Lesson 2 Flashcards
Common components of an ariel image camera and how they compare to an everyday camera:
- a lens – to gather light
- a light-sensitive surface to record the image
- a shutter – controls entry of light
- camera body - a tight enclosure that holds the other parts together in the correct position
- a film magazine (aerial specific)
- a drive mechanism (aerial specific)
- a lens cone (aerial specific)
Metric Camera – also known as the cartographic camera
Current standard camera used for aerial photography
Designed to collect high-quality images
Analog images had limits to their use:
Recorded on paper or film
Brightness is proportional to the brightness in a scene
Difficulties storing, transmission, searching, and analysis of the images.
Advantages to switching to digital technology:
Do not require film and complex mechanisms
Links images to positional and navigational systems
Lens
gathers reflected light and focuses it on the focal plane to forma an image
A curved glass disk
The design, shape, size and arrangement of a lens controls the refracted light coming from the earth’s surface to maintain color balance and minimize distortions.
Spherical aberration
imperfections in lens shape
A source of error that distorts images and causes them to loss clarity
Usually not a severe problem in modern aerial photography due to the high quality of the lens.
Compound lenses
formed from many separate lenses of different shapes, sizes, and optical properties
Most aerial cameras use this lens
Designed to correct errors that may be present in any single component.
Optical Axis
Joins the centers of curvature of the two sides of the lens.
Image principal plane
the plane that passes through the center of the lens (the center of refraction)
Reminder that refraction passes throughout a lens
Nodal point
where the image principal plane intersects the optical axis.
Focal Point
the point where the lens forms an image of the distant object.
Chief ray
passes through the nodal point without changing direction.
The paths for all other rays are deflected by the lens
Focal plane
a plane passing through the focal point parallel to the image principal plane.
Aerial photographic cameras take images at such large distances that the camera focus can be fixed to _______ and have no need to change the focus of the lens.
infinity
Focal length
the distance from the center of the lens to the focal point
Usually measured in inches or millimeters
This applies to simple positive lens. Compound lenses have a different definition.
Focal length is different for all wavelengths
Blue light is brought to a focal point at a shorter distance than red or infrared wavelengths.
Chromatic aberration
affects the focal length for all wavelengths since they are not identical.
Causes the individual colors of an image to be out of focus in the image.
Focal length is different for all wavelengths
Blue light is brought to a focal point at a shorter distance than red or infrared wavelengths.
It is corrected in high-quality aerial cameras to assure the radiation used to form the image is brought to a common focal point.
Field Stop
a mask positioned just in front of the focal plane to control the field of view of a lens.
Aperture stop
positioned near the center of a compound lens
A mask with a circular opening with an adjustable diameter.
Controls the intensity of light at the focal plane but does not influence the field of view of the size of the image.
Aperture size is measured by the diameter of the adjustable opening.
Relative aperture
where focal length and aperture are measured in the same units of length.
Large number = aperture opening is small compared to the focal length
Small number = aperture opening is large compared to the focal length
Vignetting
an effect that creates a dark rim around the center of the aerial photograph
Since lenses for aerial cameras usually have wide fields of view, light reaching the focal plane from the edges of the field of view are typically dimmer that light reflected form objects positioned near the center of the field of view.
Antivignetting filter
used to even out the brightness across the photograph
A physical filter that is darker at the center and clearer at the edges/periphery.
Shutter
controls the length of time the film is exposed to light.
“intralens” or “between-the-lens”
the simplest shutters that are often metal blades and positioned between elements of the lens.
Allows the entire focal plane to illuminate simultaneously and presents a clear image with precise measurements.
Preferred for most aerial cameras
Focal plane shutter
near the focal plane, consists of a metal of fabric curtain positioned in front of the detector array.
The curtain is constructed with several slits
The choice of shutter speed by the operator selects the opening/slit that produces the desired exposure.
Image motion compensation
Used on aircrafts that operate at a low altitude and/or high speed to reduce the blur of an image.
Analog cameras - achieved by mechanically moving the film focal plane at a speed that compensates for the motion of the image in the focal plane.
