Section 2

AERIAL PHOTOGRAPHY


 

 

 

2-01     AIRCRAFT AND CREWS

The aircraft shall be maintained and operated in accordance with the regulations of the Federal Aviation Administration and the Civil Aeronautics Board.  The overall aircraft performance shall be adequate for the satisfactory completion of all photography items and sub-items stipulated by the Proposal form and Contract and according to the guidelines and accuracies contained in these Specifications.

Crews having a minimum 400 hours experience in flying precise photographic missions for aerial surveys shall be used.  In addition, each crew shall have prior experience (50 hours minimum) with the same type of aircraft to which the crew is assigned.

2-02     AERIAL CAMERA

2-02.1  Minimum Standards

Each camera and its corresponding magazines shall have been calibrated, tested, and certified by the camera manufacturer or by a calibration center, recognized internationally or approved by the camera manufacturer within the past three (3) years.  The contracted aerial firm must provide the most recent calibration dates for its equipment for each project.  However, when there is any reason to believe that the dimensional relationship of the lens, fiducial marks, and film have been disturbed by partial disassembly or unusual mechanical shock, the camera must be submitted for recalibration at the contractor’s expense.

Any camera used on a project shall meet the following minimum standards as set forth by the USGS Calibration Certificate:

  1. Radial Distortion:  Average distortion for a given field angle is ten (10) microns or less.
  2.  

  3. Resolving Power:  Area weighted average resolution is sixty (60) cycles per millimeter or greater.
  4.  

  5. Principal Point of Autocollimation:  Lines joining pairs of fiducial (collimation) marks shall intersect at an angle of ninety degrees (90º), plus or minus thirty seconds (30”) of arc, and that intersection shall indicate the true location of the principal point of autocollimation within twenty-five (25) microns or less.
  6.  

  7. Filter Parallelism:  The two surfaces of all filters used on the camera shall be parallel to within ten seconds (10”) of arc.
  8.  

  9. Magazine Platen:  The platens of all camera magazines shall not depart from a true plane by more than thirteen (13) microns; that is, thirteen thousandths of a millimeter (0.013 MM).
  10.  

  11. Stereo model Flatness:  No test point in the stereo model shall have an average departure from flatness of more than twenty-five (25) microns at negative scale.  The stereo model flatness test results shall be provided for all camera-magazine combinations upon request.
  12.  

  13. Calibrated Focal Length:  The measurement of calibrated focal length shall be accurate to within five (5) microns.
  14.  

  15. Shutter Calibration:  Shutter efficiency shall be at least seventy-five percent (75%).  Shutter speeds shall be accurate to within ten percent (10%) of indicated value.

2-02.2  Construction and Installation

Only rigidly constructed, single lens, precision cartographic cameras exposing 230 MM x 230 MM negatives, having a nominal focal length of one hundred fifty three (153) millimeters, shall be used.  The camera shall be equipped with a between-the-lens-elements shutter and a vacuum or pressure device for holding the film flat at the instant of exposure.  The camera must produce at least four (4) fiducial (reference) marks on each negative for accurately locating the principal point of the photograph.  A total of eight (8) such marks (one in each corner and one on each side of the photographic exposure area) is preferable.

The camera shall be mounted on the aircraft so that all parts are within the outer structure and that the camera is permitted an unobstructed view.  The viewing field shall be shielded from gases, oil, and air turbulence, but no window of glass, plastic or other material shall be interposed between the camera lens and the ground to be photographed.

2-02.3  Filter

An appropriate light filter with an antivignetting metallic coating shall be used. The two surfaces of the filter shall be parallel to within ten seconds (10") of arc.  The optical characteristics of the filter shall be such that its addition and use shall not cause any unacceptable reduction in image resolution, and they shall not detrimentally alter the optical characteristics of the camera lens.

2-02.4  Fiducial Marks

A minimum of four (4) fiducial marks shall be shown, one at each corner of the format, and they shall be integral parts of the lens cone assembly.  A total of eight (8) such marks is preferable with each mark of the second quartet appearing at the midpoint of each side of the format.

