ACRES optical data processing options

This document provides an overview of ACRES satellite data processing options for optical data, e.g. all Landsat sensors and PRISM and AVNIR-2 sensors from ALOS. In depth technical information is available by contacting ACRES.

1. Introduction

The data collected and transmitted to Earth from an Earth observation satellite is in a form unsuitable for use by customers. To enable visualisation, ACRES processes this raw data in varying degrees to generate products which cover a range of processing levels. ACRES generally processes raw data only after a client has placed an order for a specific product. This is due to the large quantities of data received from a satellite and the large range of product options available. Storing data in its raw form is more efficient in terms of storage and it also facilitates improved processing of data as processing systems improve over time.

Processing options

The processing of satellite data involves two basic types of corrections:

• Radiometric (brightness calibration)
• Geometric (geometry, location)

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2. Radiometric corrections

General

The sensing instruments on Earth observation satellites contain a multitude of detectors for each band. For example, the Landsat 5 TM instrument has 16 detectors for each band. The detectors convert the observed radiance (the reflected energy from the Earth's surface) into an electronic signal. However, not all detectors will do this in an identical manner. This may lead to "striping" across the image.

The most common method for correcting the striping is based on the assumption that the statistics of the detector signals (brightness values) will be the same for all detectors. The success of this method will rely on a good sample of imagery being used to generate the statistics of the detector brightness values

A reference detector for each band is then chosen and the brightness values for the other detectors adjusted to match the brightness values of the reference detectors.

ACRES radiometric corrections

The radiometric correction of imagery described above is often referred to as relative radiometric calibration. This is the type of calibration applied to ACRES optical data.

Adjacent digital scenes of the same acquisition date will each exhibit similar brightness values for the same ground feature. However, if the acquisition dates are different, there is more likely to be a significant difference between the brightness values. These differences in brightness values may be due to changes in atmospheric conditions, sun angles and reflectance characteristics of ground surface features between the dates of acquisition.

• For details on the radiometric correction used for ACRES Landsat 5 TM products, see Radiometric corrections for ACRES Landsat 5 TM products.

The absolute radiometric calibration of data involves the conversion of the pixel brightness values into units of spectral radiance. This type of calibration is not done for any ACRES products, and if required, must be performed by the user.

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3. Geometric corrections

General

The raw data acquired by the sensor is transmitted and recorded as a sequential stream of pixels with no image structure. The geometric correction process restores this data to a two dimensional image using sets of corrections to enable it to be interpreted as a spatial representation of the Earth's surface.

The corrections are based on mathematical models which attempt to account for the various physical factors involved in the imaging process. The models are:

Sensor model which accounts for:

• Band-to-band alignment
• Detector and band offsets
• Detector delay
• Line length and mirror scan non-linearity

Satellite model which accounts for:

• Sensor-to-spacecraft alignment
• Spacecraft altitude and attitude
• Spacecraft velocity

Earth model which accounts for:

• Earth rotation
• Panoramic distortion
• Geodetic datum and map projection
• Absolute position - using Ground Control Points (GCPs)
• Relief displacement - using a Digital Elevation Model (DEM)

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Path oriented and map oriented

ACRES products may be generated as either path oriented or map oriented.

Path oriented

For path oriented products the imagery (rows and columns of pixels) is still aligned with the direction of the satellite orbit (which is about ten degrees East of North for optical sensors on a descending swath over Australia). These products are not suitable for direct input into a GIS system, and must first be "rotated" by the user to align with a map grid.

Map oriented

In map oriented products, the imagery (rows and columns of pixels) has been transformed to a map grid, and the rows and columns of image pixels are resampled to align with map-grid North. In this resampling process, the resultant pixel size of ACRES products may change from the original pixel size.

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Systematic, precision and ortho corrections

The level of geometric correction applied to a satellite image can also be divided into three other categories labelled systematic, precision and ortho corrected.

Systematic products

For systematic products the geographic location of the image is determined using a satellite model based on information about spacecraft position and attitude. This information is transmitted with the imagery, or provided by the satellite operator at the time of, or following image acquisition. It allows the imagery to be located to a limited absolute accuracy (ie. its position on the Earth).

Precision products

If precision products are offered, the satellite model is refined by the use of Ground Control Points (GCPs). The accuracy of these products depends on the accuracy of the GCPs used.

Orthocorrected products

Orthocorrected products are Precision products where the effects of relief displacement are corrected with the use of a Digital Elevation Model (DEM). The accuracy of these products depends on the accuracy of the GCPs and on the resolution of the DEM.

If precision or orthocorrected products are required over non-Australian territories, customers must supply the appropriate maps or GCPs and/or DEM.

Resampling

As part of the image correction process, brightness values must be derived for each pixel in the image output. The value is interpolated from the uncorrected satellite image. This interpolation exercise is known as resampling.

More information on resampling and resampling options.

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4. Details of processing options

ACRES has three main processing options:

• Orthocorrected Image;
• Map Oriented Image; and
• Path Image.

ACRES has two additional "non standard" options:

• Raw Image; and
• Raw (0) Data.

If available, ACRES recommends its Orthocorrected image for most applications where it can be readily and accurately related to a geographic location, and used with other geographic information.

Please check with ACRES or your preferred distributor for the most appropriate processing option for your application.

ACRES processing options

Main Options	Description
Orthocorrected Image (not currently available for ALOS)	Map oriented with systematic geometric corrections refined with the use of GCPs and the best DEM available to ACRES (which is equal to or better than the Geoscience Australia 9 second National DEM V2). Two dimensional resampling to align with a map grid. This is the most map-accurate product, suitable for input into a GIS.
Map Oriented Image	Map oriented with systematic radiometric and geometric corrections applied to the data. Two dimensional resampling to align with a map grid.
Path Image	Satellite path oriented with systematic radiometric and geometric corrections applied to the data. Two dimensional resampling to fit a specific Earth datum and map grid.
Non-standard Options
Raw Image	Radiometrically corrected but no geometric corrections.
Raw Data	No radiometric or geometric corrections. Not recommended for customer use

More details on the processing of optical data from specific sensors can be found at:

ACRES Landsat Processing Options.
ACRES ALOS Processing Options.

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5. Framing of data

ACRES products are extracted from a continuous swath of data collected by the orbiting satellite. Products can be framed from this swath in a variety of ways. ACRES and its distributors are experienced in selecting the most appropriate framing for your requirements. In some cases we will request a map of your area of interest to ensure correct framing.

The ACRES Digital Catalogue also provides the ability to view the framing of data products within the satellite swath. It also allows for the export of mid/mif files to facilitate viewing of quicklook images in MapInfo software.

Framing of Landsat products.
Framing of ALOS products.