Principles of Picture taking · Lecture 4 ... convergent for point measurements, flight planning)...

Lecture 4 - 1 - 10/23/2003 ____________________________________________________________________________________

Concept Hell/Pfeiffer February 2003 ___________________________________________________________________________________

4. Principles of Picture taking 4 hours Aim: principles of picture taking (normal case, convergent for point measurements, flight planning) flight planning (parameter, photo scale, cameras, instruments for outer orientation) ___________________________________________________________________________________ 4.1. Planning of terrestrial photogrammetry

Depending on the position of the camera we could divide photogrammetry to ground, aerial and

space. The ground photogrammetry is divided to terrestrial and close range photogrammetry.

The case of the close range photogrammetry is when the distance to the registered object is less

then 300m (in some sources no more then 100m). Close range photogrammetry includes three

main areas of application: architectural (and archeological), civil engineering photogrammetry,

industrial photogrammetry, medical photogrammetry (biomedical or bioengineering). Some

special parts of architectural photogrammetry could be separated as civil engineering

photogrammetry, when object of measurement are special engineering objects and constructions,

that are not buildings like bridges, roads, airports, tunnels, hydro technical objects, Other special

purposes of close rage photogrammetry are criminology, road accidents, micro photogrammetry.

Terrestrial photogrammetry is generally associated with objects distances in excess of 300m.

Another distinction between two categories is the fact that in terrestrial photogrammetry cameras

are focused to infinity while it is not the common case in close range photogrammetry. This type

of terrestrial photogrammetry is called also topographic terrestrial photogrametry (some times

phototopography). This is a science of surveying in which the detail is plotted entirely from

photographs taken at suitable ground stations. The photographs in terrestrial photogrammetry are

usually used for topographic purposes or for terrain modeling. They have large focal distances

and could be used for great distances to the objects. Cameras used for producing terrestrial

photographs are called terrestrial cameras or phototheodolites. Phototheodolites are combination

of theodolite and photogrammetric measuring camera in which the relationship between the

camera axes and the line of collimation of the theodolite can be measured. The single cameras

used in close range photogrammetry could also be used in terrestrial photogrammetry (when they

are usually focused to infinity).

For close range photogrammery is usually possible to determine more accurate the position and

orientation of photogrammetric cameras.

____________________________________________________________________________________ FH-KA - Master course Photogrammetry 2003 B. Marinov

Lecture 4 - 2 - 10/23/2003 ____________________________________________________________________________________

Depending on the relative orientation of main camera axes we have stereo couple with parallel

axes, with convergent axes (most often oriented to the center of the object) and with divergent

axes. This last case is usually applied when we have camera position inside of the object.

For stereo pairs, taken with parallel axes we have normal case, when axes are perpendicular to the

base, left or right tilted parallel photogrammetry.

Depending on the slope of camera axes we have cases of horizontal photos and sloped photos

(raised or dropped).

The most common use of close range photogrammetry is normal case. There are cameras

designed for taking only the normal stereo pairs (stereometric cameras).

b b

A

a ax

X

Y

O O

2

2

1

1

P

c

1

c

P2

B

Y0

x21 Figure 4.1. Normal case

The coordinate relations in normal case are very simple


Lecture 4 - 3 - 10/23/2003 ____________________________________________________________________________________

1,

,

1,

.

AA a

Ax a

AA a

YX xc

B cYpYZ zc

=

=

=

(4.1)

where parallax , 1, 2,x A ap x x= − a

The derivation of error in distance direction Yσ as a function of base-length is shown in the

Appendix 1.

The length of base B is selected from the relation

max min

20 5Y B< <

Y (4.2)

For the tilted parallel case the camera axes are parallel each other but they are tilted at the angle

Φ. The geometry of this case can be related to the normal case with different value of base b.

Very often when it is necessary to produce a model of whole object the surrounding position of

capturing cameras is applied. In this case the adjacent cameras are with highly convergent axes

and stereo measurement is not possible. In this case the arrangement of cameras is made in such

way that it is a cameras in front of the main facades of the object. Similar situation is shown on

the figure below.


