Photographic Surveying

Photographic surveying also called photograph icing is a method of surveying in which plans or maps are prepared from photographic taken at suitable camera stations or photographic is the science of making measurement from photographs.

Divisions:

Photogrammetry may be divided into two classes.

• Terrestrial or ground photogrammetery.
• Aerial photogram try.

PRINCIPAL OF THE METHOD OF TERRISTRIAL PHOTOGRAMMETRY:

The principal is exactly similar to plan table surveying, it may be stated as “The position of the object with ref to the base line is given by the intersection of the rays drawn to it form each end of the base line”

In plane tabling most of the work is executed in the field while in this method it is done in the office. The principal is explained as follows C and D = Camera stations.

CD    = Base line of length ‘B’

CE and DE = Position of vertical plane of collimation.

and B = Observed vertical angles which the vertical plane of collimation make with base line at C and D.

M = pt to be located, which is shown as ‘m’ in both the photographs.

X1 and Y1 = Coordinates of ‘M’ on pt’C’

X2 and Y2 =  ----------------------------‘D’

F = focal length of camera lens

METHOD OF LOCATING PT ‘M’:

Pt ‘M’ may be plotted graphically or analytically.

GRAPHICAL METHOD:

• First plot the base line to the given scale
• Draw CE making an angles of ‘’ wide CD with the help of a protector.
• Similarly draw DE making an angle of B with BE.
• On CE mark the pt ’e’ at a distance equal to fine font of ‘C’. Similarly setoff a distance ‘De’ equal to

‘f’ along DE in front of D as shown.

• To these pts and ‘e’ draw lines right angles ot CE and DE respectively measure ‘em’ equal to X1and

To X2 along these perpendicular lines on the same side as on the photographs. (Her they are measured on ht left of CD and DE)

• Join ‘CM’ and ‘DM’ and produce them so as to meet at ‘M’ which gives required position of ‘M’ on

Plane

TO DETERMINE THE LEVEL OF PT ‘M’

• Measure ‘y’ which gives the height of ‘m’ above the horz hair. Rise of the ray from ‘m’ to the centre of the obj glass is equal to ‘y’ in a horz dist
• Measure the distance ‘CM’ to the scale on the plane.
• The height ‘H’ of ‘M’ above the horz. Plane of collimation at ‘C’ is given by

Knowing the reduced level (RL) of horz plane of collimation, the RL of M may be obtained as R.L of M= R.L of horz plane of collimation +H

FIELD WORK The field work of terrestrial photographic surveying consists of

• Reconnaissance
• Triangulation
• Camera work

AERIAL PHOTOGRAMETRY:

Arial photogram try induces the use of photographs taken in a systematic manner from the air they are then controlled by lend survey and measured by photogrametric techniques. Since the First World War the terrestrial photographic surveying has been replaced by aerial photographic surveying or aerial surveying for most of survey work due to development of aero plane.

ADVANTAGES:

• The survey work can be carried out with great speed.
• It can be used with great success for other purposes such as classification of land an soil, geological and archeological investigations etc.

Aerial surveying is a highly technical and specialized work and most be carried out by skilled, specially trained and experienced personal. It is mainly made by government organization i.e survey of Pakistan depth.

Aerial survey consists of 4 parts.

• FLYING
• PHOTOGRAPHY
• GROUND CONTROL
• COMPILATION OR MAPPING

EQUIPMENTS REGUIRED ARE

• AN AEROPLANE
• AN AERIAL CAMERA
• ACCESSORIES REQUIRED FOR INTERPRETATION AND PLOTTING.

AIR SURVEY CAMERA:

Principal components of an aerial survey camera are.

• Lens (most important part)
• Shutter

To control the interval of time that the light passes through the lens. The shutter of  a modern air camera is capable of speeds ranging from 1/50 sec to 1/2000 sec.

The range is commonly b/w 1/100 sec to 1/1000 sec

• Diaphragm: The diaphragm control the physical opening of the lens and therefore the amount of light passing to the lens.
• Filter:  To reduce the effect of haze and distribute light uniformly over the format and also protects the lens form flying particles in the atmosphere.
• Camera Cone: To support the lens-shutterciaphram assembly with respect to the focal plane and prevent stray light form striking the film.
• Focal plane: surface on which the film lies when exposed.
• Fiducially marks: (not shown in the figure) 4 to 8 in no define the photographic principal point.
• Camera body: To house the drive mechanism, flattens the film and advances to b/w expose.
•  Magazine: It hold the supply of exposed and unexposed film.

A levees vial (level Tube) attached to camera helps keep the optical axis of the camera nearly vertical inspire of any slight tilt of an air craft. The value of the focal length determined accurately through the calibration for each camera. The most common focal length for aerial cameras is 6//(152mm)

Format size 9 Inc x 9 inch
  (Negative)            (230 mm x 230 mm)
Aerial cameras may be

• Single lens camera
• Multiple lens camera

AIR PHOTOGRAPHS:

There are two ways of taking aerial photographs.

