GELE key terms

The equipotential surface of Earth's gravity field that coincides most closely with mean sea level, continued beneath the continents; it is the reference surface from which orthometric heights are measured.

A mathematically defined oblate spheroid, fixed by a semimajor axis and a flattening, that approximates the geoid and serves as the smooth computational surface for geodetic coordinates and map projections.

The defined set of parameters, namely the ellipsoid, its origin, and its orientation, that fixes a coordinate system to the Earth, such as the Philippine Reference System of 1992 or WGS 84.

The separation between the geoid and the reference ellipsoid at a given point, equal to ellipsoidal height minus orthometric height; it is also called geoid height.

The height of a point above the geoid, measured along the curved plumb line through that point; it is the elevation obtained, approximately, by spirit leveling from a tidal benchmark.

A method of establishing horizontal control in which only distances between stations are measured and coordinates are computed by geometry, in contrast to triangulation, which relies mainly on measured angles.

A land registration system in which the State issues a certificate of title that is conclusive and indefeasible evidence of ownership, so that title need not be proved by tracing a long chain of deeds.

The official document issued under the Torrens system naming the registered owner, describing the land by its technical description, and showing liens or annotations; an original certificate is issued upon first registration.

A written statement of a parcel's boundary giving the bearing and distance of each course, the area, and the monuments or tie points that fix its position on the ground.

A survey of the lands within a municipality or other political unit that determines, marks, and maps parcel boundaries for assessment, taxation, and registration purposes.

A permanent reference monument, established by the national lands agency, whose geographic position is known and to which cadastral survey lines and lot corners are tied.

A survey line run from a parcel corner or traverse station to an established control monument or known boundary point, used to fix the survey in relation to the national control network.

The calculation of the bearing, distance, and area of each lot from field measurements and control coordinates, producing the figures that appear on the plan and in the technical description.

A legal right held by one person or by the public to a specific limited use of another person's land, such as passage, drainage, or utility lines, without taking possession of that land.

An estimation method that determines unknown parameters so that the sum of squared, properly weighted residuals is minimum, yielding the most probable values together with measures of their precision.

The difference between a measured value and the value obtained for it after adjustment; the pattern and size of residuals reveal random error, systematic error, or blunders.

A number expressing the relative reliability of an observation, usually taken as proportional to the reciprocal of its variance; more heavily weighted observations pull the adjustment more strongly.

A square symmetric matrix having the variances of the parameters or observations on its diagonal and their covariances off the diagonal, thus describing both precision and correlation.

The number of redundant observations, computed as the number of observations minus the number of unknowns; it determines whether the adjustment can be checked and how reliably precision can be estimated.

A gross mistake caused by carelessness in observing, reading, or recording, which is not random and cannot be removed by adjustment; redundancy and statistical testing are used to detect it.

The discrepancy between the value computed for a line, angle, or elevation difference and its known or geometrically required value, as when a traverse returns to a fixed point.

The ratio between a distance measured on the map and the corresponding horizontal distance on the ground, expressed as a representative fraction, a verbal statement, or a graphic bar.

A mathematical transformation that converts positions on the ellipsoid to a plane surface; because a curved surface cannot be flattened without distortion, some property such as area, shape, distance, or direction is sacrificed.

A digital representation of geographic features as points, lines, or polygons, each carrying coordinate geometry and stored attributes, well suited to discrete objects such as parcels and roads.

A digital representation of geographic space as a regular grid of cells, each holding a value, well suited to continuous data such as elevation, imagery, and land cover.

The set of rules describing spatial relationships among features, namely connectivity, adjacency, and containment, stored so that the data can be validated and used for network and overlay analysis.

The process of assigning real-world coordinates to a map, image, or dataset, usually by relating identifiable points to a known coordinate system, so that it aligns with other spatial data.

An electronic surveying instrument that combines an angle-measuring theodolite with a distance meter and an onboard computer, recording angles, distances, and computed coordinates at the press of a key.

An instrument that determines distance by measuring the travel time or phase difference of an electromagnetic signal sent to a reflector or target, giving a slope distance that is then reduced to horizontal and vertical components.

Two opposing effects in leveling: Earth's curvature makes a level surface fall away from a horizontal line of sight, while atmospheric refraction bends the line of sight downward; they are often combined into a single correction.

The northing and easting components of a survey course, computed as the course length multiplied by the cosine and sine of its bearing respectively; summed around a closed traverse they should total zero.

The direction of a line stated as an acute angle measured from north or south toward east or west in the quadrant system, or as an angle measured from a stated reference meridian.

A photograph taken with the camera axis pointed vertically downward, or nearly so, at the ground; it is the standard source imagery for topographic mapping and stereoplotting.

The ratio of a distance on a photograph to the corresponding ground distance; for a vertical photograph over flat terrain it equals the focal length divided by the flying height above ground.

The outward shift, from the principal point, of the image of an object caused by the object's height above the datum; it increases with the object's height and with its distance from the nadir.

The apparent shift in the position of an object between two overlapping photographs taken from different camera stations; its magnitude is used to compute the object's elevation.

A photograph that has been geometrically rectified for tilt, lens distortion, and relief displacement so that it has the uniform scale and positional accuracy of a map and can be used for measurement.

A point on the ground whose coordinates are known from field survey and which can be clearly identified on imagery, used to orient, scale, and georeference photogrammetric models or remotely sensed images.

An active remote sensing technique that emits laser pulses and times their return to measure ranges, building dense point clouds from which digital terrain and surface models are derived.

The ground spacing between adjacent pixel centers in a digital image, used as a measure of spatial resolution; smaller values indicate finer ground detail.