Lecture 4 - 4 - 10/23/2003 ____________________________________________________________________________________

b

A

ax

X

Y

O

1

1

P

c

1

B

Y0

b

a

O

2

2

c

P2

F

F

F

x2

1

Figure 4.3. Geometry of parallel tilted case

The right station is displaced in Y direction on the value By

.cos .sinx yB B B B= Φ = Φ (4.3)

The geometry of tilted case is related to normal by substitution

2( .cos .sin )Bcb c x= Φ − Φ (4.4)

Substituting in co-ordinate equations we obtain

2

21

21

( .cos .sin )

( .cos .sin )

( .cos .sin )

Bc

A

Bc

A

Bc

A

c xbY cp p

c xX

pc x

c

x

Z zp

Φ − Φ=

Φ − Φ=

Φ − Φ=

(4.5)

For the convergent case of taken photos the camera axes are deflected in the opposite sides in the

way that they converge. This situation is shown on figure 4.3.


Lecture 4 - 5 - 10/23/2003 ____________________________________________________________________________________

b b

A

a ax

X

Y

O O

2

2

1

1

P

c

1

cP2

B

Y0

x2

1

F1 F2 Yav

Figure 4.3. Convergent case

Some relations between average distance and convergent angle are given in the Appendix 2.


Lecture 4 - 6 - 10/23/2003 ____________________________________________________________________________________

Figure 4.4. Cameras surrounding the object

In close range photogrammetry is not possible to define a scale that is equal for the whole object.

The scale is depending on the distance to the corresponding part of the object.

The expression for scale at fixed distance d is defined as follows

1'b

b

d Xmc x M

= = = (4.6)


Lecture 4 - 7 - 10/23/2003 ____________________________________________________________________________________

d

c

PP 1

M = Mb1 b2

Figure 4.5. Cameras with different angle of view

Cameras with different frame format (different angle of view) but with equal camera constant

produce images whit the same scale from the equal distances.

For cameras with equal camera constants but different distances to the object the scales differ.

P

P2

1

M > Mb1 b2

c

c

d

d

2

1

Figure 4.6. Different distance to the object

In this case the scales for two distances are as follows

1 21 2 1 2 1 2 1 d dm m d d m m M

c c= = → < → < → 2M> (4.7)


Lecture 4 - 8 - 10/23/2003 ____________________________________________________________________________________

Cameras with different camera constants produce images with different scale from the same

distance.

P

P2

1

M > Mb2 b1

cc

d d

2

1

Figure 4.7. Cameras with different camera constants

1 2 1 2 1 2 11 2

d dm m c c m m Mc c

= = → < → > → 2M< (4.8)

In case of usage the cameras with different camera constants it is possible to obtain the same

scale at the desired object plane if the appropriate distance to the object is set.


Lecture 4 - 9 - 10/23/2003 ____________________________________________________________________________________

P

P2

1

M = Mb1 b2

c

c

d

d

2

1

2

1

Figure 4.8. Cameras with different constants may ensure equal scales

The condition for cameras with different camera constants to produce equal scales is the equality

of ratios d/c.

1 2 1 2 11 2 1 2

1 2 1 2 2 d d d d dm m m m

c c c c d= = → = → = → 2

1

cc

= (4.9)

The conditions for taking photos of the object could be select based on the above relations.

For every photogrammetric project the appropriate disposition of the cameras must be evaluated.

The disposition depends on the goals of the project and the required accuracy. For topographic

photogrammetry the full cover of the area under consideration is necessary. The camera axes are

Usually oriented through the structural lines of the terrain to ensure maximum visibility from

every station position. The vertical slope of the camera depends on the terrain height respectively

to the camera position. The camera positions are usually selected based on the planimettric

coverage of the area of mapping. The check for visibility is made after that.

For close range photogrammetry the camera positions are selected taking into account different

considerations. The main goal is to ensure coverage of the whole object with minimal number of

stations. From the other side it is necessary to ensure enough number of tie points between

photographs. They must be well visible in all the overlapping photos. Sometimes bad orientation

of some of the points gives as the final result pure estimation for point accuracy.


