Download - Introduction to photogrammetry
PGGIS-03Photogrammetry
W.N.WilsonDepartment of Geography
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Air Photograph
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Camera
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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.
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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
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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.
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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.
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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.
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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.
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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.
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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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