Topic 4: Photogrammetry 1

PHOTOGRAMMETRY DEFINITION (from Elements of Photogrammetry, Paul Wolf)

Photogrammetry: The art, science, and technology of obtaining reliable information about physical objects and the environment through processes of recording, measuring, and interpreting photographic images and patterns of recorded radiant electromagnetic energy and other phenomena.

1) metric photogrammetry: making precise measurements from photos. 2) interpretative photogrammetry: recognizing and identifying objects and

judging their signifigance through careful and systematic analysis. Also: Compare definitions from the Manual of Photogrammetry, Ed. 1 & 2. Photogrammetry: making precise measurements from images

• Close range photogrammetry: with camera focus set to a finite value. • Far range photogrammetry: with camera focus setting to indefinite (infinity)

Basic Optics: thin lens equation: 1 1 1o i f

+ =

magnification: i h ' image sizeMo h object size

= = =

Depth of Field: examples

o i

θ

imag

f

obje

ct

h

h

depth of

http://www.cs.mtu.edu/~shene/DigiCam/User-Guide/950/depth-of-

http://en.wikipedia.org/wiki/Image:DOF-ShallowDepthofField.jpg

© Piccolo Namek

F3.2 F5.6 F9.0

The 3" and 4" marks are in focus in all images. The depth of field increases as the aperture size decreases.

Topic 4: Photogrammetry

2

http://www.cs.mtu.edu/~shene/DigiCam/User-Guide/950/depth-of-field.html

• A lens focused on the yellow dot generates a yellow dot on the image plane. The yellow dot and all objects having the same subject-lens distance will appear sharp.

• The white dot, with larger subject-lens distance, will be out of focus. Its image is actually formed somewhere in front of the image plane and the image of this white dot on the image plane is a circle (the circle of confusion). As the subject-lens distance increases, the size of this circle increases.

• The same holds true for a subject in front of the yellow dot (e.g., the green dot).

• The size of a circle of confusion is proportional to the amount of light that can pass through the lens tube. Thus, smaller circles of confusion will be formed if less light can pass through. Therefore, a smaller aperture means smaller circles of confusion and a sharper image.

Depth of Field: DOF = DF – DN Near Focus Limit: DN = (H x D) / ((H - f) + D) Far Focus Limit: DF = (H x D) / ((H – f) – D) Hyperfocal Distance: H = (f x f) / (N x c)

Setting focus at the Hyperfocal Distance gives maximum depth of field from H/2 to infinity.

Where: H = Hyperfocal Distance (in mm) DN = Near Focus Limit (mm) FF = Far Focus Limit (mm) D = lens focus distance (in mm) f = lens focal length (i.e., 35mm, 105mm) N = Numerical aperture (f-stop): typical values: 1.4 (max light), 2.0, 2.8, 4, 5.6, 8, 11, 16, 22 (min light) c = diameter of circle of least confusion Digital SLRs c = 0.02 mm 35mm format c = 0.03 mm 6x6cm format c = 0.06 mm 4x5in format c = 0.15 mm

For a complete discussion, see: http://www.photo.net/learn/optics/lensTutorial also: http://en.wikipedia.org/wiki/Depth_of_field

Topic 4: Photogrammetry 3

Long range photogrammetry: focus at infinity

thin lens equation: 1 1 1o i

ff

→→∞

+ =

magnification: h 'M Scaleo hf

= = =

Aerial imaging (frame camera)

FOV – Field of View

• The total solid angle (or ground area) viewed by an imaging system or radiometer. • Commonly specified as a plane angle or length on the ground.

IFOV – Instantaneous Field of View

• The smallest solid angle (or ground area) uniquely detected by an imaging system when all motion is stopped.

• Commonly specified as a plane angle or length, as above. When given as a length it is sometimes referred to as a Ground Instantaneous Field of View (GIFOV).

i ≈ f o

Scale SHf

= =

h = altitude

optic

w

f = focal distance

FOV

Topic 4: Photogrammetry

Components of a mapping camera Lens Assembly: The lenses of aerial systems are multiple-lens systems with a between-lens

field stop and shutter. The focus is fixed at infinity. Typical focal lengths are 3.5, 6, 8.25 and 12 inches.

Focal Plane: This is a plate aligned perpendicular to the optical axis of the lens. A vacuum system is used to fix the film to the plate so the focal plane is perfectly flat during exposure.

Lens Cone: This holds the lens and filter, and covers the front part of the camera preventing light from leaking into the camera body.

Body: Encloses the camera, the mounting bolts and stabilization mechanism.

Drive Assembly: The winding mechanism, shutter trigger, the vacuum pressure system and motion compensation.

Magazine: Holds the roll of unexposed film, advances the film between exposures, holds the film in place and winds-up the exposed film. Magazines may be exchanged in-flight.

