Radiometric and Geometric Correction of Multiband Optical Data

Introduction

            Assignment tasks in remote sensing, initial processing on the raw data is usually carried out to correct for any distortion due to the characteristics of the imaging system and imaging conditions. Depending on the user�s requirement, some standard correction procedures may be carried out by the ground station operators before the data is delivered to the end-user. This process includes radiometric correction to correct for uneven sensor response over the whole image and geometric correction to correct for geometric distortion due to Earth�s rotation and other imaging condition (such as oblique viewing). The image may also be transformed to conform to a specific map projection system. Furthermore, if accurate geographical location of an area on the image needs to be known, ground control points (GCP�s) are used to register the image to a precise map (geo-referencing). (CRISP, http:// sol.oc.ntu.edu.tw/homepage/sq/crisp.htm)

            The purpose of this assignment is to introduce the procedure of image quality checking and geometric correction within ERMapper. An uncorrected SPOT multiband optical image of Bogor will be georeferenced to corrected and projected road and stream vector for the same area.

The following data has been made available through the �shared� directory:

Objectives

        จ    Image Quality Checking: Learn to check for dropped lines, bit slips and overall haze or noise in an image.

        จ    Geometric Correction: Learn and practice the required procedures for GCP collection. Perform �Image to Vector� correction. Examine            the quality of the georeferenced image by determining how the corrected vectors overlay the corrected image.

        จ    Metadata: Initial good metadata practices by creating and updating metadata records containing information about the raw data, sensor,            spectral bands, processing procedures and dates.

Any image involves radiometric errors as well as geometric errors, these errors have to corrected, then should understand about concept of radiometric and geometric correction.

    Descriptive information

1. Radiometric correction

            Radiometric correction is to avoid radiometric errors or distortions, while geometric correction is to remove geometric distortion. When a sensor on board an aircraft or spacecraft observes the emitted or reflected electro-magnetic energy, the observed energy does not coincide with the energy emitted or reflected from the same object observed from a short distance. This is due to the sun's azimuth and elevation, atmospheric conditions such as fog or aerosols, sensor's response etc. which influence the observed energy. Therefore, in order to obtain the real irradiance or reflectance, those radiometric distortions must be corrected. (Murai, 1993)

                   Radiometric correction is classified into the following three types

                  (1) Radiometric correction of effects due to sensor sensitivity

In the case of optical sensors, with the use of a lens, a fringe area in the corners will be darker as compared with the central area. This is called vignetting. Vignetting can be expressed by cost, where is the angle of a ray with respect to the optical axis. n is dependent on the lens characteristics, though n is usually taken as 4. In the case of electro-optical sensors, measured calibration data between irradiance and the sensor output signal, can be used for radiometric correction.

(2) Radiometric correction for sun angle and topography a. Sun spot

The solar radiation will be reflected diffusely onto the ground surface, which results in lighter areas in an image. It is called Sun spot: The sun spot together with vignetting effects can be corrected by estimating a shading curve which is determined by Fourier analysis to extract a low frequency component. Shading: The shading effect due to topographic relief can be corrected using the angle between the solar radiation direction and the normal vector to the ground surface.

(3) Atmospheric correction

The solar radiation is absorbed or scattered by the atmosphere during transmission to the ground surface, while the reflected or emitted radiation from the target is also absorbed or scattered by the atmosphere before it reaches a sensor. The ground surface receives not only the direct solar radiation but also skylight, or scattered radiation from the atmosphere. A sensor will receive not only the direct reflected or emitted radiation from a target, but also the scattered radiation from a target and the scattered radiation from the atmosphere, which is called path radiance. Atmospheric correction is used to remove these effects

2. Geometric correction

Digital images collected from airborne or space borne sensors often contain systematic and unsystematic geometric errors. Some of these errors can be corrected by using ephemeris of the platform and known internal sensor distortion characteristics. Other errors can only be corrected by matching image coordinates of physical features recorded by the image to the geographic coordinates of the same features collected from a map or global positioning system (GPS). (Ramsey, http://www.nr.usu.edu/Geography-Department/rsgis/RSCC/v6.2/6-2.html)

Geometric errors that can be corrected using sensor characteristics and ephemeris data include scan skew, mirror-scan velocity variance, panoramic distortion, platform velocity, and perspective geometry. Some errors that can only be accounted for by the use of GCP's include the roll, pitch, and yaw of the platform and/or the altitude variance.

Geometric correction is undertaken to avoid geometric distortions from a distorted image, and is achieved by establishing the relationship between the image coordinate system and the geographic coordinate system using calibration data of the sensor, measured data of position and attitude, ground control points, atmospheric condition etc.

The steps to follow for geometric correction are as follows (Murai, 1993)

(1) Selection of method

After consideration of the characteristics of the geometric distortion as well as the available reference data, a proper method should be selected.

(2) Determination of parameters

Unknown parameters, which define the mathematical equation between the image coordinate system and the geographic coordinate system, should be determined with calibration data and/or ground control points.

(3) Accuracy check

Accuracy of the geometric correction should be checked and verified. If the accuracy does not meet the criteria, the method or the data used should be checked and corrected in order to avoid the errors.

(4) Interpolation and resampling

Geo-coded image should be produced by the technique of resampling and interpolation. There are three methods of geometric correction as mentioned below.

Definitions

There are a number of terms that deal with geographic correction of remotely sensed data. These terms are sometimes used interchangeably and in confusion with the other. (Ramsey,_)

Ephemeris: Any tabular statement of the assigned places of a celestial body for regular intervals. For example, the solar ephemeris provides the exact location of the sun at any given time of the day or year.

Geocoding: Geographical referencing or coding of data. (Jensen, 1986)

Ground Control Point (GCP): A specific pixel on an image or location on a map whose geographic coordinates are known. GCP's are used to correct geometric distortion in an image by matching image coordinates with map coordinates. Image and map coordinates are used to compute the transformation matrix for use in rectifying the image.

Linear Transformation: The transformation of coordinates from one system to another (image to map) using a linear algebraic (1st order polynomial) formula

Non-Linear Transformation: The transformation of coordinates from one system to another (image to map) using a non-linear algebraic (Nth order polynomial) formula

Rectification: The process by which the geometry of an image is made planimetric. (Jensen, 1986) And the process of using GCPs to transform the geometry of an image so that each pixel correspond to a position in a real world coordinate system. This process is sometimes called �warping� or �rubbersheeting�.

Registration: The process of geometrically aligning two or more sets of image data such that resolution cells for a single ground area can be digitally or visually superimposed. A map coordinate system may not be involved.

Resampling: The process of extrapolating data values to a new grid. Resampling is the step in rectifying an image that calculates pixel values for the rectified grid from the original data grid.

Root Mean Square Error (RMS): The RMS is the error term used to determine the accuracy of the transformation from one coordinate system to another. It is the difference between the desired output coordinate for a GCP and the actual

  3. Metadata

Metadata are data about data. It is a way of documenting information about datasets. The information contained in metadata will document the creation of a dataset and give you an idea of what the cartographic product to which it is attached was designed to do. The following links will take you to further information and examples of metadata and explain the who, what, where, why, and when of metadata. (Oklahoma State University, www.seic.okstate.edu/gis/metadata.html) Developments in standards for metadata (data about data) that include information about the source, method of recording, resolution, data of collection, data model, and data type are making it easier for user to judge if the data offered are suitable and reliable for their purpose. (Burrough and Mcdonnell, 1998)

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