Accuracy: The degree of agreement between a measured value and the true value.
Aerial Negative: The original photo image produced by aerial photography onto reproducible film and used to produce prints and/or positive photographs.
Aerial Photograph, Oblique: Photographs taken at any angle below the horizon short of ninety degrees and used primarily for environmental analyses and outside presentations.
Aerial Photograph, Vertical: An aerial photograph with the camera’s longitudinal axis as close to truly vertical (90° below horizontal plane) as possible.
Aerotriangulation: The establishment of supplemental control points by precisely marking their locations onto glass photographic diapositives. Spatial solutions are then obtained by precise measurements and computational routines.
Air Base: The line, or length of line, joining two adjacent camera stations.
Altitude: Height above a datum. The datum is usually mean sea level.
Altitude (absolute): Height above the surface of the earth as differentiated from altitude – height above sea level.
Analytics: The densification of horizontal and vertical control using measurements made on aerial photographs through aerotriangulation methods.
Attitude: The angular orientation of a camera with respect to some external reference system.
Azimuth Line: A radial line from the Principal Point of a photograph to a similar point in an adjacent photograph in the same flight line.
Bench Mark: A monument point of known elevation.
Bird’s Eye: A directly overhead shot.
Camera Axis: A line through the camera’s rear nodal point which is perpendicular to the film plane.
Compilation: The production of a map or base plan from aerial photographs and geodetic control data using photogrammetric instruments and techniques.
Contour: A theoretical line tangent to the earth’s surface at a known elevation. Also, a line “locating” this elevation on a map or plan.
Contrast: The degree of difference between the lightest and darkest areas of a photograph.
Cover: In mapping, vegetation over a terrain.
Crab: The condition caused by failure to correctly position the camera with respect to the airplane’s line of flight. This results in the angular displacement of the photographic axis relative to the flight line.
Deformation: A change in the position of a point on a photograph, map, manuscript or print from its originally plotted position. This change is the result of differential shrinkage and/or expansion of the film or paper.
Displacement: Any shift in the position of a point, line or feature on a photograph.
Distortion: A change in the position of a point on a photograph from its original position caused by an aberration or combination of aberrations in the camera lens.
Elevation: Is the vertical distance above or below an arbitrarily assumed level surface of curved surface every element of which is normal to the plumb line. The level surface used for reference is called datum.
Flight Altitude: The average vertical distance between the aerial camera and the ground.
Flight Line or Flight Strip: (1) The flight path of the airplane carrying the camera. (2) The strip of photographs produced from a single flight.
Flight Plan: The aerial photography operational procedure in which the flight objectives and the performance criteria are specified.
Focal Length: The distance from the plane of infinite focus to the center of the camera lens.
GIS (Geographical Information Systems): A computer system for the input, storage, retrieval, analysis and display of geo-spatially referenced data. The data is typically stored as a series of layers.
Georectification: A method of digitally rectifying aerial imagery in such a way that it is map accurate.
Grid: A uniform system of equally spaced perpendicular and parallel lines superimposed onto aerial photographs, mosaics, maps, plan sheets and other representations of the earth’s surface and used to identify the positions of points.
Manuscript or Base Map: The original master reproducible sheet upon which the data gathered from the aerial photographs has been compiled.
Map, Index: (1) A small-scale map showing the position and orientation of each map or contract plan sheet with respect to all the other map or contract plan sheets in a given project. (2) A map showing the locations and numbers of flight strips and photographs. (3) A small-scale map showing geodetic control and such data comparable to that found on larger-scale topographic quadrangle maps.
Map, Photogrammetric: An orthographically projected rendering of existing land features produced with a stereoplotting instrument.
Map, Planimetric: A reproducible copy of the manuscript showing the shape and horizontal position of natural and man-made (cultural) features with no regard for elevation or measurable relief.
Map, Topographic: A reproducible copy of the manuscript showing the shape and the horizontal and vertical position of natural and man-made features. Elevations and measurable ground relief are usually delineated by contours and by spot elevations at prominent locations.
Model: The stereoscopic image of an area produced by viewing the end lapping of two successive aerial photographs depicting the same ground area from two different positions of exposure and culminating in a three dimensional image when observed through a binocular viewer.
Mosaic: A large sequential composite of individual photographs showing a continuous overview of a project site from beginning to end.
Nadir Point: (1) Photographic: where a vertical line originating from the central point of exposure in the camera intersects the plan of the photograph. The photographic nadir point coincides with the principal point of the photograph when that photograph is truly vertical. (2) Ground: where that vertical line intersects the ground surface. (3) Datum: where that vertical line intersects the reference datum surface.
Nodal Points: A camera lens has two such points: an incidental (frontal) point and an emergent (rear) point. These points lie on the optical axis of the lens and have the property that any light ray directed toward the incident point passes through the emergent point and emerges on the other side of the lens in a direction parallel to the direction of the incident ray.
Oblique Photograph: A photograph taken with the axis of the camera intentionally directed so that it is neither vertical nor horizontal.
Orthographic: Characterized by perpendicular lines or right angles.
Orthophotograph: A photographic reproduction in which each image has been extrapolated into its map-oriented (orthographic project) position.
Overlap: The amount by which two adjacent photographs show the same area. This amount is usually expressed as a percentage of the total linear dimension of the photograph in the direction of the overlapping.
Panels: Ground control points which are readily identifiable in aerial photographs. Also referred to as Targets.
Parallax: (1) An apparent change in position of one object relative to another when viewed from different positions. (2) The change in position of an image from one aerial photograph to the next as a result of the aircraft’s motion.
Photogrammetry: The science of obtaining accurate measurements through the use of aerial photographs and stereoplotting equipment.
Photographic Index: A mosaic of individual adjacent photographs in their proper relative positions and re-photographed at a reduced scale with accompanying designations.
Planimetry: The configuration of a surface and the horizontal positions of its natural and cultural features, depicted by means of lines, symbols and notations on a scaled map or plan sheet without regard to relief.
Plans: The approved plans, profiles, typical sections, cross sections, shop drawings and supplemental drawings or exact reproductions thereof, which show the location, extent, layout and dimensions as well as the scope, character, and details of the work to be done.
Rectification: The production of a truly vertical photographic print from a tilted aerial negative.
Resolution: The resolution of an image depends on several factors, including emulsion, atmospheric conditions and lens type. In general, at 1:25,000 scale roads, railways, rivers, woodland and large buildings can be seen. At 1:10,000 scale, individual trees, stone walls and vehicles are visible. At 1:5,000 scale, road markings, fence posts, people may be seen.
Scale: The ratio of a distance on an aerial photograph, map or plan sheet to its actual counterpart on the ground.
Spot Elevations: Reported elevations of high, low and other prominent points. The precise location is generally denoted by a small “x” and is thereby accompanied by the reported value of the elevation.
Stereoplotter: An instrument designed to produce stereoscopic images or models from which very precise topographic maps can be compiled by utilizing electromechanical components of the instrument.
Stereoscopy: The science of producing three dimensional images by viewing the overlap area of two photographs through a binocular viewer. The overlapping photographs are produced with the camera at a slightly different location or perspective of one exposure relative to the other.
Tilt: The departure of the camera’s line of sight from vertical.
Topography: The configuration of a surface including its relief (usually represented by contours and spot elevations) and the positions of its natural and cultural features as shown on a map or plan.