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11.1
Level 0R Product |
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Unlike earlier Landsat programs, the Landsat 7 system was not originally
designed to produce high level (i.e. Level 1) products for users. The
baselined program philosophy was to provide raw data only which would
leave the value added domain for commercial companies. A prevailing "wait
and see" position by commercial vendors prompted NASA to add a systematic
correction capability to ensure product availability. The primary product
for users and vendors seeking higher level processing, however, is 0R
data - an essentially raw data form that is marginally useful prior to
radiometric and geometric correction. This is readily apparent when viewing
a simulated 0R image . A Landsat
7 0R product, however, does contain all the ancillary data required to
perform these corrections including a calibration parameter file (CPF)
generated by the Landsat 7 IAS.
LPS spatially reformats earth imagery and calibration data into Level
0R data. This involves shifting pixels by integer amounts to account for
the alternating forward-reverse scanning pattern of the ETM+ sensor, the
odd-even detector arrangement within each band, and the detector offsets
inherent to the focal plane array engineering design. All LPs 0R corrections
are reversible; the pixel shift parameters used are documented in the
IAS CPF.
The LPs 0R output is HDF-EOS formatted and archived. Details of the archival
format can be found in the Landsat
7 System Wideband DFCB, Vol. 4.
Three options, depicted in Figure 11.2,
exist when defining the size or spatial extent of a Landsat level 0R product
ordered from the LP-DAAC.
- Standard Worldwide Reference System (WRS) Scene. The standard
WRS scene as defined for Landsats 4 and 5 was preserved as a product
for Landsat 7. The WRS indexes orbits (paths) and scene centers (rows)
into a global grid system comprising 233 paths by 248 rows. The path/row
notation was originally employed to provide a standard designator for
every nominal scene center and allow straight forward referencing without
using longitude and latitude coordinates.
The distance between WRS center points along a path is 161.1 kilometers.
A path distance of 90 kilometers before and after a WRS center point
defines the standard scene length of 180 km. This length includes
20 scans of overlap with neighboring scenes. The standard WRS scene
overlaps neighboring scenes along a path by 5% at the equator and
has a width or cross track distance of 185 kilometers.
Landsat 7 browse is framed according to WRS scenes. An ordered scene
will cover the same geographic extent observed in the browse with
the following caveat.
Standard WRS scenes have 375 scans. Partial scenes (less than 375
scans) may exist at the beginning or end of a subinterval due to the
fact that imaging events do not always start and end on scene boundaries.
Browse and scene metadata for these occurrences accurately reflect
their partial scene nature and geographic extent although partials
are currently not offered due to complexities associated with level
1 processing.
- Subinterval. An interval
is a scheduled
ETM+ image period along a WRS path, and may be from one to 90 scenes
in length. A subinterval is a contiguous segment of raw wideband data
received during a Landsat 7 contact period. Subintervals are caused
by breaks in the wideband data stream due to communication dropouts
and/or the inability of the spacecraft to transmit a complete observation
(interval) within a single Landsat 7 contact period. The largest possible
subinterval is 35 scenes long. The smallest possible subinterval is
a single ETM+ scene.
- Partial Subinterval A partial Landsat 7 subinterval can also
be ordered. The partial subinterval is dimensioned according to standard
WRS scene width, is at least one WRS scene in length, and can be up
to 10 scenes in length if ordered in 0R form or 3 scenes in length
in 1G form. A partial subinterval can float or be positioned at any
scan line starting
point
within a
subinterval.
Partial subintervals are defined by either contiguous WRS locations
or a bounding longitude/latitude rectangle. In the latter case, all
scan lines touched by the bounding rectangle are included in their
entirety.
11.1.2 Product Components
A complete scene-sized 0R product consists of
19 data sets derived from the wideband telemetry, an IAS-generated calibration
parameter file, a product specific metadata file, a geolocation index generated
by EOSDIS Core System (ECS), and an HDF directory. Therefore, if you order
a complete (i.e. all bands) scene-based 0R product it will have 23 distinct
files. A brief description of each follows.