All fiducial marks shall produce well-defined images in aerial negatives and on calibration plates so as to permit point-plotting on the images with a precision of twenty-five (25) microns or less.

2-03     FILM

2-03.1  Film Type and Size

Only a fine grain, high sensitivity, high intrinsic resolving power photographic emulsion on dimensionally stable safety film base shall be used.  Outdated film shall not be used.  Unexposed and exposed film shall be stored, handled and processed in accordance with the manufacturer's guidelines.  The film shall be suitable for photographic reproductions with sufficient stereoscopic overlap for use in precision photogrammetric instruments to compile planimetric and/or topographic maps and to measure profile and cross section elevations and heights by photogrammetric means.

The film shall yield an image area of two hundred thirty millimeters by two hundred thirty millimeters (230 MM x 230 MM) for each exposed negative.  The leader length and trailer length shall not be less than two meters (2 M) and one meter (1 M) respectively.

2-03.2  Exposure

Film exposure shall be in accordance with the manufacturer's guidelines.  The negatives shall be free from light streaks and static marks, and they shall have uniform tone and a degree of contrast permitting land features and ground details to show clearly in dark and light areas and especially so with respect to legibility in shadow areas.  Negatives which fail to meet the above requirements may be considered unsatisfactory and be subject to rejection.

2-03.3  Development and Processing

Each roll of film shall be processed as soon as possible after it is exposed.  Special care shall be taken to insure proper development and thorough fixing and washing in accordance with the film manufacturer's guidelines.

Film shall not be wound tightly on drums and shall not be stretched, shrunk or distorted in any way during processing or drying.  Film shall be free from finger marks, dirt or blemishes of any kind.  Such defects and flaws, which, in the opinion of the NJDOT, would interfere with the film's intended purpose, shall be cause for rejection.

2-03.4  Labeling

All exposures shall be labeled to read easily from left to right.  The labeling shall be oriented so as to be read in the direction from project beginning to project end.

All lettering and numbering shall be legible and uniform in presentation and shall be rendered in symbols and characters five (5) millimeters in height and shall be executed as follows:

  1. First and Last Exposures:  The first and last exposures shall be labeled as follows:
  2.  

  3. Intermediate Exposures:  All intermediate exposures shall be identified in the direction from project beginning to project end.  Exposures shall be labeled in the upper right-hand corner as follows:
    • Project number, flight line number, and the identifying number of the exposure itself.

The Contractor shall furnish the negative on spools in suitable containers.  Each container shall be labeled to show the corresponding municipalities and counties, the legislated route designation, photographic scale, date of exposure, and any applicable aerial number on the first and last exposure of each strip.

2-04     PHOTOGRAPHY METHODS AND GUIDELINES

2-04.1  Flight Line

The Contractor shall design the flight lines to insure full stereoscopic photographic coverage.  In general, flight lines shall be parallel to each other and to the lengthwise boundary lines of the areas to be photographed.

2-04.2  Weather and Sun Angle

Aerial photography shall be undertaken only when well-defined images can be obtained.  Photography shall not be undertaken when the ground is obscured by haze, snow, foliage, flooding conditions, or when clouds or cloud shadows would appear on more than five percent (5%) of the area of any one photograph.

Aerial photography shall not be undertaken when the sun angle is less than thirty degrees (30º) above the horizon.  Shadows caused by topographic features and sun angle shall be cause for rejection.

2-04.3  Crab

Crab shall not exceed three degrees (3º) in any negative.  Any two or more consecutive photographs displaying crab in excess of five degrees (5º) shall be rejected.

2-04.4  Tilt

Tilt shall not exceed four degrees (4º) in any negative.  Any two or more consecutive photographs displaying tilt in excess of five degrees (5º) are unacceptable.  Throughout the entire project, the average amount of tilt shall not exceed one degree (1º).  Any tilt in excess of the above criteria shall be cause for rejection.

2-04.5  Overlap

Overlap shall be sufficient to provide full stereoscopic coverage of the areas to be photographed.  Where there is a change in direction of the flight line(s), photographs taken at the beginning of the next flight line or segment of the same flight line shall give complete stereoscopic coverage of the area contiguous to the forward and back sections.

