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Absolute orientation
Registering an aerial photo to a ground control system.
Aerotriangulation
The method of establishing the necessary control for aerial mapping using photogrammetric methods from a sparse network of surveyed points.
Bilinear interpolation
A mathematical method for interpolating a new cell's value within a 2 x 2 neighborhood of cells.
Bounding polygon
A polygon that determines the limits of orthorectification.
Breaklines
Lines that are used to control the triangle limits on a DTM surface. They are used to indicate discontinuity.
Contrast
The actual difference in density between the highlights and the shadows on a negative or positive. Contrast is concerned only with the difference in densities, not with the magnitude of density.
DEM (Digital Elevation Modal)
A collection of points that have elevations associated with them. Usually collected on a grid.
Densitometer
Instrument for measuring the density of a photo image or step tablet.
Displacement
Any shift in the position of an image on a photograph which does not alter the perspective characteristics of the photograph (e.g., shift due to tilt of the photograph, scale change in the photograph and relief of the objects photographed).
Distortion
Any shift in the position of an image on a photograph which alters the perspective characteristics of the photograph. Causes of image distortion include lens aberration, differential shrinkage of film or paper, and motion of the film or camera.
DTM (Digital Terrain Model)
Elevation data in a 3 x 3 arcsecond grid form or a similar rectilinear form. Also can be referred to as a DEM with other information such as breaklines.
Fiducial marker
Marks located on the focal plane of an aerial camera, usually in the corners and/or mid-sides. Lines connecting opposite fiducial marks should intersect at the principal point of the photograph.
Fiducial marks
Marks which are exposed on film during aerial photography to provide a reference for photo coordinates to be used during aerial triangulation and orientation.
Focal length
Loosely, the focal distance when the lens is focused on infinity. More accurately, the distance from the rear nodal point of the lens to the image plane when the lens is focused on infinity.
Focal plane
The plane (perpendicular to the axis of the lens) in which images of points in the object field of the lens are focused.
Ground resolution
The limit of detail clarity in an image of the earth's surface collected by an aerial camera or remote sensing device, usually measured in meters. For example, an image with a ground resolution of 1O meters shows no ground features smaller than 10 x 10 meters.
Image orientation
Determining the relationship between the on-screen image and the camera calibration report.
Interior orientation
Determining the relationship between the raster coordinate system and the photo coordinate system.
Kappa
Rotation about the Z-axis of the camera. When viewed from the positive end of the axis, a plus rotation is counter-clockwise.
Mosaicing
The process of creating a large image by merging several smaller images. Involves blending the seam lines to eliminate discontinuities.
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Nearest Neighbor Interpolation
Resampling a source raster to yield a new raster with a difference cell size, raster orientation, and/or internal geometry by computing the distance between the center of each cell in the output raster and the four nearest cells in the input raster. The data value for the closest input cell is assigned without alteration to be the data value of the outputcell.
Omega
Rotation about the X-axis of the camera. When viewed from the positive end of the axis, a plus rotation is counter-clockwise.
Orthogonal projection
A perspective based on an infinite series of points, each centered directly over the object being observed, as contrasted to normal perspective in which there is a single point of view.
Orthophotograph
An orthophotograph is a photograph that has been re-assembled to function as a true map. The typical orthophoto is produced from aerial photography that has been differentially rectified (the photograph has been altered to remove both distortions caused by terrain relief and the tilt of the camera at the instant the film was exposed). Orthophotos combine the image characteristics of a photograph with the geometric qualities of a map.
Perspective projection
A perspective projection is a single point of view and is typical of an aerial photograph; areas of terrain at higher elevations lie closer to the camera at the time of exposure and therefore appear larger than corresponding areas lying at lower elevations. The tops of objects are always displaced from their bases. This distortion causes any object standing above the terrain (such as a tree or tower) to appear in the photograph as "leaning" directly away from the principal point of the photograph.
Phi
Rotation about the Y-axis of the camera. When viewed from the positive end of the axis, a plus rotation is counter-clockwise.
Photogrammetry
The art and science of obtaining precise measurements from images.
Rectification
Removing geometric distortion from a raster or a vector object. Rectification is usually achieved by aligning raster features or vector coordinate positions with features in a base map or other coordinate reference framework. Rectification may be used to bring several distorted image segments into a common framework so they can be combined into a larger image.
Resample
To interpolate cell values in a raster object and create a raster with larger or smaller cells.
Resolution
The level of object detail or sharpness determined by how many picture elements compose an area of display or corresponding raster.
Stereomodel
A stereomodel is defined as the area in common between two aerial photographs. For photogrammetric mapping, the area of overlap is usually 60 percent.
Stereoscope
An instrument which has two eyepieces and is used to view stereomodels such that the images from the two photos are combined into one stereo view of that location, showing relief and depth of features.
Triangulated Irregular Network (TIN)
DTM points are linked into a series of triangles from which the elevation of any point can be determined. Transparency - An image which is viewed by transmitted light. Specifically a film image, usually positive and often in color, intended for projection.
Vector
A geometric element, stored as a point with x,y coordinates within a computer database.
Ddefinitions are quoted from A Guide to Map and Image Processing, The GIS Sourcebook and The Glossary of Photographic Terms.
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