Digital systems – achieved electronically
Allows for a wider range of conditions to be used while also preserving the detail and clarity of an image (lower altitudes, faster flight speeds, etc.)
Orientation classifications
Oblique aerial photographs
Vertical photographs
Oblique aerial photographs
images taken by cameras oriented towards the side of the aircraft.
Able to show very large areas in a single image
Usually not sued for analytic purposes
High Oblique – show the horizon
Low Oblique – aimed more directly toward the ground surface and do not show the horizon.
Vertical photographs
images taken while camera is aimed directly at the ground providing a map-like view.
Photogrammetry
the science of making accurate measurements from aerial photographs.
Fiducial marks
adjustable index marks manufactured to an analog aerial camera so that the positions of the index marks are recorded on the photograph when it is taken.
Usually 4 or eight total
Appear as silhouettes at the edges and/or corners of the photo.
Principal Point
the point where the lines that connect opposite pairs of fiducial marks intersect.
The intersection of the optical axis with the focal plane
Ground Nadir
the point on the ground vertically beneath the center of the camera lens at the time the photo was taken.
Photographic nadir
the intersection with the photograph of the vertical line that intersects the ground nadir and the center of the lens.
Calibration
the camera’s internal optics and positional accuracy of imagery for photogrammetric applications.
Isocenter
defined informally as the focus of tilt
The center point on the “hinge” of truly vertical photograph and the almost vertical image that was actually taken.
On a truly vertical photography, the isocenter, principal point, and the photographic nadir intersect.
Positional or geometric errors in vertical aerial photography:
Optical distortions
Tilt
Relief displacement
Optical distortions
errors caused by inferior camera lens, the camera malfunctions, or similar problem.
Tilt
caused by displacement of the focal plane from a truly horizontal position by aircraft motion.
This error can throw off the scales of the image.
Since all images have tilt, measurements in one area of the image run the risk of including systematic error.
Relief displacement
Objects directly beneath the center of the camera lens with be captured so that only the top of the object is visible while all other objects will show both their tops and their sides. These objects appear to lean outward from the central perspective of the camera lens.
Positional error, cannot be corrected by selecting better equipment or more careful operation.
Amount of displacement depends on:
- Height of the object
- Distance of the object from the nadir
- Uneven terrain
Pixels
patches of the ground in an image. Together they compose an image by recording photons reflected or emitted.
An image is composed of thousands of pixels.
Each pixel represents the brightness of a small region of the earth’s surface.
Recorded as a numeric value, usually the values of the multiple regions of the EM.
Digital cameras use an array of _______ positioned at the focal plane to capture an electronic record of the image.
detectors
Detectors apply either of the 2 alternative designs:
Charged-coupled devices (CCDs)
Complementary metal oxide semiconductor (CMOS)
Charged-coupled devices (CCDs)
Formed from light-sensitive material embedded in a silicon chip
Potential well – receives photons from the scene through an optical system designed to collect, filter, and focus radiation.
Can be as small as 1µm in diameter
Sensitive to the visible and near infrared regions.
Arrays – when the detectors are connected to each other.
- Linear array – when detectors are arranged in a single line
- Two-dimensional arrays – when detectors are arranged in multiple rows and columns
Transfer gate – controls the flow of data from the detectors since the detectors capture the photons and the intensity of the radiation.
Shift registers – microcircuits connected to detectors within an array.
- these permit charges received at each detector to be passed to adjacent elements temporarily recording the info until it is transferred to another portion of the instrument.
Compact and efficient in detecting photons and responds to brightness
Is robust and reliable for recording images
Complementary metal oxide semiconductor (CMOS)
Often used for camcorders and related products to record digital imagery
Less often used for capturing aerial imagery.
Provide fine detail at low cost and at low power requirements.
Exposes a single line at a time, and while that data is transferring, the next line is exposed.
Format
area in a camera, varied in size and shape depending on camera design.
Standard: 9in x 9in (230 mm x 230 mm)
Framing cameras (frame array cameras)
formed the standard for analog aerial camera designs
Does not transfer cleanly into the digital domain.