Overlap shall be provided as follows:

  1. Boundaries:  All the area appearing on the first and last negative in each flight line or flight line segment extending over a boundary shall be outside the boundary of the project area.  Each strip of photographs shall extend over the boundary not less than fifteen percent (15%) or more than fifty-five percent (55%) of the strip width.
  2.  

  3. Endlap:  Endlap shall average not less than fifty-seven percent (57%) nor more than sixty-two percent (62%). Endlap of less than fifty-five percent (55%) or more than sixty-eight percent (68%) in one or more negatives may be cause for rejection.  However, consideration shall be given if, in the case of a stereoscopic pair, endlap exceeding sixty-eight percent (68%) was found to be unavoidable in areas of low elevation in order to attain the fifty-five percent (55%) minimum endlap in adjacent areas of higher elevation.
  4.  

  5. Sidelap:  Sidelap shall average thirty percent (30%), plus or minus ten percent (10%).  Any negative having sidelap less than fifteen (15%) or greater than fifty percent (50%) may be rejected. However, consideration shall be given if the strip area to be mapped is found to be slightly wider than the area which can be covered in one flight strip.  In that case, sidelap of up to seventy percent (70%) to take advantage of control is permissible.

2-04.6  Quality of Photography

Photography shall be executed so as to minimize image movement at the moment of exposure.  Such exposure and the subsequent processing shall be such that all negatives shall be of high quality showing all specified planimetric and topographic features at the scale stipulated by the NJDOT.

Negatives which are not clear and sharp in detail and in average contrast, and which are not free from static marks, stains and other blemishes which, in the opinion of the NJDOT, would interfere with their intended purpose, shall be rejected.

2-04.7  Scale of Negatives

The flight height above the average ground elevation or set datum shall be such that the negatives will yield photographic prints on paper or on dimensionally stable polyester-type plastic or on optically flat glass plates to the scale specified by the NJDOT.  Negatives departing from the intended scale by more than five percent (5%) shall be rejected.

Unless specified otherwise by the NJDOT, the flight height shall be six times the value of the intended aerial negative scale.  Accordingly, the photography (negative) scales and flight heights, together with the corresponding contour intervals, all recommended for the mapping scales generally employed by the NJDOT, are shown in Table 2-1.

 

Table 2-1:       Photography Scale and Flight Height Guidelines

MAPPING SCALE

CONTOUR INTERVALS

PHOTO SCALE

FLIGHT HEIGHT

1:300

0.5 M

1:3 000

459* M

1:500

0.5 M

1:4 000

612* M

1:1 000

1.0 M

1:8 400

1 285* M

1:2 000

2.0 M

1:16 800

2 570* M

* For nominal focal length of 153 MM

2-05     PHOTOGRAMMETRIC GROUND CONTROL

2-05.1  Datum’s

By definition, the horizontal datum is a rectangular plane coordinate system.  Unless approved otherwise by the NJDOT, the Contractor shall reference all horizontal control to the New Jersey State Plane Coordinate System of 1983 (NJSPCS 1983).  All horizontal control shall begin and terminate on monuments that are in the National Geodetic Reference Database System (NGRDS)

The vertical datum is normal to gravity.  Unless approved otherwise by the NJDOT, the Contractor shall reference all vertical control to the North American Vertical Datum of 1988 (NAVD 88).  All vertical control shall begin and terminate on existing benchmarks that are in the National Geodetic Reference Database System (NGRDS).

2-05.2  Ground Control Points

Horizontal control points shall be set up as station points in a closed traverse whenever practicable.  If field conditions dictate otherwise, control points shall either be tied to the traverse from two different stations or have the angles and distances for single ties measured at least twice.  Each control photograph shall be examined carefully in the field to insure that the object described in the photograph is indeed the corresponding object in the field.

Vertical control points shall be set up as turning points on differential level runs.  Side shots used for photo control points are not acceptable. Trigonometric leveling is acceptable in lieu of differential leveling if field conditions so dictate and approval is received from NJDOT.  However, all distances shall be measured using electronic distance measuring devices in order to insure that the accuracies listed in Table 2-2 can be obtained.

