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Basics of Imaging Systems II

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Leica Airborne Digital Sensor (ADS40) http://www.gis.leica-geosystems.com/products/ads40 ... Example: Leica ADS40 = 64o. if H' = 2880 m. W = 2 x 2880m tan32o ... – PowerPoint PPT presentation

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Title: Basics of Imaging Systems II


1
Basics of Imaging Systems II
  • Preparatory Session Lecture 2
  • Prepared by R. Lathrop 9/99, updated 9/01, 8/03,
    9/04
  • based on material in Avery Berlin 5th ed 1992
    Chap 4

2
Photogrammetry
  • Photogrammetry is defined as the technique of
    obtaining reliable measurements of objects from
    photographs
  • To make accurate measurements it is necessary to
    determine, as accurately as possible,
    photographic scale

3
Types of aerial photos
  • Vertical photos - camera axis vertical
  • Tilted photos - 1-3o off vertical, virtually all
    aerial photos are unintentionally tilted
  • High oblique - intentional inclination, includes
    horizon
  • Low oblique - does not include horizon

4
Mapping or metric camera
  • Single lens frame camera
  • High geometric quality
  • Film format is 230 mm (9 in) on a side
  • Focal length of 152 mm common
  • Fiducial marks for later registration and
    defining principal point of the photo

Keystones Wild RC-10 mapping camera
BW NAPP photo
5
Digital Framing/Scanning Systems
  • Charge coupled device (CCD) electronic sensor
    sensitive to a particular wavelength of light,
    that are generally physically separate on the
    focal plane
  • RGB color image generally has separate RGB CCDs
  • There can be difficulty in spatial co-registering
    of the different wavebands for the same pixel

6
Digital Mapping Camera Zeiss/Intergraph Imaging
  • 2d CCD matrix (array) to ensure a rigid image
    geometry similar to a traditional precision film
    platen
  • Panchromatic 7000 x 4000 pixels
  • Color 3000 x 2000 pixels
  • Separate lens for each band
  • Multiple smaller camera heads to create image
    rather than a single, large diameter
  • 12 bit radiometric resolution

http//imgs.intergraph.com/dmc/
7
Digital Line Sensing Systems Leica Airborne
Digital Sensor (ADS40)
  • Pushbroom linear array system rather than a 2D
    framing system
  • 3 line scanners forwards, downwards and
    backwards to provide for stereoscopic coverage
  • Three CCD sensors BW color (RGB) NIR
  • 12,000 pixels across
  • RGB co-registration through special trichroid
    filter that splits beam from single lens, rather
    than 3 different lens
  • Field of View of 64o
  • Produces up to 100GB of data per hour of flight

http//www.gis.leica-geosystems.com/products/ads40
/
8
Overlapping Stereophotography
  • Overlapping photography is needed to determine
    parallax and stereo/3D viewing
  • Endlap - 60
  • Sidelap - 20-30

9
Pushbroom Scanning vs. 2D Framing
Graphics from http//www.gis.leica-geosystems.com/
products/documents/ADS40_product_description.pdf
10
Photographic Scale
  • Scale defines the relationship between a linear
    distance on a vertical photograph and the
    corresponding actual distance on the ground
  • Photographic scale indicates proportional distance

11
Photographic Scale
  • Scale expressed as a representative fraction (RF)
    between the linear distance on the photo
    (numerator) and the corresponding distance on the
    ground (denominator)
  • Example 1/25,000 or 125,000 means that a
    length of 1 unit of measurement on the photo
    represents 25,000 units of measurement on the
    ground

12
Small vs. Large Scale
  • Small scale larger denominators objects
    appear small on the image image covers larger
    ground area e.g. 1120,000
  • Large scale smaller denominators objects
    appear large on the image image covers smaller
    ground area e.g. 110,000

13
Alternative ways to express Photographic Scale
  • 124,000 can be expressed as 1 in.
    2,000ft

1 1 in 12in 12
in 1 in 24,000 24,000 in
1ft 24,000 ft 2,000 ft 1100,000 same
as 1 cm 1 km 160,000 same as 1 in 0.95
mi 1300,000 same as 1 in. 4.7
mi 11,000,000 same as 1 in 15.8 mi
14
Photographic Scale
d
  • Scale f /H d/D
  • where f focal length H height
    above terrain d image distance D
    ground distance
  • h terrain elevation
  • H flying height (h H)