• Vertical
• Oblique

Verticals are taken with the axis of the camera pointing vertically downward. These photographs can produce more accurate maps as the variation n scale over the area is smaller and no area remains hidden. However the details in photograph can not be easily identified as the view offered is unfamiliar to eyes.

Oblique are produced by giving a camera a tilt up to 30 to the forward dir. Oblique photographs are further classified as

• High oblique
• Low oblique

HIGH OBLIQUE:

When the image of the horizon is inclined on the picture.

LOW OBLIQUE:

When the horizon is not seen and the camera tilted in axis of 30.

They provided information of the enemy territory without crossing the border. Features can be easily recognized as these provide views familiar to the eyes.

However some such as buildings, hills etc. the scale variation is large and therefore preparation of maps become more laborious and expensive.

With multiple lens camera one vertical and up to six oblique can be taken at one expose.

Vertical photographs are the principal mode of photgrammetry work oblique are seldom use for mapping or measure application but are advantageous in interpretive work and for reconnaissance.

TERMINOLOGY:

The terminology used in air photograph geometry is explained with reference to the given figure.

1: PERSPECTIVE CENTRE:

Rays form ptc on the ground pass through ‘o’ called as perspective centre.

2: PLUMB POINTS:

The vertical through the optical centre of the camera lens intersects at ground and picture plane at V and v respectively termed as the gourd and photograph plumb point.

3: PRINCIPAL POINTS:

The perpendicular to the picture plane through the optical centre of the lens meets the ground and picture plan at p and p resp, termed as the ground and photograph principal points.

The principal point ‘p’ in the photograph is located by the intersection of lines joining the fiducially marks.

4: ISO-CENTRE:

The bisector of the angle b/t the line joining the plumb point and the principal points interned the ground and picture plane at point ‘I” and ‘I’ rasp called as iso-centre. Point Ii an V and v are called homologous points.

5: PRINCIPAL PLANE:

The principal line through v and p when produce meets the ground plane at M. the vertical plane through these pts intersects the ground along the ground principal line through v and p. the iso-centres I and l also line on this vertical plane (or)

The vertical plane containing O, V, P and p is termed principal plane, its intersection with the negative plane giving principal line pv.

SCALE AND DISTORTION OF A VERTICAL PHOTOGRAPH:

SCALE OF A VERTICAL PHOTOGRAPH:

The scale of a photograph is the ratio b/t the dist measured on the photograph and the ground distance b/t the same two points.

Diff b/t map and photograph.

MAP: orthographic projection à scale in uniform.

PHOTOGRAPH: Prospective view à scale varies form pt to pt with variation in elevation.

The scale of the photograph is expressed as a representative fraction. (A scale having the importance that we can take it in any unit). Knowing the height of the airplanes above the datum and the focal length of the camera. The scale of the photograph can be dot. If the ground were level as shown in figure by the dotted line A’D the scale of the photograph would be.
From similar= S (scale) (1)

The drawing horz plane through A and B the scale at A and  B will Be
 (2)
 (3)
The scale of line ab, assuming an elevation AB equal and then.
The scale will become equal to (4)

This is now the scale which is app to both the pts A and B.

From eq 2 and 3 it is apparent that photo scale increases at higher elves and dec at lower elev. This concept is seen graphically in figure (2).

Ground lengths AB and CD are equal but photo distances ab and cd are not, cd being longer and at larger scale then ab due to the higher elves of CD.

Average photo scale is obtained by determining ground elevation of the area photograph. If N is the number of points considered with ground heights h1,h2------- hn then average  photo scale is given by

Scale for the whole photograph   

Where Havg = 

As the scale of the photograph depends on the height ‘H’ of the camera above ground, any variation in ‘H’ will change the scale. It is therefore essential that the aero plane flies at the constant variation.

Use of an average photo scale is frequently desirable but must be accepted with caution as an approximation.

Scale of a photograph can be determined if a map is available of the same area. This method doesn’t require the focal length and flying height to be known, it is necessary only to measure on a photograph a dist b.t two well defined pts identifiable on map. The photo scale is then calculated using the following relationship.

PHOTO SCALE =

Scale at average elevation of the two points considered.

NOTE:

The numerator and denominator must be expressed in the same units.

DISTORTION DUE TO HEIGHT (OR RELIEF) ON THE VERTICAL PHOTOGRAPH:

Consider the sight of a high building BC in the figure and it consequent image bc on the negative. B is vertically above C and in plane the two coincides but on photograph the sight of the building cb would by observed as well as the roof of this building would appear to be leaning outwards from the centre of the photograph.

Relief distortion on a vertical photograph occurs along radial line form the principal point an increases in magnitude with greater distance to the image.