- 1 - 9. Earth Image Data - The unique bands of ETM+ image data
comprise nine of the data sets. The data is laid out in a scan line
sequential format in descending detector order (i.e. detector 16 followed
by detector 15 and so on for the 30 meter bands). Band 6 is captured
twice - once in low and the other in high gain mode. Under nominal satellite
configuration the low gain form of band 6 will be present in format
1. All image samples or pixels are 8 bits in size.
- 10. Internal calibrator (IC) data - format 1 - IC data for
format 1 consists of scan line ordered internal lamp and shutter data
for bands 1-5 and blackbody radiance and shutter data for low gain band
6. The data is collected once per scan and structured in a band sequential
format in descending detector order (e.g. detector 16 followed by detector
15 and so on for the 30 meter bands).
- 11. Internal calibrator (IC) data - format 2 - IC data for
format 2 consists of scan ordered internal lamp and shutter data for
bands 7 and 8 and blackbody radiance and shutter data for high gain
band 6. The data is collected once per scan and structured in a band
sequential format in descending detector order (e.g. detector 16 followed
by detector 15 and so on for the 30 meter bands).
- 12. MSCD - format 1. A logical record of MSCD exists for each
data scan present in the 0R product ordered. Each logical record consists
of 3 MSCD data values - the first half scan error, the second half scan
error, and the scan line direction. This information, which actually
applies to the previous scan, is used to compute deviations from nominal
scan mirror profiles as measured on the ground and reported in the calibration
parameter file. Also included in the MSCD file are scan based values
such as time code, gain status and processing errors encountered by
LPs The MSCD is trimmed to fit the product ordered although one additional
record is added to the file during the subsetting process due to the
fact that scan error and direction information corresponds to the prior
scan.
- 13. MSCD - format 2. A duplicate set of MSCD is generated when
format 2 is processed and is kept with the product in the event format
1 MSCD is lost or corrupted.
- 14. PCD - format 1 The PCD for format 1 consists of attitude
and ephemeris profiles as well high frequency jitter measurements. PCD
for the entire subinterval is included with the 0R product regardless
of the size of the data set ordered.
- 15. PCD - format 2 A duplicate set of PCD is generated when
format 2 is processed and is kept with the product in the event format
1 is lost or corrupted.
- 16. Scan line offsets - format 1. During LPs processing image
data is shifted in an extended buffer to account for predetermined detector
and band shifts, scan line length, and possible bumper wear. The scan
line offsets represent the actual starting and ending pixel positions
for valid (non-zero fill) earth image data on a data line by data line
basis for bands 1 through 6 low gain. The left starting pixel offsets
also apply to the IC data.
- 17. Scan line offsets - format 2. During LPs processing image
data is shifted in an extended buffer to account for predetermined detector
and band shifts, scan line length, and possible bumper wear. The scan
line offsets represent the actual starting and ending pixel positions
for valid (non-zero fill) earth image data on a data line by data line
basis for bands 6 high gain through 8. The left starting pixel offsets
also apply to the IC data.
- 18. Metadata - format 1. During LPs format 1 processing metadata
is generated that characterizes the subinterval's spatial extent, content,
and data quality for bands 1 through 6 low gain. This file, in its entirety
and original form, accompanies the 0R product.
- 19. Metadata - format 2. Format 2 metadata is similar but
not identical to format 1 metadata. The subinterval-related metadata
contents are identical; the scene-related metadata is specific to bands
6 - high gain, 7, and 8. Also, the format 2 metadata does not include
cloud cover assessment data or references to browse data products. This
file, in its entirety and original form, accompanies the 0R product.
- 20. Metadata - ECS. A third metadata file generated by ECS
during order processing. This file contains product specific information
such as corner coordinates and number of scans.
- 21. Geolocation Index. The geolocation index is also produced
by ECS. This table contains scene corner coordinates and their product-specific
scan line numbers for bands at all three resolutions. Its purpose is
provide for efficient subsetting of a 0R product.
- 22. Calibration parameters. The IAS regularly updates the CPF
to reflect changing radiometric and geometric parameters required for
level 1 processing. These are stamped with applicability dates and sent
to the LP-DAAC for storage and bundling with outbound 0R products.
- 23. HDF Directory. A file containing all the pointers, file
size information, and data objects required to open and process the
0R product using the HDF library and interface routines.
A user may order a subset of the available bands which will affect the
actual file count in a 0R product. In all cases, however, every product
includes two PCD files, two MSCD files, three metadata files, the CPF,
and the HDF directory. Only the internal calibrator, scan line offset,
and earth image file counts are affected by a product possessing less
than the full complement of bands.
