introduction to photogrammetry

PGGIS-03Photogrammetry

W.N.WilsonDepartment of Geography

1

Air Photograph

2

Camera

3

Indications on photographs:

a. Fiducial Marks for determine the Principal point

b. Altimeter for determine the Flying height

c. Watch for determine the Time

d. Level for determine the Tilt (not very accurate)

e. Principal distance for determine the Scale (from the relation ship c/z)

f. Number of photograph

g. Number of camera. Enables the Camera Calibration Report to be asked.

This can be necessary for checking paper shrinkage e.g.4

Figure II 5

Photogrammetry can be defined as the

operation in which the geometrical aspects of

the photograph , such as angels, distances,

coordinates, etc. are of major importance.

6

Photo-interpretation assumes the activity of the

observer who studies the photograph with some

preliminary knowledge. If in the mind of the

photo-interpreter would not exist any knowledge

of interpretation the photograph would be a

strange combination of gray, black and white

tones. Even the observer who wants to measure

distance needs a reference level of interpretation.

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Maps

An aerial photographs is often geometrically

compared with a map. In this section therefore

some important definitions and properties of

maps are given which are considered to be

necessary for the understanding of the

following sections.

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Projection.

Maps representing a small area of land can be

considered as orthogonal projections of the terrain.

That means each terrain point is “ brought down”

to the ground by means of a plumb-bob and the

ground represented on the map.

For larger areas of land the ground cannot be

considered as a plane, but as a sphere. This sphere

has to be represented on a plane. This is the

science of map-projection.9

Unit systems of lengths and Angels.

In terrain, in maps and in photographs the position of point is expressed by means of some coordinate-system. For this unit systems for distances and for angels are needed.

For distance we have:

Kilometer….1 Km = 1000 m

Millimeter…1 mm = 0.001m

Micron…….1 µ = 0.001 mm10

Coordinates

In order to be able to indicate a point both

on a map and on the terrain a coordinate

system is used.

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ScalesThe scale of a map is the proportion between a distance on the map and the corresponding distance on the ground.

Example: 1/ distance on the map : 34.5m

" on the field : 690m

Scale 34.5 = 1

690 000 20 000

2/ distance on the map : 1 inch

" on the field : 1 mile

Scale 0.0254 = 1 (“one inch to the mile”)

1609.3  6335812

Scale: 1 or 1

AB/ab AB/a' b'

From the diagram we read:

oh' = a' o; a' b ' = oh' = c (=c)

OH = AO AB OH Z Zm

Scale = c

Zm

Zm = flying height over mean ground level.13

The triangles hOi and NOI are similar since

iho = INO = 90o

ioh = ION

io/IO = ho/OH = c/z

The scale along the isoline is :

io/IO = c/z

From figure IV we read furthermore:

Scale at a increases continuously as angle aOh decreases;

So at n scale is larger than c/z and at h scale is smaller than c/z

14

Aerial Photographs

Projection

 Figure III shows a central projection; all

projection rays which connect the corresponding

points. A and a, B and b, etc. are passing though

one point 0, the perspective center.

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Figure III 16

That the relations between (aerial) photograph

and object (terrain) is a central projection. The

points a, b, c, and d in the negative plane

correspond with the terrain points A, B, C and

D respectively.

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Definitions and mathematical relationships

Figure IV 18

Figure IV gives a diagram of a tiled photograph.

The horizontal lines represents or horizontal flat

terrain or the horizontal reference plane,

sometimes known as the datum plane, from

which all heights can be considered to be

measured.

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The vertical line ON in figure IV though the

perspective center 0 intersects the negative

plane at the nadir point or plumb point n; N

on the terrain.

20

Unit systems of lengths and Angels.

In terrain, in maps and in photographs the position of point is expressed by means of some coordinate-system. For this unit systems for distances and for angels are needed.

For distance we have:

Kilometer….1 Km = 1000 m

Millimeter…1 mm = 0.001m

Micron…….1 µ = 0.001 mm21

Coordinates

In order to be able to indicate a point both

on a map and on the terrain a coordinate

system is used.

22

ScalesThe scale of a map is the proportion between a distance on the map and the corresponding distance on the ground.

Example: 1/ distance on the map : 34.5m

" on the field : 690m

Scale 34.5 = 1

690 000 20 000

2/ distance on the map : 1 inch

" on the field : 1 mile

Scale 0.0254 = 1 (“one inch to the mile”)

1609.3  6335823

Scale: 1 or 1

AB/ab AB/a' b'

From the diagram we read:

oh' = a' o; a' b ' = oh' = c (=c)

OH = AO AB OH Z Zm

Scale = c

Zm

Zm = flying height over mean ground level.24

The triangles hOi and NOI are similar since

iho = INO = 90o

ioh = ION

io/IO = ho/OH = c/z

The scale along the isoline is :

io/IO = c/z

From figure IV we read furthermore:

Scale at a increases continuously as angle aOh decreases;

So at n scale is larger than c/z and at h scale is smaller than c/z

25

Aerial Photographs

Projection

 Figure III shows a central projection; all

projection rays which connect the corresponding

points. A and a, B and b, etc. are passing though

one point 0, the perspective center.

