diurnal age--See age of diurnal inequality.

diurnal arc-See astronomical arc.

diurnal circle-The apparent daily path of a celestial body, approximating a parallel of declination.

diurnal constituent-Any tide constituent whose period approximates that of a lunar day (24.84 solar hours). See also constituent. diurnal high water inequality-See diurnal inequality.

diurnal inequality-The difference in heights

and durations of the two successive high waters or of the two successive low waters of each day; also, the difference in speed and direction of the two flood currents or the two ebb currents of each day. Also called daily inequality.

diurnal low water inequality-See diurnal inequality.

diurnal motion-The apparent daily motion of a celestial body.

diurnal parallax-See geocentric parallax.

diurnal variation-That component of a determinable magnitude which passes through a complete cycle in one day.

divergence (leveling)—The difference between the numerical values of two runnings over the same section of a line of levels.

diverging lens-See negative lens.

dodging (photography)-The process of holding back light from certain areas of sensitized material to avoid overexposure of these

areas.

domestic map-A map of an area within the limits of the United States.

Doppler effect-The phenomenon evidenced by the change in the observed frequency of a sound or radio wave caused by a time rate of change in the effective length of the path of travel between the source and the point of observation. Also called Doppler shift. Doppler navigation-1. A system which measures ground speed and drift by means of electronically generated signals emitted from aircraft and reflected from the terrain. 2. A

system which determines positions based on the Doppler effect of satellite signals. Doppler shift-See Doppler effect. double burn-The intentional exposure of two or more line and/or halftone negatives in succession and register on the same sensitized surface. Not to be confused with double exposure, which is usually unintentional. Also called double shooting. See also composite.

double-center theodolite-See repeating theodolite.

double centering-A method of prolonging a line from a fixed point whereby the backsight is taken with the telescope direct. The instrument is plunged, and the foresight is made with the telescope inverted. The point at which the vertical cross-hair intersects the hub is then marked. The transit is then rotated to take a backsight on the fixed point with the telescope inverted, the foresight is made with the telescope direct, and a second projected point is marked on the hub. A point midway between the two marked points is the true point on the prolonged line. Sometimes called double sighting; double reversion; double reversing; reversing in azimuth and altitude; wiggling in on line; working in on a line.

double corners-(USPLS) Normally the two sets of corners along a standard parallel; the standard township, section, and quarter-section corners placed at regular intervals of measurement; additionally, the closing corners established on the line at the points of intersection of the guide meridians, range, and section lines of the surveys brought in from the south.

double interpolation-Deriving a value from tabulated computations where two proportions may be required, as in an hour-angle observation of Polaris for azimuth, where an interpolation is made suitable to the time factor, another to arrive at a result conforming with the latitude of the observing station.

double meridian distance-The algebraic sum of the perpendicular distances from the two ends of any line of a traverse to the initial, or reference, meridian.

double-model stereotemplet-A templet repre

sentative of the horizontal plot of two adjacent stereoscopic models that have been adjusted to a common, though random scale. double-optical projection stereoplotter - See double-projection direct-viewing stereoplot

ter.

double-projection direct-viewing stereoplotter -A class of stereoplotters employing the principle of projecting the images of two correctly oriented overlapping aerial photographs onto a reference datum so the resultant images may be viewed directly without additional optical system support. Sometimes called double-optical projection stereoplotter.

double proportionate measurement-A method for restoring a lost corner of 4 townships or a lost interior corner of 4 sections. It is based on the principle that monuments north and south should control the latitudinal position. of a lost corner, and monuments east and west should control the longitudinal position. In this method the influence of one identified original corner is balanced by the control of a corresponding original corner upon the opposite side of a particular missing corner which is to be restored, each identified original corner being given a controlling weight inversely proportional to its distance from the lost corner.

double reversing-See double centering. double reversion-See double centering. double-rodded line-A line of differential levels wherein two sets of turning points, one high and one low, are used to give independent measures between bench marks.

double shooting-See double burn. double sighting-See double centering. double-target leveling rod-Any target rod having graduations on two opposite faces. double zenith distance-A value of twice the zenith distance of an object, obtained by observation and not by mathematical process. doubly azimuthal map projection-An azimuthal map projection having two poles.