Nodal Points of a 4-element lens

body

lens assembly

cone

film spools

fil

focal plane

aperture stop shutte

filter

magazine

Flattening plate

incident nodal point

emergent nodal point

focal plane (plane of infinite

focal

nodal point separation

altitude (infinite

nodal pointspThe rear nodal point is the perspective center of the photo

b o a

A O B

Topic 4: Photogrammetry 5

RMK TOP - Aerial Survey Camera System CAMERA TOP

RMK TOP 15 focal length 153 mm (6 "), angular field 93° (diagonal), aperture f/4 to f/22 continuously, distortion <= 3µm RMK TOP 30 focal length 305 mm (12") angular field 56° (diagonal), aperture f/5.6 to f/22 continuously, distortion <= 3µm

SUSPENSION MOUNTT-TL (gyro-stabilization suspension mount)

• Stabilization range: • ± 5° in omega, • ± 5° in phi, • ± 6.5° in kappa

• max. angular speed: 10°/s • max. angular acceleration: 20°/s²

Intergraph DMC Specifications • 4 high-resolution 7K x 4K panchromatic cameras

– Final output image: 7,680 x 13,824 pixels – Field of view: 69.3° cross track x 42° along track – Lens system: 4: x f = 120mm/f:4.0

• Four multispectral 3K x 2K cameras: red, green, blue, and near infrared – Spectral sensitivity: Blue: 400-580 nm; Green: 500-650 nm; Red:

590-675 nm; NIR: 675-850 nm; NIR alternate: 740-850 nm. Custom filters available upon request

– Final output image pan-sharpened RGB or CIR: 7,680 x 13,824 pixels – Lens system: 4: x f = 25mm/f:4.0

• Shutters and f-stop: continuously variable 1/50 - 1/300 sec, f/4-f/22 • On-board storage capacity FDS: 864 GB (>2,200 images) • Maximum frame rate: 2.1 sec/image • Radiometric resolution: 12 bit (all cameras)

Topic 4: Photogrammetry

Focal length: The distance between the rear (emergent) nodal point and the focal plane.

Equivalent focal length: The distance along the optical axis to the plane of best average definition (measured).

Calibrated focal length: an adjusted value of the equivalent focal length, computed such that the effect of lens distortion is distributed over the entire field.

Horizontal resolution Resolution will depend on:

• inherent resolution of the film (or design of the array)

• depth of field (circle of confusion) • characteristics of the optics

– lens quality – focal length – imaging geometry

Uniformity of Scale

1. Photo taken with film plane at an angle to the building face. Note that the

roof line and ground line are not parallel. (variable scale)

2. Photo taken with film plane parallel to the building face. Note that the roof

line and ground line are parallel. (uniform scale)

3. Photo of the left end of building taken with film plane parallel to the building

face. Distance from the building is approximately the same as in photo 2.

w dScale SH FOV Df Δ

= = = =Δ

optic

FO

Δd

ΔD

f = focal length

h = altitude

w

Topic 4: Photogrammetry 7

Sector Star Target (for astigmatism)_

negativ

rear nodal front nodal

reduced contact enlarged print

datum

H

f S = f / H

Scale = image distance/ground distance

1:24,000 1" = 2,000 ft. small scale 1:250,000 1 mm = 24,000 mm large scale 1:12,000

Scale

panoramic

datum

H

f

S = f / H

imag

frame camera (with film plane // ground)

ff

Scale distortion

constant scale (for vertical image)

variable scale (for vertical image)

Topic 4: Photogrammetry

Resolution Test Patterns

Each test target comes with a chart that specifies the line pairs per mm (lppm) for each group and element.

Sector Star Target (for aerial imagery)

http://www.lacoast.gov/maps/2005doqq/2005doqq.aspx?id=C3008935.SES&quad=NICHOLSO

N

Fiducial Marks

fiducial marks

+x -

+

-

y-fiducial axis

x-fiducial axis

photographic center (principal point)

fiducial marks

Topic 4: Photogrammetry 9

Airphoto Hidalgo Cnty, TX http://www.colorado.edu/geography/gcraft/notes/remote/gif/hidalgo.jpg

Scale change with topography

•

•

•

DATU

GROUN

ELEVATION

S = f/(H -

Above datum = H Above A: HA' = H - Above B: HB' = H - hB f

PHOT••bb

a••a

A

B

B

A

h

hB

H

Topic 4: Photogrammetry

Relief displacement

Near-vertical kite aerial photograph. Notice different view of trees near scene center in comparison to trees at far right.

Cucharas Pass, Colorado; photo date 6/00, © J.S. Aber.