f
H
H
D
h
15
Scale determination from focal length and altitude
RF f / H where f focal length H
flying height above terrain Example f 210 mm
H 2,500 m MSL ground elevation 400
m RF 210 mm 1m
210 . (2,500 m - 400 m)
1000 mm 2,100,000 RF 1 or
110,000 10,000
16
Scale determination from photo-ground distance
RF PD / GD d / D where PD photo distance
between 2 points GD map distance between 2
points Example PD 5 cm GD 1,584 m RF
5 cm 1m 5 1
1584m 100 cm 158,400 31,680
17
Scale determination from Photo-Map distances
RF PD / (MD MS) where PD photo distance
between 2 points MD map distance between 2
points MS map scale denominator Example
PD 3.2cm MD 6cm MS 50,000 RF 3.2
cm 3.2 cm 1 6 cm 50,000
300,000 cm 93,750
18
Effect of flying height on ground coverage
H1 gt H2 D1 gt D2
H1
H2
x
D2
D1
Adapted from Lillesand Kiefer, 2nd edition
19
Effect of focal length on ground coverage
f1
f2
f1 gt f2 D1 lt D2
H1

x
D1
D2
Adapted from Lillesand Kiefer, 2nd edition
20
Ground Coverage
  • Ground coverage, D, of photo frame varies with f
    and H
  • as f decreases, ground coverage increases e.g.
    f1 1/2 f2 D1 2D2 A1 4A2
  • as H increases, ground coverage increases e.g.
    H1 2H2 D1 2D2 A1 4A2

21
Ground Coverage example
22
National High Altitude program (NHAP)
  • Flying Height, H 12,200 m
  • color IR camera
  • f 210 mm
    scale 158,000 area per frame 13.3
    x 13.3 km
  • panchromatic camera f 152 mm
    scale 180,000 area per
    frame 18.4 x 18.4 km

23
Ground Sample Distance (GSD)
In digital camera systems interested in Ground
Sample Distance the size of the individual
camera pixels projected onto the ground GSD
array element size H
. focal length
Example array element size 0.009mm
f 28 mm H 1800m GSD(m)
0.009mm x 1800m 0.6 m
28 mm A GSD of 0.6m does not necessarily mean we
can resolve objects 0.6m in size. General Rule of
thumb GSD should be at least one half the size
of the smallest object of interest. Example
taken from Comer et al. 1998 PERS, pp. 1139-1142.
24
Ground Coverage for Scanning Systems
  • W 2 H tan q/2 tan f opp/adj
  • where W swath width
  • H flying height above terrain
  • q/2 ½ FOV of scanner

Adj H
f
Opp ½ W
  • Example Leica ADS40
  • 64o
  • if H 2880 m
  • W 2 x 2880m tan32o 3600m

H
q/2
W
25
Determining Photo Orientation
  • Photo acquisition date, roll/frame s, and other
    annotation are almost always along northern edge
    of photo
  • Sometimes eastern edge is used
  • Only way to be certain is to compare photo to an
    appropriate map

26
Map vs. Photo Projection Systems
  • Maps have a orthographic or planimetric
    projection, where all features are located in
    their correct horizontal positions and are
    depicted as though they were each being viewed
    from directly overhead. Vertical aerial photos
    have a central or perspective projection, where
    all objects are positioned as though they were
    viewed from the same point.

27
Image Displacement
  • Relief displacement is due to differences in the
    relative elevations of objects. All objects that
    extend above or below a specified ground datum
    plane will have their images displaced.
  • The taller the object, the greater the relief
    displacement

Even satellite imagery can have relief
displacement
Quickbird image of Washington Monument
http//www.mfb-geo.ch/text_d/news_old_d8.html
28
Radial Displacement
  • A photos central projection leads to image
    displacement where objects are shifted or
    displaced from their correct positions
  • Objects will tend to lean outward, i.e. be
    radially displaced.
  • The greater the object is from the principal
    point, the greater the radial displacement.
  • Example cooling towers towards the edge of photo
    show greater radial displacement.

29
Maps vs. Aerial Photos
  • Maps Scale is constant No relief
    displacement
  • Photos Scale varies with elevation Relief
    displacement

30
Orthophotography
  • Orthophoto - reconstructed airphoto showing
    objects in their true planimetric position
  • Geometric distortions and relief displacements
    are removed
  • Orthophotoquad - orthophotos prepared in a
    standard quadrangle format with same positional
    and scale accuracy as USGS topographic maps
  • DOQ - digital orthophoto quad

31
Digital Orthophotography the new standard
Distortions removed, rectified to a standard
projection/coordinate system and in digital form
for ready input to a GIS
UTM or State Plane
2002 1 foot ground spatial resolution per pixel
32
Aerial Photographic Sources
  • National High Altitude Photography (NHAP)
    (1980-1987) 158,000 CIR or 180,000 Pan
  • National Aerial Photography Program (NAPP)
    (since 1987) 140,000 CIR
  • NASA high altitude photography (since 1964)
    160,000-1120,000 PAN, COLOR, CIR
  • These images are archived by the Eros Data Center
    as part of the USGS Global Land Information
    System. To search archive http//edc.usgs.gov/we
    bglis

