5.4　Aerial Photographing

5.4.1　Aerial photographing shall be conducted at the preliminary design stage after the routing scheme is determined.

5.4.2　Aerial photographing for line routes should be carried out in single-strip mode.When alternative schemes are available，aerial photographing can be carried out respectively for the main scheme and alternative schemes in single-strip mode or can be carried out for all these schemes in block mode.Where it is difficult to select the route scheme and for areas where substation or converter station lines are intensive，the block mode may be adopted.

5.4.3　A qualified professional aerial photographing organization shall be delegated to carry out aerial photographing.

5.4.4　The central part of objective lens of aerial camera shall have a resolution of no less than 100 line pairs/mm.

5.4.5　The strip width of flight line photographing shall not be less than 2km.

5.4.6　A technical plan shall be developed for aerial photographing，which should include the following:

1　Selecting the plane for aerial photographing.

2　Developing the arrangement plan of flight lines，containing the photographic scope and the latitude and longitude of starting & ending points of each flight segment.

3　Determining the required flying hours and the number of photographic films required when an optical camera is used.

4　Junction map of flight strips.

5　Determining the model，principal distance and picture format of the aerial camera.

6　Determining the scale and type of photographing.

7　Determining requirements on flying quality and photographic quality.

8　Determining the season and time for aerial photographing.

9　Exposure time and photographing interval，and the selected photographic materials and optical filters when an optical camera is used.

10　Specifying the designations and copies of all documents to be transferred.

5.4.7　The division of flight segments shall comply with the following provisions:

1　On a 1∶50，000 topographic map，a flight line shall be divided by turning angle，and the starting and ending point of each segment shall be determined.

2　Within a segment，the actual distance from each turning angle to the edge of flight strip shall be more than 400m.The real distance of flight endpoints to the nearest turning angle shall be more than 1000m.

3　If topographical elevation within the survey area varies greatly，the area shall be divided into several sub-areas for photographing.The topographical elevation difference within a sub-area shall not be larger than 1/4 of the relative flying height.

5.4.8　For an aerial camera with the picture format of 23cm×23cm，the lens model，principal distance and applicable topography categories shall conform to provisions in Table 5.4.8.

5.4.9　When a digital aerial camera is used for aerial photographing，the ground resolution shall not be less than 0.3m.When an optical aerial camera is used，the scale of aerial photographing shall be selected in accordance with provisions in Table 5.4.9.

5.4.10　The flying quality of aerial photographing shall comply with the following regulations:

1　The aerial photographs shall have a tilt not larger than 2°，and in rare cases，should not exceed the maximum value of 4°.

2　The aerial photographs shall have a swing angle not larger than 10°.Within a flight segment，the number of consecutive photographs with the swing angle at or close to the upper limit shall be no more than three.

3　The longitudinal overlap of aerial photographs shall be in the range of 60%-65%，and in rare cases，shall not be larger than 75%，and it shall not be less than 56% in any case.

4　The lateral overlap shall be in the range of 30%-35%，and in rare cases，shall not be less than 15%.Absolute aerial gap shall not occur in the photographic area.

5　The curvature of flight lines shall not be greater than 3%.

6　After the aerial photographing is completed，the center line of the route shall be more than 4cm away from the photograph edges.

7　The flying height difference for adjacent photographs on the same flight line shall be less than 20m，and the difference between maximum flying height and minimum flying height on the same flight line shall be less than 30m.Within a photographing sub-area，the difference between the actual flying height and the design flying height shall be less than 50m，and when the flying height is more than 1000m，the difference shall be less than 5% of the design flying height.

5.4.11　The photographic quality of aerial photographing shall meet the following requirements:

1　When visually inspecting the negatives，the images shall be clear，showing normal hue，rich gradation and appropriate image contrast，shall be free of any defects such as obvious shadow，shadow of clouds，facula，scratch，electrostatic scar，fracture，unbonding and sand grains，and the images of tiny surface features adaptable to the photographic scale are discernable.

2　The images of optical fiducial marks shall be distinct and complete，with a density corresponding to that of images of most bright surface features above the ground within the picture format.

3　The uneven deformation of aerial photographing negatives shall not be more than 3/10，000.

4　The error of negatives after being flattened shall be measured and checked with a stereocomparator.

During check，the personnel shall determine the coordinates and parallaxes of the standard configuration point and at least nine check points.When analyzing and calculating the relative orientation using six points，the vertical residual parallax of check points in the output shall not be greater than 0.02mm.

5　The fogging density of a negative shall not exceed 0.2.The maximum density of surface features should be 1.4 if they have smaller brightness and 1.8 if they have larger brightness.The density difference of surface features shall not be greater than 1.4 or less than 0.6.

5.4.12　The photographic processing quality of aerial photographing duplicates，such as transparent positives and photographs，shall satisfy the following requirements:

1　The model of photosensitive material and medicament formula shall be properly selected according to the contrast of aerial negatives.The temperature of developing solution should range from 18℃to 22℃，and the temperature difference between fixing solution and developing solution should not exceed 5℃.

2　During exposure and printing，the mechanical direction of the exposure materials shall be perpendicular to that of negatives.For magnified pictures used for purpose of annotation，the images between adjacent ones shall be properly overlapped.

3　Sufficient time shall be reserved to allow fixing and bathing and care shall be taken to prevent membrane from softening and images from shifting.The time for bathing with current water should be 30 minutes.

4　After printed，the complete picture shall have uniform photoreception and rich gradation.The images shall have a fogging density of less than 0.2 and a permissible deviation of contrast of 0.9±0.3.The asperity of the membrane surface shall be less than 0.02mm.The images of the fiducial marks shall be clear and complete.

5.4.13　The inspection and acceptance of aerial photographic materials shall be performed by means of combining visual inspection，data measurement and sample photograph comparison.

5.4.14　The requirements on flying quality，photographic quality and processing quality of photographic materials shall conform to provisions from 5.4.9 to 5.4.12.

5.4.15　After the aerial photography data is inspected and accepted，the following should be submitted:

1　All negatives or digital images and a registration form of aerial photographic negatives；

2　Three sets of photographs；

3　One negative and photo of index to photographs；

4　Technical specifications and assessment of aerial camera；

5　Aerial photography result assessment list；

6　A hand-over list of negatives，photos and index to photographs；

7　The technical and quality inspection reports of aerial photographing.