From similar triangles EBO and obv

 (1)

Similarly, from vco and VCD

 (2)

Dividing eq 1 by 2

Thus distortion due to height Bc
(A)
(b)
The expressions A and B hold good for a truly vertical photograph.

STEREOSCOPY:

MONECULAR VISION:

Monocular visions is seeing with one eye.

BINOCULAR VISION: Binocular vision is seeing the same object with both eyes is at one time.

STEROSCOPIC VIEWING:

It is defined as observing the object in SD, a process requiring a person to have normal binocular vision.

A person with vision in only one eye can not see stereoscopically.

if figure two eyes are separated y a distance ‘b’ called eye base or intraocular distance (65mm or 2.6//).

Combined image of a and as seen left and eyes appear to fuse at ‘M’ while images of b and b1at n. the distance MN is the stereoscopically depth and is the measure of the height of A over B.

and  are the paralytic angles. Stereoscopic depth depends upon the diff between  and i.e, on the sum of angles and made at the left and right eyes respectively.
If two photographs of the same obj are taken form two diff perspective or camera stations, the left print (photo) seen with the left eye and the right print seen with the right eyes simultaneously, a mental impression of the three dimensional model results.

                                                                                                         
STEREOSCOPE:

Any device which facilitates stereoscopic observation is a stereoscope. There are two main types of stereoscope.

• Lens or prism stereoscope
• Mirror stereoscope

1 LENS OR PRISM STEREOSCOPE:

In it two lenses or narrow angled prisms are used

In mirror stereoscope mirrors are used. It allows greater area of the photograph to be covered.

FLIGHT PLANNING:

In order to obtain stereo pair every part of the ground to be surveyed must be photographed at least twice. To achieve this aero plane flies in strips and takes photographs with a 60% fore and aft overlap. To secure the 50%min needed for coverage overlaps the abjection strip by 30% to make sure that no part of the gourd is left unrecorded often called side overlap. This fore and aft overlap insures stereoscopic coverage along the strip with some coverage along the strip with some margin for error; two adjacent photographs taken in this way are called the striper. The flying height depends on the following factors.

• The scale of the map or plan.
• The type of country flat or mountainous.
• The contour interval in the map
• The characteristics of the camera i.e focal light.
• The type of aero plane available

Information commonly calculated in flight planning includes.

• Flying height above mean sea level
• Dist b/t exposure
• No of photograph per flight line
• Dist b/t flight lines
• No of flight lines
• Tot no of photographs.

A flight plane is prepared based on these items.

PHOTOGRAPHS REQUIRED:

The tot no of photograph req to cover the area to be surveyed may be determined as follows.

Let

Lp = length of photograph in an or inches in the dir of flight.
Wp = width of photograph in cm or inches # at right angles to dir of flight.
Ol    = percentage of longitudinal over lap
Ow =                              side                   
Lg = net ground dist corresponding lp in ‘m’ or ‘miles’
Wg =  -------------------------------------------------------
Wp = in ‘m’ or ‘cm’or ‘miles’
S = scale of photograph (1cm = 5cm)
N = no of photograph required
Ap = Net are of each photograph in sq’m’or sq two
Ag = Area of ground to be served -------------------
Lg = S.Lp ( 1-O2)
Wg = S.Wp (1-Ow)

Net ground area covered by each photograph = Ap = Lg x Wg
No of photograph required = N =
Theoretical no of strips =

Actual no of strips = K + 1 “ one stripe cover is added to cover the sides”
Theoretical no of photograph / strip = =M

Actual no of photograph / strip = M + 1

Actual no of photograph for compel coverage of area =( K + 1) ( M + 1)

GROUND CONTROAL:

IN order to produce an accurate map from aerial photograph it is absolutely necessary to established ground control. It consists in locating the positions of a no of pts. All over the area to be surveyed det their levels. These control pts short be such that can be easily identified on the photographs.

Hor control is established by tiring or traversing. Vertical control is established through the use of ‘aneroid barometers’ or ‘Altimeters’

APPLICATIONS OF AIR PHOTOGRAPHy:

The practical uses of air photography are unlimited. Some of the application are listed below.

1. Town and country planning and developed estate man agent and economic planning are used both maps based on air survey and individual photography’s.
2. suitability of roads and rail alignments  can be studied both for traffic flow an economy of construction.
3. Forestry and geology both use air maps and photography for the study of nature of areas and changes that take place.
4. Flood control planning can be based on air survey made at suitable intervals of time.
5. Air survey provides means of mapping large undeveloped areas of the world.
6. For large scale engineering and redevelopment projects, reconnaissance can be undertake in to a large extend form air photograph.
7. Survey for accessing damage due to earth quake, crop dieses can be quickly estimated from air photograph.
8. Pollution effects form industrial wastes on land and water can be studied.