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CHAPTER 6 SECTIONS > Scientific Theory | Spatial | Temporal | Radiometric | The Landsat Niche


6.2 Spatial Characteristics

Spatial resolution is the resolving power of an instrument needed for the discrimination of features and is based on detector size, focal length, and sensor altitude. More commonly used descriptive terms for spatial resolution are ground sample distance (GSD) and instantaneous field of view (IFOV). The IFOV, or pixel size, is the area of terrain or ocean covered by the field of view of a single detector. The ETM+ ground samples at three different resolutions; 30 meters for bands 1-5, and 7, 60 meters for band 6, and 15 meters for band 8. Figure 6.3 illustrates the ETM+ IFOV for bands 1-5 and 7 relative to other sensors and a football field. IKONOS, the recently launched Space Imaging sensor, has an IFOV of 1 meter. The French SPOT panchromatic sensor an IFOV of 10 meters whereas the SPOT multispectral (XS) sensor has an IFOV of 20 meters. ETM+ has an IFOV of 30 meters for bands 1-5, and 7 of 30 meters while the Indian Remote Sensing Satellite (IRS) has an IFOV of 36.25 meters.


ETM+ Spatial Resolution

Figure 6.3 - ETM+ Spatial Resolution Relative to Other Sensors

A standard WRS scene covers a land area approximately 185 kilomenters (across-track) by 180 kilometers (along-track). A more precise estimate for actual scene size can be calculated from the 0R product image dimensions. These are listed in table 6.1

Table 6.1 Image Dimensions for a Landsat 7 0R Product
Band
Number
Resolution
(meters)
Samples
(columns)
Data Lines
(rows)
Bits per
Sample
1-5, 7 30 6,600 6000 8
6 60 3,300 3,000 8
8 15 13,200 12,000 8

It is natural to assume that one could determine a scene's spatial extent by multiplying the rows and columns of a scene by the IFOV. This would lead to a scene width of 198 kilometers (6600 samples * 30 meters) and a scene length of 180 kilometers (6000 lines * 30 meters). While this calculation applies to scene length, the scene width calculation is more complicated due to the presence of image buffers and the staggered image bands in the 0R product. Left and right image buffers were placed in the 0R product to accommodate a possible increase in scan line length over the mission's life. The staggered image bands result from the focal plane design which LPS accounts for by registering the bands during 0R processing. The end result is an increasing amount of zero-fill preamble according to the band order on the ground projected focal plane array.

The detector offsets determine the amount of zero fill preamble for each band. These are listed in Table 6.2 and can also be found in the Calibration Parameter File. Coincident imagery for all 8 bands starts at pixel location 247 for the 30 meter bands. One need only to look at at the reverse scan odd detector offset for band 6 to see that this is true. This number, 116, is actually in 60 meter IFOVs which translates to 232 30 meter pixels. Another 14 pixels must be added to this number to account for the seven minor frames of image data pre-empted by time code. Coincident imagery for all 8 bands ends at pixel location 6333 for the 30 meter bands. This number is determined by looking at the reverse even detector offset for band 8. Add to this number the value 12,626 which represents the number of band 8 pixels per line (6313 minor frames times 2). The total, 12,666, is halved to put the ending pixel number into 30 meter units. The number of coincident images pixels in a scan is therefore 6087 (6333 - 247 + 1). The nominal width for a scene is therefore 182.61 kilometers (6087 * 30 meters).


Table 6.2 ETM+ Detector Shifts
Band
Number
Forward Scan
Even Detectors
Forward Scan
Odd Detectors
Reverse Scan
Even Detectors
Reverse Scan
Odd Detectors
1 49.0 51.0 45.0 48.0
2 74.0 76.0 70.0 73.0
3 99.0 101.0 95.0 98.0
4 124.0 126.0 120.0 123.0
5 195.0 197.0 191.0 194.0
6 110.0 113.0 114.0 116.0
7 169.0 171.0 165.0 168.0
8 50.0 54.0 40.0 44.0

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