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Introduction on Photogrammetry
By: Koert Sijmons
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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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Aerial Photo Concepts
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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Aerial Photo Concepts
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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Aerial Photo Concepts
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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Aerial Photo Concepts
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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Aerial Photo Concepts
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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Photogrammetric Triangulation
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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Photogrammetric Triangulation
How do you do it?
Interior Orientation
Exterior Orientation
Image measurements
Ground Control Points (GCP)
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Interior Orientation
- 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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Interior Orientation
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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Interior Orientation
- 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
image, according to the position of the
control point in the map
Set register mark to point 2 in the left image,
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
Type in: X-coordinates: 501610.000
Y-coordinates: 1932850.000
Z-value: 23.000
for Point 2 Click: Enter
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3
3
3
3
Set register mark to point 3 in the right
image, according to the position of the
control point in the map
Set register mark to point 3 in the left image,
according to the position of the control point
in the map
502775.000 1933430.000 52.000
Type in: X-coordinates: 502775.000
Y-coordinates: 1933430.000
Z-value: 52.000
for Point 3 Click: Enter
Register Ground
Control Point
Register Ground
Control Point
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4
4
Set register mark to point 4 in the left image,
according to the position of the control point
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
Set register mark to point 4 in the right
image, according to the position of the
control point in the map
Type in: X-coordinates: 502135.000
Y-coordinates: 1932060.000
Z-value: 45.000
for Point 4 Click: Enter
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
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