introduction on photogrammetry by koert sijmons

1 Koert Sijmons

Introduction on Photogrammetry

By: Koert Sijmons

3 Koert Sijmons

Topographic map Aerial photograph

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Difference between map and photo

MAP

PHOTOGRAPH

Orthogonal projection.

Central perspective projection

Uniform scale. Variable scales.

Terrain relief without

distortion (contour

lines).

Relief displacement in the image

All objects are represented

also the non visible

Only objects that are

visible.

An abstract representation Is a real representation

of the earth surface, no legend needed.

Cont.

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Difference between map and photo

Cont.

Representation geometrically

correct

Representation geometrically

not correct

Elements appear

displaced in its real

position and in different

shapes, due to the generalization

process.

Objects appear displaced due to

geometric distortions.

MAP

PHOTOGRAPH

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Basic principles of Photogrammetry

Photogrammetry is the science and technology of obtaining

spatial measurements and other geometrically reliable derived

products from photographs.

Obtaining approximate distances, areas, and elevations using

hardcopy photographic products with unsophisticated equipment

Photogrammetric analysis procedures can range from:

Geometric concepts to generating precise digital elevation

Models (DEMs), Orthophotos,and thematic GIS data

Cont.

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Introduction

The terms digital and softcopy photogrammetry are inter-

changeable to refer to any photogrammetric operation

involving the use of digital raster photographic image data

rather than hardcopy images.

Digital photogrammetry is changing rapidly and forms the

basis for most current photogrammetric operations.

However, the same basic geometry principles apply to

traditional hardcopy (analog) and softcopy (digital )

procedures.

Cont.

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Introduction

Mapping from aerial photographs can take on numerous forms

and can employ either hardcopy or softcopy approaches.

Traditionally, topographic maps have been produced from

hardcopy stereo-pairs in a stereo-plotter device.

A stereo-plotter is designed to transfer map information

without distortions, from stereo photographs.

A similar device can be used to transfer image information,

with distortions removed, in the form of an Orthophoto.

Cont.

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Introduction

Orthophotos combine the geometric utility of a map with the

extra real-world image information provided by a photograph.

The process of creating an Orthophoto depends on the

existence of a reliable DEM for the area being mapped.

The DEM is usually prepared photogrammetrically as well.

A digital photogrammetric workstation generally provide the

Integrated functionality for such tasks as generating:

DEMs, digital Orthophotos, perspective views, and

fly-throughs simulations, as well as the extraction of

spatially referenced GIS data in two or three dimensions

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Introduction

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60% forward overlap 20 - 30% side lap

Flight strip 1

Flight strip 2

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Terrain

1

1

2

2

3

3

4

4

5

5

6

6

Flight line

Nadir line

(ground trace of aircraft)

Endlap

Photographic coverage along a flight strip

Conditions during exposures

Resulting photography

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Flight line 1

Flight line 2

Flight line 3

Exposure station

Flight paths (Photo run)

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Focal length

Focal length

E

O

Exposure station (L)

Negative

d

a b

c

e

y

x

o Positive

c d

b

a

C

D

A

B

e o

Optical axis

Geometric elements of an aerial photo

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Eustasius

June 1982

2205

Fiducial marks

Message Pad Watch Altimeter

Principle point

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Photography

central projection

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Central perspective

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L

Principle

Point

Photo Map

Orthogonal projection Central Perspective projection

Geometry of Map and Photo

Varied scale

Relief displacement

Result in:

Different size, shape and

location of static objects

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Scale at sea level (0 mtr.):

Scale at 50 mtr. Terrain elevation:

Scale at top volcano (590 mtr.)

0

50

590

S = scale

f = focal length (15.323 cm)

H = flying height (6200 mtr.)

h = local terrain height

1:40.462

1:40.136

1:36.612

Closer to the camera = larger scale

Scale S = H h f

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Positive f

o

h

L

H

O A

A

A

a a

D

d r

Relief displacement Occurs for terrain points Whose elevation is above

or below the reference

Elevation (at O).

Can be used for height

Calculation (h):

h = d H

r

d = 2.01 mm.

