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
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
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
Mapping or metric camera
• Single lens frame camera
• High geometric quality
• Film format is 230 mm (~9 in)
on a side
Keystone’s Wild RC-10
• Focal length of 152 mm
mapping camera
common
• Fiducial marks for later
registration and defining
principal point of the photo
B&W NAPP photo
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
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/
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: B&W 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/
Overlapping Stereophotography
• Overlapping
photography is needed
to determine parallax
and stereo/3D viewing
• Endlap - ~60%
• Sidelap - ~20-30%
Pushbroom Scanning vs. 2D Framing
Graphics from http://www.gis.leica-geosystems.com/products/documents/ADS40_product_description.pdf
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
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 1:25,000 means that
a length of 1 unit of measurement on the
photo represents 25,000 units of
measurement on the ground
Small vs. Large Scale
• Small scale: larger denominators
objects appear small on the image
image covers larger ground area
e.g. 1:120,000
• Large scale: smaller denominators
objects appear large on the image
image covers smaller ground area
e.g. 1:10,000
Alternative ways to express
Photographic Scale
• 1:24,000 can be expressed as 1 in. = 2,000ft
1
=
1 in * 12in = 12 in =
24,000
24,000 in 1ft 24,000 ft
1:100,000 same as 1 cm = 1 km
1:60,000 same as 1 in = 0.95 mi
1:300,000 same as 1 in. = 4.7 mi
1:1,000,000 same as 1 in = 15.8 mi
1 in
2,000 ft
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
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
(2,500 m - 400 m) 1000 mm
RF =
1
10,000
or 1:10,000
=
210 .
2,100,000
Scale determination from photoground 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
Scale determination from PhotoMap 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
6 cm * 50,000
=
3.2 cm
300,000 cm
=
1
93,750
Effect of flying height on ground coverage
H’1 > H’2
H’1
D1 > D2
H’2
x
D2
D1
Adapted from Lillesand & Kiefer, 2nd edition
Effect of focal length on ground coverage
f1
f2
f 1 > f2
H’1
D1 < D2
x
D1
D2
Adapted from Lillesand & Kiefer, 2nd edition
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. H’1 = 2H’2 D1 = 2D2
A1 = 4A2
Ground Coverage example
Case 1
film size = 9.0” = 230mm
f1 = 210 mm
H’ = 12,200 m
Scale = ?
D=?
1 = 210 mm =
MS 12,200m
Case 2
film size = 9.0”
f2 = 152 mm
H’ = 12,200 m
Scale = ?
D=?
1
58,000
D = 230mm x 58,000 = 13.3km
1 = 152 mm =
MS
12,200m
1 .
80,000
D = 230mm x 80,000 = 18.4km
National High Altitude program
(NHAP)
• Flying Height, H’ = 12,200 m
• color IR camera
f = 210 mm
scale 1:58,000
area per frame 13.3 x 13.3 km
• panchromatic camera
f = 152 mm
scale 1:80,000
area per frame 18.4 x 18.4 km
Ground Sample Distance (GSD)
In digital camera systems interested in Ground Sample Distance =
the size of the individual camera pixels projected onto the ground
array element size * H’
GSD =
.
focal length
Example: array element size = 0.009mm
f = 28 mm
GSD(m) =
H’ = 1800m
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.
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
f
Adj
= H’
Opp =
½W
Example: Leica ADS40
q= 64o
if H’ = 2880 m
H’
W = 2 x 2880m tan32o = 3600m
q/2
W
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
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.
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
Radial Displacement
• A photo’s 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.
Maps vs. Aerial Photos
• Maps: Scale is constant
No relief displacement
• Photos: Scale varies with elevation
Relief displacement
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
Digital Orthophotography: the new standard
Distortions removed, rectified to a standard projection/coordinate
system and in digital form for ready input to a GIS
2002
1 foot ground spatial resolution per pixel
UTM or State Plane
Aerial Photographic Sources
• National High Altitude Photography (NHAP):
(1980-1987) 1:58,000 CIR or 1:80,000 Pan
• National Aerial Photography Program (NAPP):
(since 1987) 1:40,000 CIR
• NASA high altitude photography: (since 1964)
1:60,000-1:120,000 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/webglis
Aerial Photographic Sources
• USDA: (since 1955): mainly PAN of
1:20,000-1:40,000. These photos are
archived by the Aerial Photography Field
Office
http://www.fsa.usda.gov/dam/APFO/airfto.h
tm
• National Archives and Records
Administration archives older (pre- 1950’s)
aerial photography
http://www.nara.gov/research/ordering/map
ordr.html
Aerial Photographic Sources
• National Ocean Survey (NOS) coastal
photography: (since 1945), color, scales of
1;10,000 - 1:50,000
• The photos are used for a variety of geopositioning 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
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
Contract Photography
• Existing aerial photographs may be
unsuitable for certain projects
• Special-purpose photography - may be
contracted through commercial aerial
survey firms
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
Seasonal considerations
• Cloud free conditions, ideally < 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
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
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 < 1o
for entire project
Example: Flight planning for aerial
photography of submerged aquatic vegetation
• Color film gives better
water depth
penetration
Example: Flight planning for aerial
photography of submerged aquatic vegetation
•
•
•
•
Other considerations
Scales of 1:12,000 to 1:24,000 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
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
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’ = 0.5m * 12 mm / 0.00465mm = 1290 m
.
H’ = ?
Example: Flight planning for aerial
photography of submerged aquatic vegetation
• What will be the image width(m)?
FOV = 28.1o
• 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
adj
H’ = 1290m
opp