Aerial Photographs - Pictures usually taken from an airplane. It could be taken from directly overhead or at an angle.

Aerial Photography Field Office (APFO) - The U.S. Department of Agriculture's APFO is managed by the Farm Service Agency (FSA). FSA was formerly known as the Agricultural Stabilization and Conservation Service (ASCS). APFO is the repository for all of the USDA's aerial photography. The archive contains over 50,000 rolls of film acquired over the last 40 years and includes over 14 million frames of coverage of the conterminous U.S., Alaska, and Hawaii. APFO provides photographic products to local county, State and Federal offices within the U.S. Department of Agriculture (USDA) including FSA, National Resources Conservation Service (NRCS), and the Forest Service. They also serve the general public with similar products upon request at the cost of reproduction.

Aerial Survey - The study of the earth’s surface using images taken from aircraft. Aerial survey is used in mapmaking, agriculture, environmental studies, and military operations.

Albers Equal Area projection - A method of projection on which the areas of all regions are shown in the same proportion of their true areas. The meridians are equally spaced straight lines converging at a common point, which is normally beyond the pole. The angles between them are less than the true angles. The parallels are unequally spaced concentric circular arcs centered on the point of convergence of the meridians. The meridians are radii of the circular arcs. The poles are normally circular arcs enclosing the same angle as that enclosed by the other parallels of latitude for a given range of longitude.

Albers Equal Area is frequently used in the ellipsoidal form for maps of the United States in the National Atlas of the United States, for thematic maps, and for world atlases. It is also used and recommended for equal-area maps of regions that are predominantly east-west in extent.

American National Standards Institute (ANSI) - An ANSI standard implies a consensus of those substantially concerned with the scope and provisions of a product and is intended as a guide to aid the manufacturer, consumer, and the general public.

Arc Second - 1/3600th of a degree (1 second) of latitude or longitude.

Azimuth - The angle of horizontal deviation, measured clockwise, of a bearing from a standard direction.

Base Map - A map containing visible surface features and boundaries, essential for locating additional layers, or types, of georeferenced information.

Bathymetry - The measurement of depths of water in oceans, seas, and lakes. Also, the information derived from such measurements.

Benchmark - A point on the ground with a known elevation relative to mean sea level.

Bilinear - The term bilinear is referring to a bilinear interpolation. This is simply an interpolation with two variables instead of one.

Cartography - The art or work of making maps.

Contour Interval - The difference in elevation represented by two adjacent contours.

Contour Line - A line on a map representing an elevation contour.

Control Point - CPTs are features of known ground location that can be accurately located on imagery.

Contrast - The actual difference in density between the highlights and the shadows on a negative or positive. Contrast is not concerned with the magnitude of density, but only with the difference in densities.

Contrast Index - Contrast Index is the slope of a line between two particular points of the D-Log E curve. These two points are the minimum and maximum densities on the curve that are normally used to make high-quality negatives. The minimum point falls somewhere on the toe of the curve, meaning the shape of the toe influences Contrast Index. It is unlike gamma, which is the slope of the straight-line portion only.

Control Points - A set of points on the ground whose horizontal and vertical location is known. Control points are used as the basis for detailed surveys.

Department of Natural Resources - Are dedicated to the preservation, protection, effective management of the State’s natural resources. The primary use of aerial photos by resource professionals of DNR is to aid in managing activities on state lands. It also is used to map geology, the types and extent of vegetation communities, watersheds, and fish and wildlife habits, etc.

Digital Elevation Model (DEM) - Terrain elevation data organized by quadrangle and provided in digital form.

Digital Terrain Model (DTM) - A three-dimensional model of the Earth's surface, provided in digital form.

Displacement - Any shift in the position of an image on a photograph which does not alter the perspective characteristics of the photograph (i.e., 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.

Dodging - A process used to hold back light which passes through certain parts of the negative or positive and prevents exposure to sensitized material. Manual dodging is done by holding a piece of opaque material between the enlarger lens and the easel. Electronic dodging is produced by feedback of signal voltage through the negative or positive to be printed to minimize density variations of produced material.