dove prism-A prism which reverts the image but does not deviate or displace the beam. A given angular rotation of the prism about its longitudinal axis causes the image to rotate through twice the angle. Also called a rotating prism.

draconic month-See nodical month.

drafting-The art of drawing from given specifications.

drafting guide-See guide.

drainage-In mapping, all features associated with water, such as shorelines, rivers, lakes, marshes, etc.

drainage pattern-Rivers, streams and other inland water features portrayed on a specific map.

drag-1. (theodolite) A slight movement of the graduated circle of a theodolite produced by the rotation of the alidade. 2. See atmospheric drag.

drift-1. The lateral shift or displacement of an aircraft from its course, due to the action of wind or other causes. 2. Aerial photography obtained under this condition produces successive photographs whose edges are parallel but sidestepped. 3. (precession) See total drift.

drift angle-The angle, in degrees, between the heading of an aircraft and the track made good.

drift station-A term sometimes used in shoran operations to designate the ground station about which the aircraft flies during arc navigation. The second ground station is then referred to as the rate station.

dummy-1. A preliminary drawing or layout showing the position of illustrations and text as they will appear in the final reproduction. 2. A set of blank pages made up to show the size, shape, and general style of a book, booklet, or pamphlet.

dummy pendulum-A pendulum of similar construction to the working pendulums except that it is equipped with a thermometer and is fastened rigidly in the receiver so that it cannot swing during observations. The dummy pendulum is subject to the same

temperature conditions as the working pendulums, and is used in determining their temperature when in use.

dumpy level-A leveling instrument which has its telescope permanently attached to the leveling base, either rigidly or by a hinge that can be manipulated by a micrometer

screw.

duplex base-line measuring apparatus-A contact base-line measuring apparatus, composed of two disconnected bars, one brass and the other of steel, each 5 meters in length and so arranged as to indicate the accumulated difference of length of the measures from the brass and steel components.

duplicate level line-A line of spirit leveling composed of two single lines run over the same route, but in opposite directions, and using different turning points.

duplicate negative—A negative made from an original negative or from a positive. The duplicate negative may be a true reproduction of the original or a reproduction possessing greater or less contrast. With direct positive materials, chemical reversal process, and duplicating film, it is not always necessary to make a positive to obtain a duplicate negative.

dynamic correction-The quantity that must be added to the orthometric elevation of a point to obtain its dynamic number.

dynamic gravity meter-A type of gravity instrument in which the period of oscillation is a function of gravity and is the quantity directly observed.

dynamic map-A map designed to show motion, action, or change. Contrasted with static map.

dynamic number-The work required to raise a unit mass from sea level to a given point, expressed in absolute units.

dynamic temperature correction (pendulum)— The correction to the observed period of a pendulum for the rate of change of its temperature.

dynamical mean sun-A fictitious sun conceived to move eastward along the ecliptic at the average rate of the apparent sun.

dyne A force which, acting on a mass of one gram, imparts to that mass an acceleration of one centimeter per second per second. The dyne is the unit of force of the c.g.s. system of units. Until about 1930, the dyne was used by the U.S. Coast and Geodetic Survey in stating values of gravity. Since that time, gravity has been reported in terms of the gal, the c. g. s. unit of acceleration.

earth-centered ellipsoid-A reference ellipsoid whose geometric center coincides with the earth's center of gravity and whose semiminor axis coincides with the earth's rotational axis.

earth-fixed coordinate system-Any coordinate system in which the axes are stationary with respect to the earth.

earth inductor-An instrument designed for use in magnetic surveys to determine magnetic dip. In principle, the instrument is a small dynamo by which the electrical flow can be introduced to the coil of the instrument. The presence or absence of current is indicated by a galvanometer which provides for direct reading of magnetic dip.

earth inductor compass-A compass depending

for its indications upon the current generated in a coil revolving in the earth's magnetic field.

earth satellite-A body that orbits about the earth; specifically, an artificial satellite placed in orbit by man.

earth tide-A periodic movement of the earth's crust caused by tide-producing forces of the moon and sun.

easement curve-See spiral curve.

easting-A reference on a map with a military grid is usually given in terms of rectangular grid coordinates, an easting and a northing, which represent the x-coordinate value and the y-coordinate value, respectively. See also departure; false easting; false northing.

east point-See prime vertical plane.

ebb tide-The portion of the tide cycle between high water and the following low water. Also called falling tide.

eccentric-Not having the same center.

eccentric anomaly-See anomaly, definition 3.