Digital Scanning of Analog Images
This form of image collecting has advantages for storage, transmission, and analysis.
Great for office use or casual use, but the positional accuracy and the low detail visible in the original, not good enough for scientific or photogrammetric applications.
Advantages of digital systems:
Doesn’t require film scanning to provide the digital format and is more of a direct path to analytical processing.
Better storage, processing, and transmission of data
Better operational costs
Versatile applications
Greater range of brightness in digital imagery
True multispectral coverage
Disadvantages of digital systems:
Varied camera designs to not replicate the optical and radiometric proprieties of an analog framing camera and need to have the brightness adjusted across the image to ensure the proper registration of the separate bands. Many experts consider digital imagery to be inferior to the high-quality metric cameras.
Typically, smaller footprints of digital images require more images relative to analog systems.
Linear systems are especially dependent on high-quality airborne GPS/internal measurement unit data.
Linear scanners have sensor models that are less widely supported in softcopy photogrammetric software.
Digital systems require high initial investments
There is less inherent stability than in metric film cameras – can require reflying.
Component-level calibration and quality control can be difficult
- Digital camera designs vary from manufacturer to manufacturer that calibration must be completed by the specific manufacturer rather than by a centralized service like analog cameras.
Emulsions
used by color films, sensitive over a range of wavelengths.
Max sensitivity lies in the red, green, and blue regions, but also sensitive to radiation beyond desired limits.
Digital sensors can be designed to have spectral sensitivities to focus on narrow ranges of wavelengths therefore providing high precision in measurement of color brightness.
- No practical for current aerial cameras because they are extremely expensive, difficult to manufacture, and may require long readout times to retrieve the image data.
Byer filter
a specialized filter that uses a single array to acquire data in the three primaries to select the wavelengths that reach each pixel.
- 50% receives green
- 25% receives red
- 25% receives blue
Disadvantage: the color filters reduce the amount of energy reaching the sensor and the interpolation required to construct the several bands reduces image sharpness.
Foven Technology
exploits the differential ability of the sensor’s silicon construction to absorb light.
Foveon detectors are designed as three separate detector layers encased in silicon:
- Blue-sensitive detectors at the surface
- Green-sensitive detectors below blue
- Red sensitive detectors below green
Allows each pixel to be represented by a single point that portrays all three primaries without using filters.
Not present in aerial imagery, there are concerns that colors captured deeper in the chip may receive weaker intensities of the radiation and may have higher noise levels.
Band combinations
a term used to refer to the assignment of colors to represent brightness in different regions of the spectrum.
Black-and-White Infrared imagery
Imagery acquired in the near infrared region are largely free of atmospheric scattering and show vegetation regions and land-water distinction.
One of the most valuable regions of the spectrum
Panchromatic
means “across the colors”, indicates that the visible spectrum is represented as a single channel (without distinguishing between the three primary colors).
This view provides a black-and-white image that records the brightness using radiation from the visible region without separating the colors (PAN).
Captured when the decision is made that the added detail will provide more value that a color representation would.
Blue can degrade the quality of the image, so some instruments are designed to capture green, red, and near infrared regions to provide a sharper image.
Natural-color assignment model (RGB Model)
blue is assigned as blue, green as green, and red as red.
False-color images
the colors on the image do not match their true colors in nature.
Used by analysts to represent patterns on imagery needed to attain specific information.
It is necessary to do this since remote sensing uses radiation outside of the visible spectrum and must use false colors to view this radiation.
Color infrared model (CIR)
creates a three-band color image by discarding the blue band form the visible spectrum and adding in near Infrared
Developed in WWII as camouflage detection film
- Detects differences between actual vegetation and surface painted to resemble vegetation.
Shows living vegetation and water bodies very clearly and reduces atmospheric effects.
- Important for studying agriculture, forestry, and water resources
Useful for high-altitude aerial photography
Drift
when a planes course is deflected by crosswind and the positions of ground areas shown by successive photographs form a “stairstep” affect
Crab
caused by correction of the flight bath to compensate for drift without a change in the orientation of the camera.
