IMAGE DISPLACEMENTS
(PARALLAX)
On some photographs, the images of objects do not appear uprightly vertical as are in real life. They are observed to appear displaced by occupying apparently linear position. This in the study of photographs is knows as image displacement.
It is thus; image displacement is defined as the photographic setting of objects on the photo to occupy apparently linear position.
The scientific term used to describe and explain image displacement is known as parallax. Hence; parallax is defined as the apparent shift in position of an object on the photo with respect to reference point. By reference point refers to the position of an object on the photo.
Causes of image displacements
(a) The change of viewing position
If a photograph is taken by the tilted photographic view, the images of objects on the photo are made to have both top and side views and thus; appear displaced. With respect to this; oblique photographs are the ones which have highest degree of image displacement compared to horizontal and vertical photographs.
(b) Height of the viewed objects.
This makes variation in the degree of displacement of objects on a photograph. The higher objects are more displaced that the smaller objects on the photo. For instance; the high buildings on oblique photographs appear more displaced compared to the smaller buildings.
(c) Position of the object on the photo.
Usually objects at a nadir (principle point) are less displaced compared to those that are away from the Nadir (principle point) this is observed on vertical photographs, the objects on the fore ground have low degree of displacement compared to those on the back ground. But this factor is mostly considered to objects of the same height.
Note
Image displacement is one among the sources for image displacement on photographs.
SCALE DISTORTION
On photograph scale is changed by getting progressively smaller from one section to other sections. This in the study of photographs is referred as photo scale distortion.
It is thus; photo scale distortion defined as the variation in relation ship (ration) of distance between the photograph and ground by getting progressively smaller from the principle point outwards on vertical photographs or from the fore ground to backwards on horizontal and oblique photographs.
However; the degree of photo scale distortion is much higher on oblique and horizontal photographs than on vertical photographs.
CAUSES OF SCALE DISTORTION
Scale distortion is made by the following causal factors.
(i) Non uniformity coverage of the ground.
The section on photograph that covers small area is subjected to large scale, while that covers wider area is subjected to small scale.
(ii) Distortion in size of the objects.
Objects that appear apparently smaller make small scale compared to those which appear apparently larger.
(iii) Image displacement.
More displaced objects make small scale compared to objects which are less displaced.
PARTS OF PHOTOGRAPHS
Photographs are used to make description and analysis of the geographical features observed with reflection to be actual areas represented. The features described should give their position location on the photographs.
The location position of the features is by pointing out parts of the photographs where the features are found. It is thus important to understand the major parts of photographs in such a way that the features can be described and analyzed clearly.
Parts of photographs are recognized by dividing the photographs horizontally and vertically into three dimensions
Horizontal division makes a photograph to have three equal parts of; fore ground, middle, ground and back ground.
| Back ground |
| Middle ground |
| Fore ground |
Vertical divisions makes a photograph to have three equal parts of; left, centre and right.
| Left | Centre | Right |
If both horizontal and vertical divisions combined, a photograph is made to have nine equal parts. The main parts include the following:-
(a) Fore ground:
This develops; left fore ground, centre fore ground, and right fore ground.
(b)Middle ground:-
This develops; left back ground, centre ground and right ground.
(c)Back ground:–
It develops; left back ground , centre back ground and right ground.
| Left back ground | Centre back ground | Right back ground |
| Left middle ground
|
Centre middle ground | Right middle ground |
| Left foreground
|
Left back ground | Right fore ground |
Note:
The nine parts of the photographs as already pre described have the following vital significance.
·They are used for position location of the features on the photograph in the course of making description.
· They are considered for diagrams drawing from the photograph. i.e. they are used to place features on the diagram at right positions.
VERTICAL PHOTOGRAPHS SCALES
Photograph is much smaller in size, while objects represented are much large in size. It is thus; photographs being much smaller subjected to have considerable relation of distances to the much large objects represented. This is what referred to photo scale.
It is thus; photo scale is defined as the relation ship of distance between the photograph and their respective object represented.
The scale of a photograph is not much up on the decision of the photographer. It is thus; once a photograph produced, its scale should be assessed to allow further assessment of the geographical features.