Table 2-2:       Recommended Accuracies

MAPPING SCALE

HORIZONTAL

VERTICAL

1: 300

60 MM

15 MM

1: 500

90 MM

20 MM

1: 1 000

150 MM

30 MM

1: 2 000

300 MM

90 MM

Note:

Standard error, defined as the square root of the sum of the squares of the errors from “n” measurements divided by “n”, in position and elevation of each control point shall not exceed the recommended accuracies shown.

2-05.3  Targeting Control Points

Either control points can be pre-targeted (prior to flight), or photo-identifiable points can be selected for use upon viewing existing aerial photographs.  Unless approved otherwise by the NJDOT, the Contractor shall prepare and establish targets in the field for a permanent photographic record to be made by means of aerial photography.

Targets serve to make evident the locations of control points so that the existence and position of each point is easily and accurately discernible when its corresponding image is viewed in an aerial photograph.  Targets also pinpoint supplemental control points which enable aerial photographs to be oriented within photogrammetric instruments for use in the stereoscopic compilation of map manuscripts.  Additional targets will be provided over existing baseline and right of way monuments or control points.  This will permit orienting the maps to plan stationing and plan right of way lines.

Targets shall be placed in the median and shoulder zones of the roadway in question and on flat ground whenever practicable.  Steep slopes, sharp ridges and ditches should be avoided.  All targets shall be placed on contrasting background so as to be readily distinguishable in aerial photographs.

Each target shall be placed with its center directly over and at the exact elevation of the steel rod or other appropriate manifestation of the control point in question.  The target legs should not slope appreciably from the center.

Normally, target spacing shall be at an interval equal to one-fifth (1/5) the flight height.  However, for those projects where the required flight height is 365 M or less, targets shall be placed so that at least two (2) will appear in the overlap between adjacent photographs. Accordingly, unless approved otherwise by the NJDOT and as noted above, the guidelines for sizes and center-to-center intervals of white targets shown in Table 2-3 are recommended.  The linear dimensions of a black target should be two to three times those tabulated below to allow for image spread in the aerial negatives.

Target shape shall be in the form of either a symmetrical cross, a "T”, or a "Y" in that order of preference.  The stem of the "T" and each leg of the "Y" shall be equal in length to one half (1/2) the recommended leg length.  For Recommended Target Sizes, see Figure 2-1.  For Sample Target Plan with Full Field Control, see Figure 2-2.

Targets shall be prepared by painting or printing them on cardboard, muslin or similar cloth, or they shall be constructed of lime placed on the ground, or they shall be painted on the roadway surface.  In all cases, a cross, "T" or "Y" template shall be used as a guide.

 

Table 2-3:       Design Guidelines for White Targets

MAPPING SCALE

FLIGHT HEIGHT

MAXIMUM INTERVAL

TARGET LEG WIDTH

LEG LENGTH

1:300

459 M

110 M

0.15 M

0.6 M

1:500

612 M

192 M

0.15 M

0.9 M

1:1 000

1 285 M

384 M

0.20 M

1.5 M

1:2 000

2 570 M

768 M

0.46 M

3.0 M

2-05.4  Photo-identifiable Control Points

Upon approval by the NJDOT, photo-identifiable control points may be used in lieu of targeting control points.  The use of photo-identifiable control points may be authorized if existing aerial photographs are readily available and if the project area is urban or suburban in character where such points would exist in abundance.

Photo-identifiable control points shall be established on permanently fixed objects and shall be of sufficient clarity and definition as to provide the same quality and reliability in aerial photographs as targeted control points.  Points that are indefinite or not permanent (e.g. bushes, logs; intersections, roadway centerlines or building corners at a large scale; etc.) are not acceptable.

Photo-identifiable control points shall be maintained at the maximum intervals or less as specified for the placement of new targets with respect to their corresponding flight heights.  Such points shall be established in the central zone of the roadway in question and on flat ground whenever practicable.  In all other respects, requirements for these points shall correspond to the specifications affecting the layout of targeted control points.

FIGURE 2-1:   RECOMMENDED TARGET SIZE

FIGURE 2-2:   SAMPLE TARGET PLAN WITH FULL FIELD CONTROL