11.1.3 Product Format
The product delivered to Landsat 7 data users is packaged in HDF - an
open standard selected by NASA for Earth Observing System (EOS) data products.
HDF is a self-describing format that allows an application to interpret
the structure and contents of a file without outside information. HDF
allows Landsat 0R products to be shared across different computer platforms
without modification and is supported by a public domain software library
consisting of access tools and various utilities.
Product users are directed to the
Landsat 7 0R Distribution Product Data Format Control Book, Volume 5 (PDF)
for details regarding the HDF design used for the 0R product. Included
are references to NCSA-authored documentation. New users should begin
with Getting Started with HDF while the HDF User's Guide
and HDF Reference Manual are excellent resources for the HDF programmer.
11.2
Level 1R Product |
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The 2008 single-product data policy changes at EROS made the Level
1R product option obsolete. The following paragraphs are only relevant
only from a historical perspective.
The Level 1R product is a radiometrically
corrected 0R product. Radiometric correction is performed using either
the CRAM gains in the CPF or gains computed on the fly from the IC data.
The choice is available to a user when the product is ordered. The biases
used are always calculated from the IC data. Image artifacts such as
banding, striping, and scan correlated shift are removed prior to radiometric
correction. Radiometric corrections are not reversible. The 1R product
geometry is identical to the input Level OR data.
During 1R product rendering image pixels are converted to units of absolute
radiance using 32 bit floating point calculations. Pixel values are then
multiplied by 100 and converted to 16 bit integers prior to media output.
Two digits of decimal precision are thus preserved. One merely divides
each pixel value by 100 to convert the 1R image data back to radiance
units. The 16 bit 1R product is twice the data volume of an alike 0R product.
Note for band 6: A bias was found in the pre-launch calibration
by a team of independent investigators post launch. This was corrected
for in the LPGS processing system beginning Dec 20, 2000. For data processed
before this, the 16 bit image radiances are 0.31 w/m2 ster um too high.
See the official announcement
for more details.
11.2.1 Product Size
Two options existed for users when defining the size or spatial extent
of a Landsat level 1R product ordered from the LP-DAAC.
- Standard Worldwide Reference System (WRS) Scene. The standard
WRS scene, as defined above for the 0R product, could be ordered in
1R form. Partial scenes that may exist at the beginning and end of
subintervals could also be ordered.
- Partial Subinterval A partial subinterval could also be ordered
in 1R form, although this capability is not scheduled for release
until early 2000. Unlike the 0R product the 1R was limited to a maximum
of 3 WRS scenes in size. The variably sized 1R product could float
or be positioned at any scan line starting point within a subinterval.
Alternatively, the product could be defined by up to three contiguous
WRS locations.
11.2.2 Product Components
A complete scene-sized 1R product consists
of 17 data sets derived from the wideband telemetry, an IAS-generated
calibration parameter file, a product specific metadata file, a geolocation
index generated by EOSDIS Core System (ECS), and an HDF directory. Therefore,
if you order a complete (i.e. all bands) scene-based 0R product it will
have 21 distinct files. There are two fewer data files than an alike
0R product due to the fact that the multiple PCD and MSCD files are merged
into single consensus files. Please reference the 0R file product for
individual file descriptions.
A user could order a subset of the available bands which affected
the actual file count in a 1R product. In all cases, however, every product
included one consensus PCD file, one consensus MSCD files, three metadata
files, the CPF, and the HDF directory. Only the internal calibrator,
scan line offset, and earth image file counts were affected by a product
possessing less than the full complement of bands.
11.2.3 Product Format
The 1R product was delivered to users only in the HDF format. The HDF
0R and 1R formats are nearly identical. Exceptions include the united
PCD and MSCD files and an enhanced product specific metadata file that
reflects 1R correction characteristics. Please refer to the
Landsat 7 0R Distribution Product Data Format Control Book, Volume 5
(PDF)
for details regarding HDF specifics. Additional information unique to
the 1R product can be found in the
ESDIS Level 1 Product Generation system Output Files DFCB (PDF).
11.3
Level 1G Product |
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The 1G product available to users from EROS is a radiometrically
and geometrically corrected Level 0R image.