26

Figure III 27

That the relations between (aerial) photograph

and object (terrain) is a central projection. The

points a, b, c, and d in the negative plane

correspond with the terrain points A, B, C and

D respectively.

28

Definitions and mathematical relationships

Figure IV 29

Figure IV gives a diagram of a tiled photograph.

The horizontal lines represents or horizontal flat

terrain or the horizontal reference plane,

sometimes known as the datum plane, from

which all heights can be considered to be

measured.

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The principle point is defined as the foot of the

perpendicular from the perspective center 0 to the

negative. Notation: h on the negative and H on

the terrain.

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The line iOJ bisects the angel between the

line perpendicular on negative plane and the

vertical line (perpendicular on terrain; I is

the iso-center and I is the ground iso-center.

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The principal distance is the distance from

perspective center to photograph. We always

indicate it with "c";"f" is the focal length of the

lens. Principal distance (a mathematical

definition) and focal length (a physical

definition) are not always equal. The line hOH

is sometimes known as the principal axis.

The angle NOH is called angel of tilt.33

The plane containing perspective center, nadir point,

principal point and iso-center is know as the principal

plane. The principal plane is perpendicular to nevigative

plane and perpendicular to reference plane.

The intersection line of principal plane and negative plane ,

' nih' is the principal line.

The line passing though the iso-center I and perpendicular

to the principal line is called the isoline .

The length ON represents the flying height. i.e. the height

of the lens above the reference plane; the flying height is

referred to as Z; ON = Z.34

Photographs will rarely be taken entirely free

from tilt. The expression “vertical photographs”

is commonly used for photographs with tilt less

than 400 .Thus the displacement of the nadir

point n and the iso-center I from the principal

point h will be very small.

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Types of photographs

The following expressions are used for different types of photographs according to the direction of the principal axis.

High oblique: horizon on photograph;

Greatest coverage;

Treapezoidal area;

The scale decreases from foreground to back ground.

Low oblique : no horizon on photograph ;

Less coverage;

Trapezoidal area;

The scale decreases from foreground to back ground. 36

Vertical photograph:

Tilt smaller than 40;

Least coverage;

Rectangular area;

The scale is uniform for one level.

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The advantage of the oblique photograph is

the large coverage and certain features of

the terrain are better illustrated. The vertical

photograph has the advantage that it is easier

to plot on the map and a vertical photograph

looks almost like a map.

38

Standard or normal angel photography.

The field of the view is of the order of 600

Examples:

Size 18 x 18 cm ,c = 21 cm

Size 14 x 14 cm ,c = 17 cm

Some names of lenses:

Aviotar (Wild)

Topar (W.Germany)

Wide angel photography.

The angel of view is of the order of 900

Examples: (23 x 23 cm)

Size 9 "x 9“ c = 6"=15 cm

Size 18 x 18 cm , c = 11.5 cm

Size 14 x 14 cm, c = 10 cm39

Some names of lenses:

Aviogon (Wild)

Lamegon (E.Germany)

Pleogon (W.Germany)

Super wide angel photography.

The angle of the view is of the order of 1200.

 Examples:

Size 23 x 23 cm ,c = 81 mm

Size 18 x 18 cm ,c = 70 mm

Some names of lenses:

Super Aviogon (Wild)

Russar (U.S.S.R.) 40

Figure IV 41

Difference between map and nearly vertical photograph.

A map is an orthogonal projection of terrain details on a horizontal plane. This projection is then reduced several thousand times.

A nearly vertical photograph is a central projection on a slightly tilted plane.

An exact vertical photograph of completely flat terrain is the same as a map apart from scale. If differences in terrain height occur, then relief displacement becomes apparent. If the photograph is tilted, although this tilt may be small, tilt displacement becomes apparent.

42

Relief displacement.

Relief displacement is the distance between

the position of a point on the photograph if

it were on the reference plane and its actual

position due to relief.

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Figure IV 44

From figure IV we read;

∆R = ∆Z = ∆ r

R Zm r

Thus: ∆r = r ∆Z

Zm

∆ r = relief displacement

∆ Z = Height difference over / under reference plane

Zm = fling height over reference plane.

 In words: relief displacement ∆ r is proportional with the distance from the nadir point r and with the ratio height difference ∆ Z over fling height Zm.

Relief displacement increases from the nadir point outwards.45

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