Edge Fog - Fog on film caused by leakage of light between the flanges of the spool on which it is wound.

Efficiency of a Shutter - The relationship between the total time a shutter remains open (counting from half-open to half-closed position) and the time required for the shutter to reach the half-open and the half-closed positions.

Emulsion Speed - A property of photographic emulsions which determines how long they must be exposed to a given light source to secure equal density when developed. This speed may be given in USASI, DIN, Weston, Scheiner, or AEL scales.

Family of Curves - A display of characteristic curves obtained from a film type that has been subjected to varying development parameters.

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.

Focal-Plane Shutter - A shutter located near the focal plane; usually consisting of a curtain with a slot which is pulled across the focal plane to make the exposure.

Fog - A darkening of negatives of prints by a deposit of silver which does not form a part of the image. Fog tends to increase density and decrease contrast. It may be caused by exposure to unwanted light, exposure to air during development, forced development or impure chemicals.

Forced Development - Development deliberately prolonged considerably beyond the normal time to compensate for underexposure.

Gamma - Gamma is a measure of the contrast of a negative and it is symbolized by the Greek character g. Gamma is the slope of the straight-line portion of the curve. Slope refers to the steepness of a straight line, determined by taking the change in density from two points on the curve and dividing that by the change in log exposure for the same two points. Degree of developing affects the steepness, or contrast, of the curve. If we change the degree of development, the relationship between exposure and the degree of darkening also changes.

GIS (Geographic Information System) - Describes any automated system for spatially managing and analyzing geographic information.

GPS (Global Positioning System) - A technology that enables an individual to identify the location of an object by triangulating his/her coordinates from a network of satellites.

Graininess - The impression of non-uniformity in the image which is produced on the consciousness of an observer by the granular structure. It is most noticeable in areas of uniform density of middle values. It becomes less apparent in light and in dark areas and in areas that contain considerable detail. Graininess is subjective in nature.

Granularity - An objective measurement which correlates with the subjective appearance of graininess. This and similar objective measurements indicate the ability of a film to record an image in terms that are meaningful from the standpoint of the appearance and usefulness of the photograph. Granularity is the value published in the film data sheet for Kodak aerial films, and it is given as the root-mean-square (rms) granularity value.

Gray Scale - A strip of film or paper displaying individuality uniform areas of density ranging from light to dark in a series of steps. Also sometimes called step tablets, they are used in testing the sensitivity and contrast characteristics of photographic materials.

Halation - The fringe or halo which sometimes occurs around very heavily exposed image points as the excess light, penetrating the emulsion layer, is reflected back to the emulsion from the surfaces of the film base. It is reduced in modern films by dyeing the film base material itself or by the application of a dyed gelatin layer on the film back, called the anti-halation or anti-halo backing.

Infrared Films - In addition to panchromatic sensitivity, infrared-sensitive films have a sensitivity that extends into the infrared region of the spectrum to about 900 nm. They are generally used with a filter that prevents short-wavelength radiation from reaching the film. Their value lies in their ability to penetrated haze, to show good contrast between land and water, and to differentiate various types of vegetation .

International Date Line - An imaginary line drawn on the map of the Pacific Ocean, near, and in many places coincident with, the Prime Meridian. It marks the place where navigators change their date by one day on a transpacific voyage. East of the line it is one day earlier than to the west.

Latent Image - The invisible impression on the sensitized emulsion exposed to light upon a photographic emulsion in a camera. Development converts the latent image to a visible one.

Latitude and Longitude - Geometrical coordinates for designating the location of places on the surface of the earth. Latitude, which gives the location of a place north or south of the equator, is expressed by angular measurements ranging from 0 degrees at the equator to 90 degrees (Decimal Degree Range -90.0 to 90.0) at the poles. Longitude, the location of a place east or west of a north-south line called the prime meridian, is measured in angles ranging from 0 degrees at the prime meridian to 180 degrees (Decimal Degree Range -180.0 to 180.0) at the International Date Line.