The correction algorithms employed model the spacecraft and sensor using
data generated by onboard computers during imaging events. Primary inputs
are the PCD, which includes the attitude and ephemeris profiles, the definitive
ephemeris (if available) and the MSCD. Refined parameters from the
CPF, ground control points and a digital elevation model are also
used to improve the overall geometric fidelity of the standard level-one
terrain-corrected (L1T) product.
The L1T correction process utilizes both ground control points (GCP)
and digital elevation models (DEM) to attain absolute geodetic accuracy.
The WGS84 ellipsoid is employed as the Earth model for the Universal Transverse
Mercator (UTM) coordinate transformation. Associated with the UTM projection
is a unique set of projection parameters that flow
from the USGS General Cartographic Transformation Package. The
end result is a geometrically rectified product free from distortions
related to the sensor (e.g. jitter, view angle effects), satellite (e.g.
attitude deviations from nominal), and Earth (e.g. rotation, curvature,
relief).
Geodetic
accuracy of the L1T product depends on the accuracy of the GCPs
and the resolution of the DEM used*. The 2005 Global Land Survey is used
as the source for GCPs while the primary terrain data is the Shuttle
Radar Topographic Mission DEM. Scenes that have a quality
scores of 99 and less than 40 percent cloud cover are automatically processed,
and any archived scene, regardless of cloud cover, can be ordered through
one of two EROS web portals (Product
Ordering.)
During L1T processing the 0R image data undergoes two-dimensional resampling
according to the following set of parameters:
- Correction level - L1T*
- Pixel Size - 15, 30, 60 meters for panchromatic, VNIR/SWIR, and thermal
- Resampling kernel - Cubic
Convolution (CC)
- Map projection - UTM
with Polar Stereographic projection used for Antarctica scenes
- Ellipsoid - WGS84
- Image orientation
- north up
- Output format - GeoTIFF
- File transfer protocol (FTP) download only
|
* While most scenes are processed to L1T, some lack GCPs and/or
DEMs required for precision and terrain correction processing.
In these cases, the best level of correction will be applied -
Level 1GT-systematic terrain (GCPs absent) or Level 1G-systematic
(DEMs and GCPs absent). |
|
11.3.1 Conversion to Radiance
During 1G product rendering image pixels are converted to units of absolute
radiance using 32 bit floating point calculations. Pixel values are then
scaled to byte values prior to media output. The following equation is
used to convert DN's in a 1G product back to radiance units:
Lλ = Grescale * QCAL + Brescale
which is also expressed as:
Lλ = ((LMAXλ - LMINλ)/(QCALMAX-QCALMIN))
* (QCAL-QCALMIN) + LMINλ
where: |
Lλ |
= Spectral Radiance at the sensor's aperture in watts/(meter
squared * ster * μm) |
|
Grescale |
= Rescaled gain (the data product "gain" contained in the Level
1 product header or ancillary data record) in watts/(meter
squared * ster * μm)/DN |
|
Brescale |
= Rescaled bias (the data product "offset" contained in the Level
1 product header or ancillary data record ) in watts/(meter
squared * ster * μm) |
|
QCAL |
= the quantized calibrated pixel value in DN |
|
LMINλ |
= the spectral radiance that is scaled to QCALMIN in watts/(meter
squared * ster * μm) |
|
LMAXλ |
= the spectral radiance that is scaled to QCALMAX in watts/(meter
squared * ster * μm) |
|
QCALMIN |
= the minimum quantized calibrated pixel value (corresponding
to LMINλ) in DN
= 1 for LPGS products
= 1 for
NLAPS products processed after 4/4/2004
= 0 for NLAPS products processed before 4/5/2004 |
|
QCALMAX |
= the maximum quantized calibrated pixel value (corresponding
to LMAXλ) in DN
= 255 |
The LMINs and LMAXs are the spectral
radiances for each band at digital numbers 0 or 1 and 255 (i.e QCALMIN,
QCALMAX), respectively. LPGS used 1 for QCALMIN while
NLAPS used 0 for QCALMIN for data products processed
before April 5, 2004. NLAPS from that date now uses 1 for the QCALMIN
value. Other product differences
exist as well. One LMIN/LMAX set exists for each gain state. These values
will change slowly over time as the ETM+ detectors lose responsivity.
