realization of NAD83.
The JNC series provides worldwide coverage at 1:2,000,000 scale. The information on these charts are suitable for aeronautical long-range, high-altitude, high-speed travel; map features include cities, roads, railroads, lakes, principal drainage, and permanent snow/ice areas. The polar regions are in a Transverse Mercator projection. All other regions are presented in the Lambert Conformal Conic projection.
Azimuthal projections are formed onto a plane which is usually tangent to the globe at either pole, the Equator, or any intermediate point. The Lambert Azimuthal Equal Area projection is a method of projecting maps on which the azimuth or direction from a given central point to any other point is shown correctly and also on which the areas of all regions are shown in the same proportion of their true areas. When a pole is the central point, all meridians are spaced at their true angles and are straight radii of concentric circles that represent the parallels.
This projection is frequently used in one of three aspects: The polar aspect is used in atlases for maps of polar regions and of the Northern and Southern Hemispheres; the equatorial aspect is commonly used for atlas maps of the Eastern and Western Hemispheres; and the oblique aspect is used for atlas maps of continents and oceans.
The Lambert Conformal Conic Projection is derived by the projection of lines from the center of the globe onto a simple cone. This cone intersects the Earth along two standard parallels of latitude, both of which are on the same side of the equator. All meridians are converging straight lines that meet at a common point beyond the limits of the map. Parallels are concentric circles whose center is at the intersection point of the meridians. Parallels and meridians cross at right angles, an essential of conformality.
To minimize and distribute scale errors, the two standard parallels are chosen to enclose two-thirds of the north to south map area. Between these parallels, the scale will be too small, and beyond them, too large. If the north to south extent of the mapping is limited, maximum scale errors will rarely exceed one percent. Area exaggeration between and near the standard parallels, is very slight; thus, the projection provides good directional and shape relationships for areas having their long axes running in an east to west belt.
In general: Angular distance, in degrees, minutes, and seconds of a point north or south of the Equator.
There are three principle values for the latitude.
Geocentric Latitude:
The angle made
by the line to the center of the earth from the surface point and the
equatorial plane. This is the only latitude in a spherical model of the
earth.
Geodetic Latitude / Geographic Latitude:
The line defined is defined by the perpendicular to the ellipsoid. The
angle between this line and the equatorial plane is the geodetic latitude.
This is the latitude found on maps.
Astrodetic Latitude
The line is defined by the local "up", that is the local gravity field.
This is very close to the geodetic latitude.
longitude
Mercator is a conformal map projection, that is, it preserves angular relationships. Mercator was designed and is recommended for navigational use and is the standard for marine charts. Mercator is often and inappropriately used as a world map projection in atlases and for wall charts where it presents a misleading view of the world because of the excessive distortion of area in the higher latitude areas.
Unit of angular measure equal to one-thousandth of a radian. A radian is the angle subtended at the center of a circle by an area of length equal to the radius of the circle.
NAD27 is defined with an initial point at Meads Ranch, Kansas, and by the parameters of the Clarke 1866 ellipsoid.
NAD83 is an Earth-centered datum and uses the Geodetic Reference System 1980 (GRS 80) ellipsoid, unlike NAD27, which is based on an initial point (Meades Ranch, Kansas). Using recent measurements with modern geodetic, gravimetric, astrodynamic, and astronomic instruments, the GRS 80 ellipsoid has been defined as a best fit to the worldwide geoid. Because the NAD83 surface deviates from the NAD27 surface, the position of a point based on the two reference datums will be different.
Point on the ground vertically beneath the center of a remote sensing platform.
Reference surface established by the U.S. Coast and Geodetic Survey in 1929 as the datum to which relief features and elevation data are referenced in the conterminous United States; formerly called "mean sea level 1929."
Specifications promulgated by the U.S. Office of Management and Budget to govern accuracy of topographic and other maps produced by Federal agencies.
The NGDC at Boulder, Colorado, was created in 1965. It is one of three data and information centers of NOAA's National Environmental Satellite, Data and Information Service (NESDIS). NGDC brings together in one data center activities in solar-terrestrial physics, marine geology/geophysics, and solid earth geophysics.
The agency of the Department of Defense responsible for map and map like products. This includes paper and digital products.
An image taken with a camera or sensor with the axis intentionally directed between the vertical and horizontal planes. A high oblique image includes the horizon in the field of view, while a low oblique shows only the Earth's surface.
Correction applied to satellite imagery to account for terrain-induced distortion.
An abbreviation of picture element. The minimum size area on the ground detectable by a remote sensing device. The size varies depending on the type of sensor.
Precision is a statistical measurement of repeatability that is usually
expressed as a variance or standard deviation, root mean square or
RMS, of repeated measurements.
Note see also accuracy.
In computer systems, the number of significant digits. Two standards are common.