Maximum Useable Density - The highest image density having some detail (highlight).

Minimum Useable Density - The lowest image density having some detail (shadow).

Neutral Density Filter - A thin sheet or disc of glass, plastic or gelatin having plane and parallel faces, toned to some uniform and specific shade of gray, and intended to be used over the camera lens during exposure for the purpose of reducing the intensity of the exposing light without changing its color.

Orthochromatic - Type of emulsion which is sensitive to visible blue and green light, but not to red.

Orthophotography - Aerial photographs that have been rectified to produce an accurate image of the Earth by removing tilt and relief displacements which occurred when the photo was taken.

Orthorectified / Orthorectification - The process by which the geometric distortions of the aerial photograph caused by topographical variations in the surface of the earth and by the tilt of the aircraft camera lens are modeled and accounted for, resulting in an image with the characteristics of a photograph and the geometric qualities of a map.

Panchromatic - Type of emulsion which is sensitive to light of all colors, as a film or plate emulsion.

Photogrammetry - The science of deducing the physical dimension of objects from measurements on photographs.

Prime Meridian - The meridian (line of longitude) at which longitude is defined to be 0°.

Quadrangles - The area of land charted on each of the atlas sheets published by the USGS. A four-sided area bounded by parallels of latitude and meridians of longitudes.

Relative Aperture - For a photographic or telescopic lens, the ratio of the equivalent focal length to the diameter of the entrance pupil. Expressed as f/4.5 or f.4:5, also called f number or speed of lens.

Remote Sensing - Any of the technical disciplines for observing and measuring the Earth from a distance, including satellite imaging, Global Positioning Systems, RADAR, SONAR, aerial photography, etc.

Resolving Power - An expression of lens definition, usually stated as the maximum number of lines per millimeter that can be resolved (i.e., seen as separate lines) in the image. The resolving power of the lens varies with the contrast of the test chart and normally varies also with the orientation and position of the chart within the field.

Resolving Power Target - A test chart used for the evaluation of photographic, optical, and electro-optical systems. The design usually consists of ruled lines, squares, or circles varying in size according to a specified geometric progression.

Sensitometry - The science of the measurement of the sensitivity, and related characteristics, of photographic materials.

Shoulder - The upper, diminishing-gradient portion of a film characteristic curve, which represents the region of overexposure.

Spectral Sensitivity - The description of a film's photographic response to radiation of various wavelengths. Silver halides, the fundamental photosensitive elements of all photographic emulsions, are sensitive to high-energy radiations. However, they are sensitive only to the ultraviolet, violet, and blue in the visible and near-visible spectrum. Sensitizing dyes are commonly used to extend the sensitivity of an emulsion to the green, red, and near-infrared portions of the spectrum.

State Plane Coordinate System - A method developed by the National Geodetic Survey for mapping each state within a grid system.

Step Wedge - A strip of film or a glass plate whose transparency diminishes in graduated steps from one end to the other; often used to determine the density of a photograph. Also called gray scale or step tablet.

Tonal Scale - The range of grays or densities of a photographic image. Gradation.

Transparency - An image which is viewed by transmitted light. Specifically a film image, usually positive and often in color, intended for projection.

Triangulation - A method of surveying in the location of an object may be calculated from the known locations of two other objects. Creating a triangle from the three items, the angles and sides of the triangle can be measured and the location of the unknown object is calculated algebraically.

United States Geologic Survey (USGS) EROS Data Center - This agency is responsible for receiving, processing, archiving, distributing, researching and applying remote sensing data taken of the Earth.

Vector - A geometric element, stored as a point with x.y coordinates within a computer database.

Vignetting - A gradual reduction in density of parts of a photographic image due to the stopping of some of the rays entering the lens. A lens mounting may interfere with the extreme oblique rays. An antivignetting filter is one that gradually decreases in density from the center toward the edges; it is used with many wide-angle-lenses to produce a photograph of uniform density by cutting down the overexposure of the center of the photograph.

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