H (Flying Height) = 1220 mtr.

r = 56.43 mm.

h = 43.45 mtr.

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o o

Change in positions of

stationary objects caused by a

change in viewing position

Parallax of point A

Pa = xa xa

DATUM

y

x

L

y

x

L

a b a b

x a

o

x a b a

o

A

B

o

a b

o

Pa = the parallax of point A

x = The measured x coordinate

of image a on the left photo a

x = the x coordinate of image a

on the right photo a

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Y

X

Y

Y

X

O X

Y

X

O

a b a b

x a x a

Pa = x x a a

Pa = 54.61 (- 59.45) = 114.06 mm

x b

x b

Pb = x x b b

Pb = 98.67 (- 27.39) = 126.06 mm

P = 12.00

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H

O

o

O

A

f

O A

Y A

A x X A

h A

L

o

f

B = Air base H = Flying height f = Focal length Pa B

f H - h A

= __ _____ Pa = parallax of point A h = Height above datum

A

H h = Bf

P a

____

A

Also from similar triangles:

LOA A x

and Loa x

H - h A

X A _____ a x =

__ f

From which:

L

x a

a x

a y a a

a x

x a

X A

x (H h ) a A

= _________

f

X A

= B

x a

p a

____

Y A = B y a

p a

____

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X A

= B

x a

p a

____ Y A = B

y a

p a

____ Parallax equations

are ground coordinates of a point with respect to an arbitrary

coordinate system whose origin is vertically below the left

exposure station and with positive X in the direction of flight

X and Y

p Is the parallax of the point in question

x and y are the photocoordinates of point a on the left-hand photo

The major assumptions made in the derivation of these

equations are that the photos are truly vertical and that they

are taken from the same flying height.

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Aerial Photo Concept

Digital Orthophotos are generated from the same type of

Aerial photo as conventional hardcopy Orthophotography.

The difference lies in the scanning of the airphoto, converting

the photo to a digital image product that will be manipulated

and processed with a computer.

Cont.

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Aerial Photo Concepts

The relationship between photo scale, scanning resolution

and final scale must be considered.

Final resolution of the Orthophoto product is based on the

application that the Orthophotos are being used for, and also

the limitations of disk space that may be encountered during

the project.

It is not always beneficial to scan an airphoto at the highest

number of dots per inch (DPI), if the application does not

warrant such high resolution.

Cont.

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A simple equation can be used to calculate the scanning

resolution necessary based on the original scale, final

output pixel size and the size of the hardcopy photo.

The equation is: where:

p = output pixel size (cm)

W = photo size (cm)

r s = scanning resolution (DPI)

d = Foot print size (cm)

Cont.

= ______ r s W p *

d

* 2,54 cm/inch

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Example:

A photo is 9 inches (22.86 cm) across, and covers a ground

distance of 8 Km. The final resolution required is 3 meter

the scanning resolution in dots per inch (DPI) would be:

r s =

800000 cm

* 2.54 cm/inch = 296 DPI 22.86 cm * 300 cm _________________

Cont.

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The scanning resolution can also be determinated from

the photo scale, without having calculate the ground distance.

photo scale is more commonly quoted in the aerial survey

report.

= ______ r s W p *

d From the previous mentioned equation:

we derive:

r s =

d

W * S *

2.54

p

____ ___ = 2.54 ____

p

where S = photo scale Cont.

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For example, a typical aerial survey might consist of photos

at 1:4,800 scale. The desired output resolution for the

orthophotos is approx. 30 cm. For 22.86 cm airphoto,

a reasonable scanning resolution would be:

r s =

_____ * * S

2.54 2.54

p = 4800

_____

30 = 406 DPI

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The St. Eustasius demonstration dataset was flown at an

average photoscale of 1:40,500

The photos are 22.86 cm x 22.86 cm.

We want a ground resolution of 3m., so we must calculate the

scanning resolution.

r s = S * *

2.54

p = 40.500

300 = 342.9 DPI ____

2.54 ____

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Photogrammetric Triangulation

What is it?

- Increasing the density of whatever ground control you have;

called Control Point Extension

What does it do?

- Computes coordinate values for any point measured on two

or more images (tie points)

- Computes positions and orientation for each camera station

Cont.

