applied to each image pixel to correct several types of distortions. Some image
processing operations, such as geometric correction (translating pixel coordinates
into a known geographic coordinate system, projection and datum), are essential if
you are planning to link field data or other spatial data with your remotely sensed
images. A good starting point to explain what these are and why they are essential
can be found at: www.ga.gov.au/earth-monitoring/geodesy/geodetic-datums.html.
Additional processing operations (e.g., atmospheric correction) are required if the
image is going to be used to estimate biophysical properties of the water column or
corals (e.g., depth and pigment concentrations).
Raw image data: This is the first output from an imaging sensor, which typically has no coordinate system, projection or datum, and cannot be used or displayed with other spatial data such as field survey GPS points. The image pixel
values also represent relative measures of reflected light, and cannot be related to
light interactions on the water surface or reef. Nonetheless, this data can still be
used for basic visual assessments.
Corrected data (geometric, radiometric and atmospheric): These are the first
stages in image processing, referred to as image preprocessing steps. Geometric
correction involves aligning the image to an established coordinate system, projection and datum, which allows the image to be used or overlaid with other spatial
data and field data. An accuracy or error level should be provided as part of this
correction. Radiometric correction translates the relative pixel values to absolute
measures of radiance per unit wavelength of light. Atmospheric correction
removes atmospheric effects and thereby transforms the radiometric values into
surface radiance or reflectance. This allows field based measurements or biophysical parameters to be estimated. In some cases, additional corrections may be
required to remove sunglint or attenuation due to the water column.
1.3.2 Processing Types
Transforming photographic, multispectral and hyperspectral images from preprocessed, or corrected, images to maps showing specific coral reef biophysical
properties requires the application of manual and/or software driven image processing operations. Details of these operations and their output map products for
reef science and management are provided in Chaps. 2– 4. This section introduces
b Fig. 1.6 Complete remote sensing image processing flow from image collection to map
production. (Source Phinn et al. 2010) Steps in the processing sequence: a Browse image from
Google Earth (Landsat TM/QuickBird combination); b Raw QuickBird image with no
corrections; c Corrected QuickBird image after atmospheric and air–water interface corrections:
d Georeferenced QuickBird image after atmospheric and air–water interface corrections; e Fully
corrected image d, with non-reef areas masked out; f Shallow water and exposed reef image with
calibration and validation field data; g Benthic cover map produced by image classification of f;
h Benthic cover map overlaid on the original image
1 Visible and Infrared Overview
19
processing operations, such as geometric correction (translating pixel coordinates
into a known geographic coordinate system, projection and datum), are essential if
you are planning to link field data or other spatial data with your remotely sensed
images. A good starting point to explain what these are and why they are essential
can be found at: www.ga.gov.au/earth-monitoring/geodesy/geodetic-datums.html.
Additional processing operations (e.g., atmospheric correction) are required if the
image is going to be used to estimate biophysical properties of the water column or
corals (e.g., depth and pigment concentrations).
Raw image data: This is the first output from an imaging sensor, which typically has no coordinate system, projection or datum, and cannot be used or displayed with other spatial data such as field survey GPS points. The image pixel
values also represent relative measures of reflected light, and cannot be related to
light interactions on the water surface or reef. Nonetheless, this data can still be
used for basic visual assessments.
Corrected data (geometric, radiometric and atmospheric): These are the first
stages in image processing, referred to as image preprocessing steps. Geometric
correction involves aligning the image to an established coordinate system, projection and datum, which allows the image to be used or overlaid with other spatial
data and field data. An accuracy or error level should be provided as part of this
correction. Radiometric correction translates the relative pixel values to absolute
measures of radiance per unit wavelength of light. Atmospheric correction
removes atmospheric effects and thereby transforms the radiometric values into
surface radiance or reflectance. This allows field based measurements or biophysical parameters to be estimated. In some cases, additional corrections may be
required to remove sunglint or attenuation due to the water column.
1.3.2 Processing Types
Transforming photographic, multispectral and hyperspectral images from preprocessed, or corrected, images to maps showing specific coral reef biophysical
properties requires the application of manual and/or software driven image processing operations. Details of these operations and their output map products for
reef science and management are provided in Chaps. 2– 4. This section introduces
b Fig. 1.6 Complete remote sensing image processing flow from image collection to map
production. (Source Phinn et al. 2010) Steps in the processing sequence: a Browse image from
Google Earth (Landsat TM/QuickBird combination); b Raw QuickBird image with no
corrections; c Corrected QuickBird image after atmospheric and air–water interface corrections:
d Georeferenced QuickBird image after atmospheric and air–water interface corrections; e Fully
corrected image d, with non-reef areas masked out; f Shallow water and exposed reef image with
calibration and validation field data; g Benthic cover map produced by image classification of f;
h Benthic cover map overlaid on the original image
1 Visible and Infrared Overview
19