Source: http://academic.emporia.edu/aberjame/airphoto/p_gram/p_gram.htm

fa

abt

rbrb ra

ra

da

RA RB

A

A

H

h

hB h• •

• •

•• • •

datum

photo

nadir

• a Aa

r hdH

=•b

rt

Topic 4: Photogrammetry 11

Source: http://www.photoscience.com/airphoto.htm#Sample Air Photo

Topic 4: Photogrammetry

Tilted Aerial Photograph

Tilt displacement A point that would have been imaged at a' on a vertical photo is actually imaged at a on the "up side" of the tilted photo.

The tilt displacement of points on the "up side" of the tilted photo is then toward the isocenter while points on the "down side" are displaced away from it.

• Tilt displacement is always relative to the isocenter. • Scale change is in the direction of tilt. • The nadir point is always on the down side of the axis of tilt and opposite the principal

point from the isocenter

•

•

p

in

principal

isometric

photo

photograph perpendicula

perspective center(rear nodal point)

principal plane

+x

-y

-x

t

s

ground

f

t/ t/

•

•

n - nadir point i - isocenter p - principal point f - focal length t - angle of tilt s - swing

for t < 5°, pi = pn/2

Topic 4: Photogrammetry 13

The direction of tilt displacement is radial relative to the isocenter. The amount of displacement is proportional to the distance from the isometric parallel.

Oblique photography: Extreme tilt displacement

Image areas on the upper side of the tilt are displaced further away from the ground than is the isocenter and are at smaller scales than the nominal scale.

Image areas on the lower side of the tilt are displaced closer to the ground than the isocenter and are at larger scales than the nominal scale.

Source: http://www.aboveallphoto.com/oblique_photography.html

perspective center

t

ff

d'

d d''

n'

n

Pa a''

a'equivalent vertical photo

•

•

•

••••i

tilted photo •

AB

•

perspective center

principal line

isometric parallel

equivalent vertical photo

tilted photo

d' i

•

••

a''•

•

•

•a

a'

d d''

t t

Topic 4: Photogrammetry

Tilt displacement The direction of tilt displacement is radial relative to the isocenter. The amount of displacement is proportional to the distance from the isometric parallel.

Relief & Tilt displacement

1. Location of an object on the datum plane for an untilted photo 2. Position of the object on a vertical photo due to relief displacement. (Object is above the

datum plane.) 3. Position of the object on a tilted photo due to tilt displacement.

isometric parallel

principal a a'

b b'

pi I

n d d'

e e'

c,

•• •

•

•

•

•

••

••

•

•

•

•

•

•

•

•

•••

••

ip

n

isometric

principal

1 231

23

123

12,

123

"up side"

"down side

"

Topic 4: Photogrammetry 15

Definitions

Stereo Air-photo terminology

Principal point: Geometric center of photograph. Literally the point on the ground in line with axis of camera lens.

Fiducial marks: Marks on the photograph margins used to locate principal point in photo.

Conjugate principal point: Point in overlapping photo that is equivalent to principal point of adjacent photograph.

Photo base: Distance between principal point and conjugate principal point measured on a single photograph.

Ground (air) base: Ground (air) distance between principal points of overlapping photographs.

Parallax: Apparent shift in relative positions of objects when viewed (photographed) from different vantage points.

Stereo Imagery from a frame camera

Determining height from stereo imagery

Determining the height of the Washington Monument using stereo parallax 555 ft 5.9 in (169.314 m)

air baseelevation H above datum

1 2

n1'

a b

A

B

a'n1 n2 n2' b'

DATU

N1

N2

Topic 4: Photogrammetry

Windsor, Ontario, 1931 For instructions in stereo viewing on-line, see: http://rst.gsfc.nasa.gov/Sect11/Sect11_3.htm

http://airphotos.nrcan.gc.ca/index_e.php Stereo Imagery from Mars

http://mars.jpl.nasa.gov/MPF/mpf/stereo-arc.html

Topic 4: Photogrammetry 17

Height Measurement

O1, O2 = nadir points of photo 1 and photo 2, respectively X1, X2 = location of the base of the tree x'1, x'2 = position of the base of the tree along the flight path dP1, dP2 = relative parallax Change in height

Flight Planning

flight O1 O O O2

dP1 dP2

x' x'

XX2

f

H-

dp

oo1 x

h

H

dp

o2 ox

image

rear node (lens

O1 O

dP = absolute parallax

dPdP

parallax difference: dP = dP1 – dP2

h dPH h P

=−

H dPhP dP

=+i

1

1

dpSdP

=

Topic 4: Photogrammetry

Factors to consider:

1. General issues • focal length • film size (format) • photo scale / FOV • overlap / sidelap (continuous coverage, stereo, …)