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Computes position of

Each camera station

- X,Y and Z (where Z is

flying height)

- Omega ()

- Phi ()

- Kappa ()

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f

Aerial photographs f Deformations

X

Y

Z

X

Y

Z

X

Y

Z

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How do you do it?

Interior Orientation

Exterior Orientation

Image measurements

Ground Control Points (GCP)

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- Lens focal length

- Origin of co-ordinate system (principal point)

- Radial lens distortion

Objective: Interior Orientation models the geometry inside the camera

Coordinate systems

- Establish the relationship between positions in the image

(pixel) and the corresponding position in the camera (mm.)

The coordinates of the fuducial points in the camera are

known.

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left right

Principle point Principle point

Aerial photographs en stereo

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Fiducial marks

Interior Orientation: Image used

during demonstration

Principle point

Image details:

Average photo scale:

Scanning resolution:

Ground resolution per pixel:

1:40,500

300 DPI

(2.54 / 300)*405 =

3.43 m.

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Film: coordinate position are measured in

microns (Image coordinate system)

Digital image: coordinates positions are

measured in pixels (Pixel coordinate system)

Using fiducial points a linear relationship can

be established between film and image

coordinate postions

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1: 106.004

2: -105.999

3: -106.004

4: 106.002

X and Y coordenates of

the fuducial points

-106.008

-105.998

106.005

106.002

-X

1 2

3 4

Principal point

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Column

X

Y

Relation between

Pixel coordinates

(Line,Column)

and

Image coordinates

(in the camera in millimeters)

(x,y)

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0

Col pixel 0

Lin pixel 0

A

Col pixel A

Lin pixel A

Pixel coordinate system

Image coordinate system (film)

Colum 0,0

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- Camera calibration information - Obtained from camera calibration certificate

- Calibration elements:

- Focal Length

- Fiducial coordinates

- Principal point location

- Radial lens distortion

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Exterior Orientation

Objective: Establishing a relationship between the digital image

(pixel) co-ordinate system and the real world (latitude and longitude)

co-ordinate system

Ground Control Points

Visually identifiable

Preferably on multiple images

Larger image blocks need less control per image

Need to be well distributed in X,Y and Z

Ground control types:

Full (X,Y,Z)

Horizontal (X,Y)

Vertical (Z)

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O: Projection centre

A: Point on the ground

a: Image of A on the

photograph

from similar triangles:

O (Uo, Vo, Wo)

colinearity condition

a (Ua, Va, Wa)

A (UA, VA, WA)

oa

oa

oa

a

oA

oA

oA

a

oa

oA

oa

oA

oa

oA

WW

VV

UU

s

WW

VV

UU

:or

sWW

WW

VV

VV

UU

UU

UA -Uo

Ua -Uo

Wo -Wa

Wo -WA

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angles

Z

(Kappa)

X (Omega)

Y (Phi)

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What do these letters mean?

Position of a point in the image: x, y

Position of the corresponding terrain point: U, V, W

Known after interior orientation: xPP, yPP , c

From Exterior orientation: Uo, Vo , Wo,

r11, r12, r13, r21, r22, r23, r31, r32, r33 (computed from of , , )

For each point in the terrain its position in the image

can be computed from these two equations. (Different

for the left and the right image.)

PP

o33o32o31

o23o22o21

PP

o33o32o31

o13o12o11

y)WW(r)VV(r)UU(r

)WW(r)VV(r)UU(rcy

x)WW(r)VV(r)UU(r

)WW(r)VV(r)UU(rcx

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Resampling one pixel

Center of the orthophoto-

pixel in the original image

Nearest neighbour:

the value of this pixel

Bilinear: interpolated

between these 4

pixelcenters

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Example St Eustatius: How to accurately transfer interpretation from photo to map!!!

Shoreline from topographical map Aerial photo

?