Lecture 4 - 10 - 10/23/2003 ____________________________________________________________________________________

Figure 4.9. Close range photogrammetry project

Another problem is the selection of control points. Control points for close range

photogrammetry may be natural elements on objects – vertexes of the edges, small elements,

elements on the drawings or ornaments on the object. Some time it is possible to fixed signalized

control points at the suitable positions. Depending on the object this is always possible. After

signalizing the object the problem is exact positioning and orientation of cameras. For positioning

the cameras are used traditional survey methods. For orientation are usually used the supplied


Lecture 4 - 11 - 10/23/2003 ____________________________________________________________________________________ instrument for angle measurements. For orientation of photogrammetric cameras is typical that

usually the angles of theodolite axes are known relatively to the camera axes. This angle is fixed

and after that camera is oriented by targeting to specified target on the object or on the tripod. For

vertical orientation are usually used bubble levels to ensure base vertical position of camera axis

of rotation during exposure. For near facades are supplied special adapters that allow to incline

camera vertically (upwards or downwards) at the fixed angles ( more often ±30 and ±70gon).

When for taking photos are used non-metric cameras without adapters for precise leveling and

orientation the more sophisticated procedures for processing are applied. In this case only the

observation of control point in the area of viewfinder is possible.

4.2. Planning and executing the Aerial Photogrammetric Project

4.2.1. Types of Aerial Photography Types of aerial photos are classified depending on the orientation of camera axes

Vertical case. The camera axes are vertical or near vertical. This is main often used case.

Advantages are:

- easy measurements;

- normal shape of images and easily recognition;

- less hidden ground objects.

Oblique case. The camera axes have greater angles relatively to horizon.

The oblique photography can be low-oblique or high-oblique.

Advantages are:

-coverage of large areas;

- increasing base-height ratio.

This photography is widely used for military purposes for reconnaissance.

Special case is horizontal photography. It is usually used for orientation of camera systems.

Non frame cameras usage. There are two main cases:

- panoramic cameras – slit is moving perpendicularly to the direction of light.

Special case of panoramic cameras is horizon-to-horizon camera.

- slit cameras – the slit movement is result of aircraft flight. The film is moving synchronous

with the projection of the ground at the focal distance.


Lecture 4 - 12 - 10/23/2003 ____________________________________________________________________________________ 4.2.2. General requirements Aerial photogrammetry is widely used for purposes of mapping. The basic operations in

conducting a photogrammetric mapping are:

-Photography: obtaining suitable photography for mapping;

- Control: obtaining sufficient control through field surveys and/or extension by

photogrammetric methods;

- Map compilation: the plotting of planimetric and/or topographic features by

photogrammetric methods;

- Map Completion: the refinement of the map editing in the office and further, special

surveys in the field;

- Final Map Drafting: the completion of the map by drafting/scribing.

The first step, as given in the preceding outline is to prepare a complete plan that will make

possible to start the actual mapping operations and that will guide these operations as they are

carried out. The plan includes the following essential steps.

- Conversion of requirements: region to be mapped, the scale at which it is to be

mapped, the accuracy of he final map, the date by which the map should be

completed, the approximate cost of the project;

- Gathering of materials and people for the planning: photographs, maps, survey data,

instruments, and personnel;

- Determining Specifications and conditions for Operations: where to establish control,

how much photography is necessary, kind of equipment.

- Preparing Final Plans: scheduling, instructions for surveying, photography,

compilation, quality control;

- Costing and Replanning: redone the plan until costs are set within limits.

The information that is gathered at the second step of planning includes:

a) aerial photographs of the region to be mapped;

b) b) old maps of the region;

c) c) survey data showing the locations of horizontal and vertical control in region, the

accuracy of control and its accessibility.

4.2.3. Planning and execution of aerial photography project (flight planning) The aerial photography is base upon which the photogrammetric project is build. The success of

the project consequently depends greatly on the availability of suitable photographic coverage.


Lecture 4 - 13 - 10/23/2003 ____________________________________________________________________________________ Suitable coverage of the project depends upon many factors of which several are of particular

importance:

a) scale of photographs;

b) overlap between exposures;

c) optical and mechanical characteristics of the taking camera;

d) film base and emulsion type used;

e) date of photography.