Forward Overlap
flight plans call for a certain amount of forward overlap to duplicate coverage by successive frames in a flight line.
- About 50-60% of each frame overlap
Conjugate principal points
when there is forward overlap of 50% or more and the image of the principal point of one image is visible in the next image in the flight line.
Flight line
several parallel strips of photography. Used when it is necessary to photograph large areas.
- Sidelap between adjacent flight lines may vary from about 5-15% to prevent gaps in coverage.
Holidays
the gaps that occur as pilots collect complete photographic coverage.
- Due to equipment malfunction, navigation errors, or cloud cover.
Stereoscopic Parallax
the difference in appearances of objects due to change in perspective.
The amount of parallax decreases as objects increase in distance from the observer.
Can be used as a basis for measuring distance or height by using a stereo triplet (using 3 adjacent frames in with 50% or more overlap).
- The overlapping images allows the entire area to be viewed in stereo
You can manually measure the parallax by:
- Tape photos together so the axis of the flight line is oriented from right to left
- Measure the distance between two principal points (X)
- Measure the distance between separate images of the base of the object as represented on the two images (Y). Subtract this distance from the found in (1) to get P.
- Measure top-to-top distances (B) and base-to-base distances (A), then subtract to find dp.
Orthophoto
shoes the photographic detail without the errors caused by tilt and relief displacement.
Orthophotoscope
an optical-mechanical instrument developed in the 1970s. Developed to optically project a corrected version of a very small portion of an aerial photograph.
- Mid-1980s – able to scan an entire image piece by piece to generate a corrected version of that image showing an image that has the same detail as the original aerial photo but without the geometric errors of tilt and relief displacement.
Orthophotomaps
shows the image in its correct planimetric form with place names, symbols, and geographic coordinates.
Routinely used in GIS
Digital Orthophoto Quadrangles (DOQs)
orthophotos prepared in a digital format designed to correspond to the 7.5-minute quadrangle of the USGS.
Either black-and-white or color
Prepared by from National Aerial Photography Program (NAPP)
Analytical Stereoplotters
first designed in 1920s, uses optical and mechanical instruments to reconstruct the geometry of the images at the time they were acquired so they can be viewed as stereo images.
Softcopy Photogrammetry
named because it does not require a hardcopy form of the photograph.
A digital version of the image is used through a series of mathematical models that reconstruct the orientation of each image to create planimetrically correct representations.
This is possible through the use of GPS and the aircraft’s navigational system to record the orientation of the photos.
Requires specialized computer software
Advantages: speed and accuracy, generates output data easily integrated into other systems including GIS.
Multi-ray photogrammetry
can be used to exploit the collection of so many independent views of the same features to extract highly detailed positional elevation data.
Advantages of satellite sensors over aerial platforms:
Observe a large area in a single image
Fine detail
Systematic, repetitive coverage
Perturbed orbits
satellites follow these orbits. These orbits are due to the distortion of the earth’s gravitational field by earth’s shape.
Normal orbit forms an ______ with the earth being it’s center.
ellipse
Apogee
point farthest from the earth in the satellite’s orbit
Perigee
point closest to the earth in the satellite’s orbit
Ascending node
point where the satellite crosses the equator moving south to north
Descending node
point where the satellite crosses the equator moving south to north
Inclination
the angle that a satellite track forms with respect to the equator at the descending node.
Can also be defined as the angle between the earth’s axis at the north pole and line drawn perpendicular to the plane of the satellite orbit.
Orbit Period
the time it takes for a satellite to complete one orbit.
Geostationary orbit
if the satellite is positioned in the equatorial plane, it remains stationary with respect to the earth’s surface.
Ideal for meteorological and communications satellites designed to maintain a constant position.
Sun-synchronous orbits
designed to reduce variations in illumination by systematically moving the orbital track so that it moves westward 360 degrees each year.
Allows the satellite to observe each part of the earth at the same local sun time each day and thereby removing time of day as a source of variation in illumination.