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Available: 2 digital stereo Aerial Photos at scale 1:40,000

of the Island of Sint Eustasius (Caribbean Sea)

Left Right

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Available: Topographic map at

scale:1:10,000 of St. Eustasius

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Software: ERDAS IMAGINE 8.6

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Create New Block File

Working Directory

Type: Block File name

Sint_eustasius.blk

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Setup of Geometric Model

Frame Camera

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Select Projection

Set Projection

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Select Projection

UTM Zone 20 (Range 66W-60W)

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Select Spheroid Name

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Set Horizontal/Vertical Units in:

Meters

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Set Fly Height in meters

V 6200

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Loading images

Load left and right images

From your working directory

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Loading Left and Right image

d:/het mooie eiland st eustasius/left img

d:/het mooie eiland st eustasius/right img

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Set up for Interior Orientation

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Set Focal Length

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Type: 4

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Indicating: left.img

Interior orientation for left image

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Load left image

1st Fiducial point Jumps automatically to next fiducial point

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2753.202 2655.394

1st fiducial point

Set fiducial mark

Coordinades 1st. Fiducial point

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Measure 2nd fiducial point, as

done for point 1

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Measure 3rd fiducial point, as

done for point 1 and 2

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Measure 4th fiducial point, as

done for point 1, 2 and 3

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Should be less than 1 pixel

All 4 fiducial points are measured

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Make adjustments for the fiducial points in

order to get less than 1 pixel RMSE

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Green infill indicates, that Interior orientation

has been carried out for left.image

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Indicating: left.img Indicating: right.img

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Interior Orientation for right image

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Measure the 4 fiducial points for the

Right image, starting with point 1,as

done for the Left image

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The measurement for the 4 fudical points

are completed with less then 1 pixel RMSE

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Both images have their interior

orientation (green)

Set Ground Control

Points (GCPs)

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2

3

4

5

6

7 8

9

10 11

12

13

14

15 16

17

1

Control Points

X = 502865

Y = 1932070

Z = 107 m.

Coordinates:

1

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1 1

Control Point in map with corresponding point in left image

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32 1931430 502400 7

20 1935180 502265 6

55 1933750 503780 5

45 1932060 502135 4

52 1933430 502775 3

23 1932850 501610 2

107 1932070 502865 1

Z Value Y Coordinates X Coordinates Pnt nr.

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0 1936998 502450 14

0 1934460 503515 13

20 1931880 506030 12

35 1930600 504340 11

10 1930820 505190 10

62 1933420 505250 9

46 1930760 503260 8

Z value Y coord. X coord. Pnt. Nr.

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0 1934310 500570 17

0 1937315 500730 16

0 1936998 501480 15

Z value Y coord. X coord. Pnt. Nr.

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Measuring Ground Control Points

(GCPs)

Set Ground Control

Points (GCPs)

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Add 1st. Ground

Control Point (GCP)

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1

1 Set register mark to point 1 in the right

image, according to the position of the

Ground Control Point in the map

1

1

Set register mark to point 1 in the left image,

according to the position of the Ground

Control Point in the map

502865.000 1932070.000 107.000

Register Ground

Control Point

Type in: X-coordinates: 502865.000

Y-coordinates: 1932070.000

Z-value: 107.000

for Point 1 Click: Enter

Register Ground

Control Point

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2

2

2

2

Set register mark to point 2 in the right


control point in the map


according to the position of the control point

in the map

501610.000 1932850.000 23.000

Register Ground

Control Point

Register Ground

Control Point



Z-value: 23.000


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3

3

3

3






in the map

502775.000 1933430.000 52.000



Z-value: 52.000


Register Ground

Control Point

Register Ground

Control Point

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4

4



in the map

Automatically display the

Image positions of Control

Points on the overlap areas

of 2 images. This capability

Is enabled when 3 or more

Control Points have been

measured

4

4






Z-value: 45.000


502135.000 1932060.000 45.000

Register Ground

Control Point

Register Ground

Control Point

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Continue the same

Procedure for the Remaining Ground

Control Points according to map and

Coordinate list

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Click right button Click right button

Control

Full

Change type none into Full

and

Change Usage into Control

For all GCPs

introduction on photogrammetry by koert sijmons

Documents

koert sijmons difference

koert sijmons terrain

koert sijmons flight

digital photogrammetry

map information

digital orthophotos

terms digital

softcopy digital procedures