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Lecture 20 Content

Flight preparation– Equipment– Weather conditions– Flying height– Coverage

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Flight preparation– a single flight of an aircraft includes a

take off to touch down

– To secure satisfactory air cover, the following must be considered:

Proper equipment must be used

Weather conditions, including height and sun intensity must be favorable

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The flying height is so chosen that, with the given lens, contact prints of a suitable mean photo scale are produced

The accuracy for contouring with certain instruments in the United States is often defined by:

C-factor = Flying Height (H) / Contour Interval (CI)

Used for vertical wide-angle photography with nominal focal length of 150mm

Scale of photograph = f / (H-h)

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The area of interest must be completely and systematically covered so that every bit of ground appears on at least two photographs

In addition, the optical axis must be nearly vertical at the time of exposure

Considerations in the flight planning are covered in the following slides:

– Equipment– Weather conditions– Flying height– Coverage

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– EquipmentThe aircraft:

– The aircraft must be able to carry the aerial camera

– The aircraft must be capable of flying at the height required (high or low)

The camera:– Mounted on the gimbal to keep the camera

stationary, ensuring that the optical axis passes through the center of the lens

– Free to rotate about the optical axis to facilitate different camera orientation

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– Weather conditionsThe following affect the stability of the aircraft and should be avoided:

– Dense, whitish, turbulent clouds– Cumulus clouds are turbulence– Air pockets, in which there is a sudden downward

current of air

Photographs cannot be normally taken through cloud or fog, smoke, dust or haze.High sun intensity implies that the shadows of the photograph become too dense, and obscure detail Rainy conditions must be avoided

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Shadows obscure details, so that the sun should be fairly high in the sky to give a reasonably short shadow

Necessary for aircraft, camera and crew to stand by for many days before the desired weather conditions is obtained

Cost of flying is very high

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– Flying heightThe mean average scale of the air cover should be approximately the same as the desired scale of the compilation map

Given focal length of the camera lens, and a given compilations scale, the necessary height of the aircraft can be calculated using:

Scale of photograph = f / (H-h)

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– An example:The photo scale 1/10,000 is required, focal length is 150mm and h is 100m above mean sea level

H

f

S

A B

b a

h

Horizontal planeRepresenting mean ground level

Mean sea level

Scale of photograph = f / (H-h)

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Applying the formula1/10,000 = 0.150 / (H – 100) m

i.e.

H – 100 = 1500m

H = 1600m

Therefore the flying height must be 1600m

Given the same focal length and the same size of format, the higher the aircraft flies the greater the area of ground covered, and the less the number of photographs required to cover any particular area of land

Typical aerial photo format is 9inx9in (230mmx230mm)

Therefore the greater the flying height, the more economical the coverage

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Alternatively, the less the flying height, the greater the scale, the more detail that can be readily discerned and the greater the plotting accuracy

Greater the flying height increases the thickness of haze and dust that light rays must penetrate, with consequent loss in the photograph quality

Normal flying heights maybe between 1000m to 4500m above ground level, therefore we expect flying cost to increase in mountainous areas

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– CoverageSystematic coverage is obtained by flying the aircraft at a fixed height, in a series of straight lines, and taking photographs at regular intervals

The photographs taken during the time that the aircraft flies along any one of these straight lines are known collectively as a “run”

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Each photograph in a run should overlap its predecessor by 60%

60%overlap double

overlap

1st 2nd 3rd1st 2nd

Aircraft Aircraft

Double overlap usually is 20%of each of the three photographs

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Having flown one complete run the aircraft must turn around and fly back another straight line, again taking photographs at regular intervalsThe second strip of photographs must also overlap the first run, this is known as the “lateral overlap”Lateral overlap is usually between 20 to 30%Runs will normally flown in a direction parallel with the longest dimension of the land to be covered

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{{{

{

1st run

2nd run

3rd run

4th run

Plan showing later overlaps

Lateraloverlap

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Overlap between Runs or SwathsOverlap between Runs or Swaths

30% overlap between runs or swaths

Run #1

Run #2

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Exposure for 1st strip

Exposure for 2nd strip

Exposure for 3rd strip

Exposure for 4th strip

A typical flight plan

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Other factors to be considered in planning aerial photography include: The intended use of photography (quantitative or interpretive) The desired product (Map, orthophoto, mosiac, etc) The specified accuracy The size and shape of the interested area The amount and disposition of relief The scale of photography (influences the final scale of the end

product, camera to be used, and so on)

Photogrammetry …. Lecture 14Photogrammetry …. Lecture 14Dr. Steve RamroopDr. Steve Ramroop04/19/23

GEM 336620

… The End …