4.2.4. Flight planning Practical aerial photogrammetry is limited to the approximately normal case; but the exact normal

case cannot be achieved. The deviations of individual photographs from the strict normal case

are, in practice not more than Dw=±5 gon, DF=±3 gon and Dk=±15 gon. A tolerance of ±2% in

the flight height is usual. The track of the aircraft can, with visual navigation and good navigation

information, be held within ±1 cm in the photograph space.

The simple geometric relations required for flight planning are shown 4.9, where flat ground is

being assumed.

Figure 4.10. Flight plan

The relations between main parameters of flight project (according to Kraus) are formulated later

on. In formulas and figure 4.10 are used the following symbols.

A – Distance between flight lines B – Base c – Principal distance s – Image side (to edge) h – Flying height above the ground Z – Ground height


Lecture 4 - 14 - 10/23/2003 ____________________________________________________________________________________

Z0 – Absolute flying height v – Flying speed over the ground L – Length of the strip or block Q – Side length of the block mm – Scale of mapping

The relations between parameters of flight project are defined.

Table 4.1

Photo scale number .pm k m= m k depends on apparatus

Flying height above the ground . ph c m=

Image side in the ground . pS s m=

Base in the photograph

p

Bbm

=

Absolute flying height 0Z Z h= +

Overlap between photos [%] .100 1 .100S B Bl

S S− = = −

Side lap (%) .100 1 .100S A Aq

S S− = = −

Ground area of one photograph 2 2.p p2F S s m= =

Base length for l% overlap . 1

100lB S = −

Distance between strips for q% side lap . 1

100qA S = −

Number of models in the strip .( /50 1) 1 for 50mL S ln l

B− − = + >

Number of photographs in the strip 1p mn n= +

Number of strips in a block 1 for 50s

Q Sn qA− = + <

Area of stereoscopic model ( )m .F S B S= −

New area for each model in a block .nF A B=

Time between photographs [ ][s] = 2/0 [ / ]B mt

v m s∆ ≥


Lecture 4 - 15 - 10/23/2003 ____________________________________________________________________________________ [ ] – Largest integer number

The values of l and q are usually taken l=60% and q=30%. When GPS control is used for aircraft

tracking it is possible to decrease the values of overlap and lap but not too much due to the

requirements of triple overlapping in strip and overlapped tie points between the strips.

The value of side lap allows for:

• Errors in holding the track of the aircraft along the strip (~±5%)

• Variations in lateral tilt Dw (~±5%)

• Residual, uncorrected drift (~±3%)

• Variations in rotation of the photograph about its axis (~±3%)

• Smaller variations in terrain heights

Lightening of the image at the edges in electronic dodging, which make it impossible to observe

stereoscopically right to the edges.

By satisfying the above conditions it is possible to remain minimum 10% coverage of side lap

between strips, in which homologous points can be found to serve as tie points between strips,

and mapping is possible without gaps.

Flight planning is inseparably bound up with project planning. The particular points to be

observed are:

• The performance limits of the aircraft

• The ranges f the stereoplotter to be used

• The type of product – line map or photo map

• The ground relief

• The size of the map sheets of the finished product

• The accuracies required

Two methods are used to allow for the map sheet limits with Very good navigation data are

required for aimed photographs to ensure that the photographs really are aimed strip: aimed

single photography or a high overlap. A global positioning system is a very good help in this task.

If a high overlap is chosen, usual value of 90% and the best situated photographs and

stereoploters are then chosen.

A navigation plan is drawn up as part of the flight planning. This can consist either of enlarged

aerial photographs or better a good topographic map. The navigation plan must show:

The area of interest, which absolutely must be covered by stereopairs;

Any obstacles of flying;


Lecture 4 - 16 - 10/23/2003 ____________________________________________________________________________________ Any prohibited areas, which are forbidden for flying or allowed under strict precautions (military

training areas, foreign countries).

The requirements to graphical presentation of navigation plan:

Area of presentation must be extended at least 5 km at each end of the strips, for turning of the

aircraft;

The flight line is shown with solid lines over the strips and with dashed lines for manoeuvres.

