Intro to Remote Sensing- Airborne Remote Sensing

Camera Systems

Passive optical sensors that use a lens (B) to acquire a snapshot of an area (A) and to form an image at the focal plane (C)

• Optical Sensors: 0.3-15 μm

Vantage Point

The orientation of the camera relative to the ground during acquisition; camera orientation

Optical Axis

The straight, imaginary line passing through the geometric center of an imaging sensor's lens system along which light rays are focused to form an image

Vertical Aerial Photography

Obtained when the camera’s optical axis is within ±3° of being vertical to the earth’s surface

• Used to create planimetric, topographic, and orthophotomaps, and DEMs using fundamental photogrammetric principles

Oblique Aerial Photography

Obtained if the camera’s optical axis deviates more than 3° from vertical

• Useful for covering very large areas in a single image and for depicting terrain relief and scale

Flight Height (Altitude)

Altitude of the camera (H) above ground level (AGL) for a given focal length

• The higher the altitude, the larger the area

• The lower the altitude, the smaller the area

Focal Length (f)

Distance between the lens and focal plane

Scale

View Angle

The field of view of a camera

• Given the same flight height, viewing angle is determined by the focal length

• The smaller the focal length, the larger the view angle

Ground Coverage

The actual area on the ground that one air photograph covers

• Determined by the altitude and the camera’s viewing angle

Advantages and Disadvantages of Aerial Photography (Films)

• Advantages

  • High resolution (ground)

  • Flexibility

  • High geometric reliability

  • Relatively inexpensive

• Disadvantages

  • Day light exposure (10:00 am - 2:00 pm) required

  • Poorer contrast at shorter wavelengths

  • Film non-reusable

  • Inconvenient

  • Inefficient for digital analysis

Panchromatic Imaging

Sensor is a single channel detector sensitive to radiation in a broad wavelength range

• Where the wavelength range coincides with the visible range, the resulting image resembles a “black-and-white” photograph

Multispectral Imaging

With multispectral, or multiband data, there are several layers, or values for each pixel, each representing a different “cannel/band” or reflectance in a wavelength spectrum

• Each “band” or “channel” is sensitive to radiation within a different band of wavelength

Displaying Multispectral Bands Separately

• These bands can be looked at separately

• One band from a multispectral image would be displayed as a grayscale image, with each pixel represented by a grayscale value

• These bands can be combined to make “composite” images by assigning three of the bands to the red, green, and blue channels (on computer monitor)

Color Types

• Additive Primaries- red, green, and blue

• Subtractive Primaries- cyan, magenta, and yellow

• Displaying image is based on additive mixing of primary colors

True Color Composite

Bands are assigned to color channels in such a way that colors in the image roughly correspond with the colors in the real world

False Color Composite

Shows colors that don’t really exist in that location

• Ex. Color Infrared Composite