In this aspect; it is possible to asses’s scales for photo graphs which make uniformity coverage of ground. Hence it is difficult to assess specific scale for horizontal and oblique photographs whose specific scales can be assessed.
Photo scale extremely varied in size and the most common types are large and small photo scales depending on the size of the ground covered. Coverage of smaller area make large scale, while coverage of wider area make small scale.
The scale of the vertical photograph relatively to the size of the ground covered is the function of the following determinant factors:-
·Focal length of the camera used.
· Flying height of the air craft above the ground.
PHOTO SCALE EXPRESSION
The assessed scale can be expressed in one of the following techniques.
I. In ration form. 1:50,000
II. In unit equivalent. e.g. 1cm on the photo = 0.5 km on the ground.
III. In statement; e.g. 1cm on the photo represents 0.5 km on the ground.
PHOTO SCALE DETERMINATION
Scales of the vertical aerial photographs are determined by two methods and they include the following:-
1. By relating the corresponding distances between the photograph and ground. The distances should be measured and the application is as follows:-
| Photo scale = Photo distance Ground distance |
Ground distances obtained in the one of the following two ways:-
· By making actual measurement at the photographed area.
· The use of scaled topographical map representing the same photographed area.
Example:-
Ground distance from area A to B is represented by 5cms on the photo. The same ground distance is represented by 10 cm on a topographical map drawn on the scale of 1:50,000. Determine the photo scale.
Solution.
The photo scale is determined as follow:-
| Photo scale = Photo distance Ground distance |
According to the given data:-
· Photo distance = 5 cm
· Map scale = 1:50,000
· CD on the map = 10 cm.
Thus; the photo scale; 1:100,000
2. By relating the focal length and flying height above the ground. The technique (method) is applied as follows
While:-
· F = focal length of the used camera.
·Hg = flying height of the air craft above the ground.
Note:-
Flying height of the air craft above the ground does not on the photography. What directly appears on the photograph is the flying height of the air craft above the sea level. It is thus; the flying height above the ground is obtained by subtracting the ground height from the flying height of air craft above the sea level.
| Hg = H – g |
Where by:-
· Hg = The height of the aircraft above the ground.
·H = Height of the air craft above the sea level.
·g = Ground height above the sea level.
Example:-
Determine the scale of the vertical aerial photograph taken with the camera of 6” if the flying height are 7,000m and ground height 4,000m respectively.
Solution:-
Scale for vertical aerial photography up on the given data is determined as follows:-
| Photo scale = f Hg |
While; Hg = H – g
According to the given data:-
f = 6’
H = 7,000m
g = 4,000m
6”
7,000m – 4,000m
15 cm
3,00m
15
300000
1
20,000
Thus; the photo scale; 1: 20,000
Note:-
1.The focal length of the used camera is determined as follow.
| Focal length = photo scale x Hg |
2.The height of the air craft above the ground is determined by the following application.
3.The height of the aircraft above the sea level is determined as follow.
| H= Hg + G |
Where by:-
·H = Height of the air craft above the sea level
·Hg = Height of the air craft above the ground.
·G = Ground height above the sea level.
Exercise.
An air craft flying at the same level over the terrain ground took two photos over the coastal plain that was at the sea level and mountain peak respectively, using the camera of the same focal length.
The first photo scale was 1; 37,000 taken at 4500m a.s.l. and the second photo scale was 1:20,000
(a) Calculate the camera focal length.
(b) Determine the altitude of the mountain peak.
Calculate the height of an area shot by a camera whose focal length is 132mm mounted on an air craft which is flying at 10500m above the sea level. The suggested scale is 1:25000.
A vertical photograph with suggested scale of 1:20,000 has been taken by photographic view set from air craft at altitude of 5000m. The used focal length was 6” (15cm).
(a) Determine the imperial altitude of the photographed area.
(b) What metric scale the photograph may have if the photographic view set at altitude of 400m.
Calculate the flying height of an aircraft for the vertical aerial photograph at the scale of 1:20,000 whose mean ground level is 500m above the mean sea level with the focal length of
i. 210mm
ii. 152mm