Single-precision coordinates have up to seven significant digits of precision. This allows for a level of accuracy of approximately 10 meters for a region whose extent is 1,000,000 meters across.
Double-precision coordinates have up to 15 significant digits; this allows for the precision necessary to represent any desired map accuracy at a global scale.
Orderly system of lines on a plane representing a corresponding system of imaginary lines on an adopted terrestrial or celestial datum surface. Also, the mathematical concept of such a system. For maps of the Earth, a projection consists of (1) a graticule of lines representing parallels of latitude and meridians of longitude or (2) a grid.
A radian is a unit of angular measure equal to the angle subtended at the center of a circle by an arc of length equal to the radius of the circle equal to approximately 57 degrees, 17 minutes, 44.6 seconds.
A raster image is a matrix of row and column data points whose values represent energy being reflected or emitted from the object being viewed. These values, or pixels, can be viewed on a display monitor as a black and white or color image.
Reflectance is the fraction of the total radiant flux incident upon a surface that is reflected and that varies according to the wavelength distribution of the incident radiation.
A way of modeling information in a database by relations between the features. Relations are usually represented as a collection of tables where each table contains the occurrences of a particular feature. Each column of the table corresponds to an attribute and each row is an instance of the feature. For example, two related tables might be created to describe types of transportation networks in a data set. The first table has columns that uniquely identify the transportation feature, and another that contains codes that describe the transportation type (trails, roads, railways, ferries, etc.). A second table, which relates to the first, might contain columns that list the transportation codes used in the first (related) table, and a second column that defines, in further detail, the definition of the code (gravel, asphalt, concrete, etc) and road maintenance schedule for that type of road surface. The benefit of a relational database is that repetitive information is not recorded numerous times in a table, but instead is pointed to in related tables. Also referred to as Relational Database Management Systems (RDBMS).
The RMSE statistic is used to describe accuracy encompassing both random and systematic errors.
The square of the difference between a true test point and an interpolated test point divided by the total number of test points in the arithmetic mean. The square root of this value is the root mean square error.
Mathematical figure closely approaching the geoid in form and size and used as a surface of reference for geodetic surveys. A reference spheroid or ellipsoid is a spheroid determined by revolving an ellipse about its shorter (polar) axis and used as a base for geodetic surveys of a large section of the Earth (such as the Clarke spheroid of 1866 which is used for geodetic surveys in the United States).
Note: The terms spheriod and ellipsoid are used interchangely in geodesy.
A measure of the dispersion of random errors about the mean value. If a large number of measurements or observations of the same quantity are made, the standard deviation is the square root of the sum of the squares of deviations from the mean value divided by the number of observations less one.
Thematic data layers in a data set are layers of information that deal with a particular theme. These layers are typically related information that logically go together. Examples of thematic data would include a data layer whose contents are roads, railways, and river navigation routes.
Map that presents the horizontal and vertical positions of the features represented; distinguished from a planimetric map by the addition of relief in measurable form.
Configuration (relief) of the land surface; the graphic delineation or portrayal of that configuration in map form, as by contour lines; in oceanography the term is applied to a surface such as the sea bottom or a surface of given characteristics within the water mass.
UTM is a widely used map projection that employs a series of identical projections around the world in the mid-latitude areas, each spanning six degrees of longitude and oriented to a meridian. This projection is characterized by its conformality; that is, it preserves angular relationships and scale plus it easily allows a rectangular grid to be superimposed on it. Many worldwide topographic and planimetric maps at scales ranging between 1:24,000 and 1:250,000 use this projection.
Any quantity which has both magnitude and direction, as opposed to scaler which has only magnitude.
Vector data, when used in the context of spatial or map information, refers to a format where all map data is stored as points, lines, and areas rather than as an image or continuous tone picture. These vector data have location and attribute information associated with them.
The definition of DMA DEMs, as presently stored in the USGS data base, references the WGS 72 datum. WGS 72 is an Earth-centered datum. The WGS 72 datum was the result of an extensive effort extending over approximately three years to collect selected satellite, surface gravity, and astrogeodetic data available throughout 1972. These data were combined using a unified WGS solution (a large-scale least squares adjustment).
The WGS 84 datum was developed as a replacement for WGS 72 by the military mapping community as a result of new and more accurate instrumentation and a more comprehensive control network of ground stations. The newly developed satellite radar altimeter was used to deduce geoid heights from oceanic regions between 70 degrees north and south latitude. Geoid heights were also deduced from ground-based Doppler and ground-based laser satellite-tracking data, as well as surface gravity data. This system is described in "World Geodetic System 1984," DOD DMA TR 8350.2 September 1987. New and more extensive data sets and improved software were used in the development.
Zenith is the point on the celestial sphere vertically above a given position or observer.
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