33
Aerial Photographic Sources
  • USDA (since 1955) mainly PAN of
    120,000-140,000. These photos are archived by
    the Aerial Photography Field Office
    http//www.fsa.usda.gov/dam/APFO/airfto.htm
  • National Archives and Records Administration
    archives older (pre- 1950s) aerial photography
    http//www.nara.gov/research/ordering/mapordr.html

34
Aerial Photographic Sources
  • National Ocean Survey (NOS) coastal photography
    (since 1945), color, scales of 110,000 -
    150,000
  • The photos are used for a variety of
    geo-positioning applications, which include
    delineating the shoreline for Nautical Chart
    creation, measuring water depths, mapping seabed
    characteristics, and locating obstructions to
    marine and air navigation.
  • http//mapfinder.nos.noaa.gov

35
Digital Orthophotography Sources
  • New Jersey 1995/97 2002 digital orthophotos are
    available from the USGS Eros Data Center and the
    NJ Office of Information Technology. Individual
    images can be downloaded
  • http//gisdata.usgs.net
  • http//njgin.nj.gov
  • Or viewed interactively http//mapping.usgs.gov

36
Contract Photography
  • Existing aerial photographs may be unsuitable for
    certain projects
  • Special-purpose photography - may be contracted
    through commercial aerial survey firms

37
Contracting Photography Considerations
  • Camera focal length
  • Camera format size
  • Photo scale/ground coverage desired
  • Film/filter
  • Overlap/sidelap
  • Photo Alignment/tilt
  • Seasonal considerations
  • Time-of-Day considerations/ cloud cover

38
Seasonal considerations
  • Cloud free conditions, ideally lt 10
  • Leaf-off spring/fall when deciduous tree leaves
    are off and ground free of snow used for
    topographic/soils mapping, terrain/landform
    interpretation
  • Leaf-on summer when deciduous trees are leafed
    out or late fall when various tree species may be
    identified by foliage color used for vegetation
    analyses

39
Time-of-day considerations
  • Quantity of light determined by solar elevation
    angle no shadows - 2 hrs around solar
    noon shadows desired early or late day
  • Spectral quality possibility of sun/hot
    spots causing image saturation

40
Flight Alignment
  • Flight lines are planned to be parallel
  • Usually in a N-S or E-W direction. For maximum
    aircraft efficiency, they should be parallel to
    the long axis of the study area (minimize
    aircraft turns).
  • Crab or drift should be minimized
  • Tilt , 2-3o for any single photo, average lt 1o
    for entire project

41
Example Flight planning for aerial photography
of submerged aquatic vegetation
  • Color film gives better water depth penetration

42
Example Flight planning for aerial photography
of submerged aquatic vegetation
  • Other considerations
  • Scales of 112,000 to 124,000 needed
  • Time of year late spring-early summer
  • Time of day sun angles 15-30o, generally early
    morning to reduce wind/surface waves
  • Tides - 2 hours of lowest tide

43
Example Flight planning for aerial photography
of submerged aquatic vegetation
  • GeoVantage Digital Camera
  • 4 bands Blue, Green, Red, NIR
  • Pixel Array Size 0.00465mm
  • Focal Length 12mm
  • Field of View 28.1o crossrange, 21.1o along
    range
  • Easily mounted on wheel strut
  • Coordinated acquisition with Inertial Measurement
    Unit to determine precise geodetic positioning to
    provide for georegistration and
    orthorectification

44
Example Flight planning for aerial photography
of submerged aquatic vegetation
  • What Flying Height (m) needed to resolve
    individual SAV beds of 1m wide x 10 m long (0.001
    ha in size)?
  • General Rule of Thumb GSD at a minimum of ½ the
    size of smallest feature. In this case need, GSD
    of 0.5m.
  • GSD array element size H
    .
  • focal
    length
  • Example array element size 0.00465mm
  • f 12 mm GSD 0.5m H ?
  • H 0.5m 12 mm / 0.00465mm 1290 m

45
Example Flight planning for aerial photography
of submerged aquatic vegetation
  • What will be the image width(m)?
  • Remember your basic trigonometry? Tan opposite
    / adjacent
  • Tan FOV/2 (1/2 image width)/H
  • Image width 2 tan14.05 1290m
  • 2 0.250 1290m
  • 645 m

FOV 28.1o
H 1290m
adj
opp
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