2. Issues specific to the application • spectral considerations (film / filter) • time of day (illumination, sun orientation, tidal stage, …) • season (crop calendar, leaf on, leaf off, …) • sun orientation, sun angle

Focal length

• The nominal scale of the photo is the ratio of the focal length to the altitude:

scale = f / altitude

• If the image medium can resolve 1 line pair / d, the equivalent ground resolution is:

rgnd = d / scale

• The FOV of the image is related to the film format. (Film formats: 35 mm, 23 cm (9"), etc.):

• Vertical resolution, a function of the distance between images, the altitude and the film resolution, may be estimated as:

Overlap

• Plan for 60% overlap (endlap), especially for stereo flights. • Can be less if stereo is not required

1 2 3 4 5 6 7 8

1 2 3 4 5 6 7 8

coverage of a single photo endlap

nadir line (ground flight path)

horizontal shift and rotation due to drift and correction for crabbing

consecutive frames collected by camera at equal time intervals

photos aligned to fit a base map

photo centers

Topic 4: Photogrammetry 19

Sidelap • Plan for 30% overlap (sidelap) in order to insure complete coverage (no gaps).

Sources of Aerial Photography USGS National Aerial Photography Program: http://edc.usgs.gov/products/aerial/napp.html

• Standardized images, cloud-free, every 5-7 years • Collected at 20,000 ft; about 1 m resolution • Centered on one-quarter section of a 7.5-minute USGS quadrangle, and covers

approximately a 5.5 x 5.5 mile area USDA Aerial Photography Field Office http://www.apfo.usda.gov/

• Imagery dated beginning with 1955 to the present at this site. • Imagery prior to 1955 are held by the National Archives but must be acquired

through commercial vendors. National Ocean Service (Coastal Aerial Photography) http://oceanservice.noaa.gov/dataexplorer/welcome.html National Air Photo Library (NAPL) of Canada http://airphotos.nrcan.gc.ca/collection_e.php Listing of commercial sources: http://www.puredirectory.com/Recreation/Roads-and-Highways/Photography/Aerial/

flight path

Topic 4: Photogrammetry

Spectral Considerations

What spectral bands will highlight the target in the expected background?

vegetation: NIR/Red is characteristic of vegetation

mineral exploration: Specific band selection will depend on the minerals in question, but most will be in the Mid-IR or SWIR.

water quality: visible channels will dominate.

Seasonal Considerations

Will the target be more detectable at some times of year?

vegetation: discrimination between oak and maple may be most effective in early spring when maple has leafed out but oak has not.

mineral exploration: any season will do if there is no cloud cover (or snow).

water quality: - wet season vs. dry season - temperature regime (thermocline, plankton growth) - seasonal land use changes (tourism, industry, recreation)

Time of day considerations

Will the target be more detectable at certain times of day?

vegetation: discrimination between oak and maple may be most effective in early spring when maple has leafed out but oak has not.

mineral exploration: shadows may be an advantage (low sun angle) in some cases.

water quality: - tidal stage - relatively high sun angle (to maximize the amount of light entering the water).

Flight alignment

• Flight lines are usually planned to be parallel to each other and parallel to the long axis of the study area. (Minimizes aircraft turns which are very time consuming.)

• Complicating factors: – wind (causes the aircraft to crab or drift across the flight path). – topography (low altitude flights in mountainous areas may result in flight lines

that are not parallel to the long axis of the study area. – restricted zones (airports, military bases), national borders,

• Issues specific to line scanning systems – sun angle effects (BRDF) may be minimized by selecting a flight line into or out

of the sun. –

Topic 4: Photogrammetry 21

Other interesting topics • motion detection/measurement

– bio-mechanics – fluid dynamics

• synthetic apertures/depth of field

– partially occluded targets – viewing through an obscuring medium

Motion detection: bio-mechanics

• high-speed, synchronized cameras • cameras fixed, object in motion

http://it.uku.fi/biosignal/research/motion.shtml

Motion detection: fluid dynamics

• close-range photogrammetry • high-speed, synchronized cameras • cameras fixed, object in motion

http://www.tu-dresden.de/ipf/photo/publikationen/aeltere/Maas_Virant_Becker_Boesemann_Gatti_Henrichs_ActaAstronautica2002.pdf

Topic 4: Photogrammetry

Camera Arrays: obscured targets

Synthetic Aperture Confocal Imaging (2002?) Levoy et al., Stanford University, SIGRAPH? Synthetic aperture image: partially occluded target

Synthetic Aperture Confocal Imaging (2002?) Levoy et al., Stanford University

Topic 4: Photogrammetry 23

Synthetic aperture: imaging underwater

Synthetic Aperture Confocal Imaging (2002?) Levoy et al., Stanford University

Topic 4: Photogrammetry