Each flight line is accomplished with notes on the flying height and course (azimuth in degrees)

at each end.

Additional information is given on separate sheet. It contains:

-Project name, purpose, dates,

-Photo scales, focal length, absolute flying height, overlap, side lap,

-Minimum length of film required and type of film;

Organization details:

-agreements on signalisation of points,

-closeness of international boundaries,

-flying only under full cloud cover.

The typical navigation and personnel equipment of survey aircraft includes:

• special glass of camera port or remotely operating camera;

• navigation instrument is inside with overlap regulator;

Central disposition of camera ports with possibilities to close the doors during take off and

landing

Good view downwards for the camera operator and possibilities for communication;

good navigation utility – GPS, Dopler or Inertial Navigation systems (INS)

Flight crew duties:

- pilot – coarse navigation;

- copilot – pilot support

- (navigator –navigation),

- camera operator – fine navigation and camera control.

4.2.5. Aerial camera equipment Modern aerial cameras are complex system. A schematic diagram of such camera is shown

below.


Lecture 4 - 17 - 10/23/2003 ____________________________________________________________________________________

Figure 4.11. Camera equipment

Camera cycle (min 1.6-2.0 sec) includes:

exposure (with motion compensation), rising the pressure platen, releasing the vacuum

transport the film , advanced the photo number, apply the vacuum, press the platen.

Modes of exposure:

aimed single photograph – by operator control; serial photographs – with automatic overlap control

Content of the Viewfinder used for visual control:

-field of view with interchangeable frames, depending on objectives;

- level bubble;

- central cross of target point;

- set of moving eccentric spiral for overlap control;

- central line for angle orientation and drift control.


Lecture 4 - 18 - 10/23/2003 ____________________________________________________________________________________

Figure 4.12. Navigation view-finder

If it exists wind during the flight, the aircraft trace on the ground does not coincide with the

direction of flight. By that reason the angle orientation of camera is due to be in the opposite

direction (but with the same size as) the angle aircraft flight according to the flight line.

The influence of the wind is shown on the next figure (according to Kraus K., 1983)

Figure 4.13. Drift compensation


Lecture 4 - 19 - 10/23/2003 ____________________________________________________________________________________

v0 – vector of own velocity vw – vector of wind v – resultant vector of velocity over the ground

Some cameras have separated overlap regulator with viewfinder

Figure 4.14. Overlap regulator

Barometric differential height measuring device (statoscope)

The possibility to exposure the coordinates from navigation system (if persists)

Some cameras have unit for forward motion compensation.

Appendixes

Appendix 1

The derivation of error in distance direction Yσ as a function of base-length is shown below. It is

derived from the formula for Y coordinate in object space.

.x

BY cp

= (4.10)

The error in Y co-ordinate is function of the standard error of the parallax


Lecture 4 - 20 - 10/23/2003 ____________________________________________________________________________________

2. .Yx

B cp pσ σ= (4.11)

The error is estimated for maximal distance because for it the error is maximal. From this error

the value of B can be computed.

2max( ) p

Y

YBc

σσ

=

(4.12)

The length of base B is selected from the relation

max min

20 5Y YB< < (4.13)

The stereo overlapping zone stats at the distance Y0 from the middle of the base and has wide-

angle 2.β. The distance of the vertex of stereo overlapping zone is defined by the equation

0 2 .cotBY gβ= (4.14)

As angle β could be expressed by photo size lx and camera constant c,

/ 2tan xlc

β = (4.15)

it is possible to obtain other presentation for Y0

02. .Y

x

c Bl

= (4.16)

Appendix 2

The length of base B and average distance Yav with overlapping of kov are connected with

relation

(1 ). .tan( ) (1 ). .tan( )ov av ov avB k Y k Yβ β= − + Φ + − − Φ (4.17)

The minimal distance at which the overlapping area begins is given by the equation

01 2tan( ) tan( )

BYβ β

=+ Φ + − Φ

(4.18)


Principles of Picture taking · Lecture 4 ... convergent for point measurements, flight planning)...

Documents

Transcript of Principles of Picture taking · Lecture 4 ... convergent for point measurements, flight planning)...