E

eccentric error-Centering error.

eccentric reduction (triangulation)-The correction which must be applied to a direction observed by an instrument with either the instrument or signal (swing), or both, eccentric, to reduce the observed value to what it would have been if there had been no eccentricity. Also called eccentricity correction. eccentric signal-A signal (target) which is not in the same vertical line with the station which it represents.

eccentric station-A survey point over which an instrument is centered and observations made, and which is not in the same vertical line with the station which it represents and to which the observations will be reduced before being combined with observations at other stations. In general, an eccentric station is established and occupied when it is impracticable to occupy the station center, or when it becomes necessary in order to see points which are not visible from the station center.

eccentricity-1. Amount of deviation from a center. See also eccentric station. 2. (surveyor's compass) An effect caused by one or a combination of the following conditions. A straight line through the ends of the magnetic needle fails to pass through the center of rotation of the needle; the center of rotation of the needle is not coincident with the center of figure of the graduated circle; the line of sight fails to pass through the vertical axis of the instrument.

eccentricity correction (triangulation)-See eccentric reduction (triangulation).

eccentricity of alidade-The distance between the center of figure of the index points on an alidade and the center of figure of the graduated circle. See also eccentricity of instrument.

eccentricity of circle-The distance between the center of figure of a graduated circle and its center of rotation. See also eccentricity of instrument.

eccentricity of ellipse-The ratio of the distance between the center and a focus of an ellipse to the length of its semimajor axis. eccentricity of instrument-The combination of eccentricity of circle and eccentricity of alidade.

eccentricity of spheroid (ellipsoid) of revolution-The eccentricity of an ellipse forming a meridional section of the spheroid. echo-See blip.

echogram-See fathogram.

echo sounder-An echo sounding instrument used for depth measurements in water. echo sounding-A method for measuring depths indirectly by recording the time interval required for sound waves to go from a source of sound near the surface to the bottom and back again.

echo timing-The measurement of time required for a short train of energy waves to travel the round-trip path from an originating station to a reflector, or transponder. eclipse-1. The reduction in visibility or disappearance of a non-luminous body by passing into the shadow cast by another non-luminous body. 2. The apparent cutting off, wholly or partially, of the light from a luminous body by a dark body coming between it and the observer. See also annular eclipse; lunar eclipse; solar eclipse.

eclipse year-The interval between two successive conjunctions of the sun with the same node of the moon's orbit.

ecliptic-The great circle formed by the intersection of the plane of the earth's orbit around the sun (or apparent orbit of the sun around the earth) and the celestial sphere. ecliptic coordinate system-See ecliptic system of coordinates.

ecliptic latitude-See celestial latitude.

ecliptic longitude-See celestial longitude.

ecliptic meridian-A circle on the celestial sphere containing points of the same celestial longitude.

ecliptic node-See node.

ecliptic parallel-A circle on the celestial sphere containing points of the same celestial latitude.

ecliptic polar distance-The complement of the celestial latitude.

ecliptic pole-One the celestial sphere, either of the two points 90° from the ecliptic. ecliptic system of coordinates A system of curvilinear celestial coordinates which uses the ecliptic as the primary reference plane and the ecliptic meridian through the vernal equinox as the secondary plane. The points 90° from the ecliptic are the north and south ecliptic poles. Angular distance north or south of the ecliptic analogous to latitude, is celestial latitude. Celestial longitude is measured eastward along the ecliptic from the vernal equinox through 360°.

editing The process of checking a map or chart in its various stages of preparation to insure accuracy, completeness, and correct preparation from and interpretation of the sources used, and to assure legible and precise reproduction. Edits are usually referred to by a particular production phase, such as compilation edit, scribing edit, etc.

effective area-For any aerial photograph that is one of a series in a flight strip, that central part of the photograph delimited by the bisectors of overlaps with adjacent photographs. On a vertical photograph, all images within the effective area have less displacement than their conjugate images on adjacent photographs.

effective earth radius-See effective radius of the earth.

effective focal length (EFL)-See principal distance, definition 1.

effective radius of the earth-A fictitious value for the radius of the earth, used in place of the geometrical radius to correct for atmospheric refraction when the index of refraction in the atmosphere changes linearly with height.