GELE flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Geoid undulation

The separation N between the geoid and the reference ellipsoid, computed as ellipsoidal height minus orthometric height: N = h − H.

GRS80 ellipsoid

Reference ellipsoid with semimajor axis a = 6,378,137 m and flattening 1/f = 298.257222101, adopted for NAD83 and modern geodetic datums.

Deflection of vertical

The angle between the gravity vertical and the ellipsoid normal at a point, resolved into meridian (ξ) and prime-vertical (η) components.

Helmert seven-parameter transformation

Datum transformation using three translations, three rotations, and a scale factor: X′ = T + (1 + s)·R·X.

UTM central meridian

Each 6° UTM zone has central meridian scale factor 0.9996, false easting 500,000 m, and false northing 0 or 10,000,000 m.

Torrens system

A land registration system in which the state registers and guarantees title, so a registered proprietor holds indefeasible title subject to statutory exceptions.

Monument precedence rule

In boundary retracement, natural monuments control over artificial monuments, which control over courses and distances, which control over area.

Accretion

Gradual and imperceptible addition of land by water action; the new land belongs to the riparian owner without a new deed.

Avulsion

A sudden change in a watercourse, such as a flood cutting a new channel; the original boundary line remains unchanged.

Adverse possession

Acquisition of title by open, notorious, hostile, exclusive, and continuous possession of another's land for the statutory period.

Normal equations

The least-squares system (AᵀPA)x = AᵀPL, solved for the unknown parameters that minimize the weighted sum of squared residuals.

Reference variance

The a posteriori variance of unit weight, σ0² = vᵀPv/(n − u), where n is the number of observations and u the number of unknowns.

Compass (Bowditch) rule

Traverse adjustment that distributes latitude and departure misclosures in proportion to each side's length, correcting total closure.

Transit rule

Traverse adjustment that apportions misclosure in proportion to the magnitude of each latitude or departure rather than to side length.

Error propagation

For y = f(x), the covariance propagates as Σy = J Σx Jᵀ, where J is the Jacobian matrix of partial derivatives.

Traverse angular closure

For a closed polygon with n sides, the sum of interior angles equals (n − 2)·180°; the difference from the observed sum is the angular misclosure.

Representative fraction

Map scale expressed as 1:N, meaning one unit on the map represents N of the same units on the ground, for example 1:24,000.

Conformal projection

A map projection that preserves local angles and small shapes; scale is the same in all directions at a point, but area is distorted.

Equal-area projection

A projection that preserves area ratios on the map, usually at the cost of distorting shapes and angles.

Contour interval

The constant vertical distance between adjacent contour lines on a topographic map, chosen to suit terrain and map scale.

Vector data model

GIS representation of features as points, lines, and polygons with explicit coordinates and topology relationships.

NMAS horizontal accuracy

United States National Map Accuracy Standard: 90 percent of well-defined map points must be within 1/30 inch of true position at map scale.

Curvature and refraction

Combined correction to a level sight, C = 0.0675·K² metres, where K is the sight distance in kilometres.

Prism constant

The fixed offset, typically −30 mm or 0 mm, added to an EDM-measured distance to account for the reflector's optical path.

Temperature tape correction

Correction Ct = α·L·(T − T0), where α is the coefficient of thermal expansion and T0 is the standardization temperature.

Sag correction

Correction for an unsupported tape span, Cs = −w²L³/(24P²), where w is weight per unit length and P is tension.

Law of cosines

For any triangle, c² = a² + b² − 2ab·cos C, used to solve a side when two sides and the included angle are known.

Photo scale

The ratio S = f/H, where f is camera focal length and H is flying height above ground; also photo distance divided by ground distance.

Relief displacement

The radial shift d = r·h/H of an object's image from the nadir point, where r is radial distance and h is object height.

Parallax flying height

For a stereo pair, flying height above ground H = B·f/p, where B is the airbase, f the focal length, and p the parallax.

NDVI

Normalized Difference Vegetation Index, (NIR − Red)/(NIR + Red), ranging from −1 to +1 and increasing with green vegetation vigor.

Ground sample distance

The ground spacing of one image pixel, GSD = H·pixel size/f, where H is flying height and f is the focal length.