Most satellites acquire images between 9:30 – 10:30 a.m. local sun time
Support Subsystems
Units that are necessary to maintain proper operation of the sensors
Attitude control subsystem
maintained orientation of the satellite with respect to the earth’s surface and with respect to the orbital path.
Orbit adjust subsystem
maintained the orbital path within specified parameter after the initial orbit was attained.
Power subsystem
supplied electrical power required to operate all satellite system by means of two solar panels and eight batteries.
Thermal control subsystems
controlled the temps of satellite components by means of heaters, passive radiators, and insulation.
Communications and data-handling subsystem
provides microwave communications with ground stations for transmitting data from sensors, commands to satellite subsystems, and info regarding satellite status and location.
Multispectral Scanner Subsystem (MSS)
a scanning instrument utilizing a flat oscillating mirror to scan from west to east to produce a ground swath of 185 km.
became the primary Landsat sensor
Tailored to provide multispectral data without as much concern for positional accuracy.
Found to be good quality images and clearly demonstrated the merits of satellite observation for acquiring earth resources data.
Versions of the MSS were placed on Landsats 4 and 5
Instantaneous field of view (IFOV)
part of a scanning unit, can be informally defined as the ground area viewed by the sensor at a given instant in time.
The IFOV for the MSS is 79m x 79m
MSS scene
defined as an image representing a ground area approximately 185 km in the east-west (across-track) direction and 170 km in the north-south (along-track) direction.
A MSS scene is an array of pixel values consisting of about 2,400 scan lines, each composed of 3,240 pixels.
Skew
an effect of an image acquired due to spacecraft movement and rotation of the earth.
When these effects are removed, the image assumes the shape of a parallelogram.
Fill pixels
added to preserve the correct shape of the image
Convey not information, they are assigned values of zero as necessary to attain the desired shape
False-color composite
when the green, red, and infrared bands are combined into a single color image.
- Green = red band 2
- Blue = green band
- Red = near infrared band
– Vegetation appears bright red, turbid water is blue, and urban areas are gray or pinkish gray.
Information provided in the annotation block at the lower edge of the image:
Date
Format center and ground nadir in degrees and minutes of lat and long.
Ground location of the center point of the image
Spectral band given in form “MSS 1”
Sun angle and sun elevation in degrees
Solar elevation
Azimuth of the solar beam from true north at the center of the image.
Scene ID – unique number that specifies the scene and band
Worldwide reference system (WRS)
a concise designation of nominal center points of Landsat scenes
Used to index Landsat scenes by location
There are 233 north-south paths represented by long and 119 rows represented by lat
- The combination of path number and row number uniquely identifies a nominal scene center.
Thematic mapper (TM)
can be considered as an upgraded MSS
Carried on the Landsats 4 and 5 as a replacement for MSS.
Same concept of MSS but a more complex design
Finer resolution, improved geometric fidelity, greater radiometric detail and more detailed spectral information in more precisely defined spectral regions.
Records 7 spectral bands
Provides a much larger range of brightness values than MSS
Acquires data as it moves in both east-west and west-east directions
Has a slower speed of mirror movement allowing move length of time for the detectors to respond to the brightness in the scene.
Landsat-Class Sytems
satellite systems designed to acquire regional overviews at moderate-to-coarse levels of spatial detail.
- Landsat 7 and Enhanced Thematic Mapper Plus
- SPOT – SPOTs 1, 2, 3, 4, - and 5
- India Remote Sensing
– IRS-1A, IRS-1B, IRS-1C, IRS-1D, Resourcesat 1, and Cartosat-1
Broad-Scale Coverage
satellite systems that provide coarse levels of detail for very large regions
- AVHRR
- SeaWiFS
- VEGETATION – an auxiliary sensor on SPOT 4 and 5
Fine-Resolution Satellite Systems
satellite systems designed to provide detailed coverage for small-image footprints
- GeoEye-1
- IKONOS
- QuickBird
- WorldView-1
- WorldView-2
- RapidEye
- ImageSat International