Table 11.2 lists two sets of LMINs and LMAXs. The first set should be
used for both LPGS and NLAPS 1G products created before July
1, 2000 and the second set for 1G products created after July
1, 2000. Please note the distinction between acquisition and processing
dates.
Use of the appropriate LMINs and LMAXs will ensure accurate conversion
to radiance units. Note for band 6: A bias was found in the
pre-launch calibration by a team of independent investigators post
launch. This was
corrected for in the LPGS processing system beginning Dec 20, 2000. For
data processed before this, the image radiances given by the
above transform are 0.31 w/m2 ster um too high. See the official announcement for more details.
Note for the Multispectral Scanner (MSS),
Thematic Mapper (TM), and Advanced Land Imager (ALI) sensors: the
required radiometry constants are tabulated in this PDF file.
Table 11.2 ETM+ Spectral Radiance Range
watts/(meter squared * ster * μm) |
Band Number![](images/one_pixel.gif) |
Processed Before July 1, 2000 |
Proccessed After July 1, 2000 |
Low Gain |
High Gain |
Low Gain |
High Gain |
LMIN |
LMAX |
LMIN |
LMAX |
LMIN |
LMAX |
LMIN |
LMAX |
1 |
-6.2 |
297.5 |
-6.2 |
194.3 |
-6.2 |
293.7 |
-6.2 |
191.6 |
2 |
-6.0 |
303.4 |
-6.0 |
202.4 |
-6.4 |
300.9 |
-6.4 |
196.5 |
3 |
-4.5 |
235.5 |
-4.5 |
158.6 |
-5.0 |
234.4 |
-5.0 |
152.9 |
4 |
-4.5 |
235.0 |
-4.5 |
157.5 |
-5.1 |
241.1 |
-5.1 |
157.4 |
5 |
-1.0 |
47.70 |
-1.0 |
31.76 |
-1.0 |
47.57 |
-1.0 |
31.06 |
6 |
0.0 |
17.04 |
3.2 |
12.65 |
0.0 |
17.04 |
3.2 |
12.65 |
7 |
-0.35 |
16.60 |
-0.35 |
10.932 |
-0.35 |
16.54 |
-0.35 |
10.80 |
8 |
-5.0 |
244.00 |
-5.0 |
158.40 |
-4.7 |
243.1 |
-4.7 |
158.3 |
11.3.2 Radiance to Reflectance
For relatively clear Landsat scenes, a reduction in between-scene variability
can be achieved through a normalization for solar irradiance by converting
spectral radiance, as calculated above, to planetary reflectance or albedo.
This combined surface and atmospheric reflectance of the Earth is computed
with the following formula:
Table 11.3   ETM+ Solar Spectral
Irradiances
(generated using the Thuillier solar
spectrum) |
Band |
watts/(meter squared * μm) |
1 |
1997 |
2 |
1812 |
3 |
1533 |
4 |
1039 |
5 |
230.8 |
7 |
84.90 |
8 |
1362. |
Table 11.4 Earth-Sun Distance
in Astronomical Units |
Day of Year |
Distance |
Day of Year |
Distance |
Day of Year |
Distance |
Day of Year |
Distance |
Day of Year |
Distance |
1 |
.98331 |
74 |
.99446 |
152 |
1.01403 |
227 |
1.01281 |
305 |
.99253 |
15 |
.98365 |
91 |
.99926 |
166 |
1.01577 |
242 |
1.00969 |
319 |
.98916 |
32 |
.98536 |
106 |
1.00353 |
182 |
1.01667 |
258 |
1.00566 |
335 |
.98608 |
46 |
.98774 |
121 |
1.00756 |
196 |
1.01646 |
274 |
1.00119 |
349 |
.98426 |
60 |
.99084 |
135 |
1.01087 |
213 |
1.01497 |
288 |
.99718 |
365 |
.98333 |
11.3.3 Band
6 Conversion to Temperature
ETM+ Band 6 imagery can also be converted from spectral radiance (as described
above) to a more physically useful variable. This is the effective at-satellite
temperatures of the viewed Earth-atmosphere system under an assumption
of unity emmissivity and using pre-launch calibration constants listed
in Table 11.5. The conversion formula is:
Where: |
![](images/one_pixel.gif) |
T |
=   Effective at-satellite temperature in Kelvin |
K2 |
=   Calibration constant 2 from Table 11.5 |
K1 |
=   Calibration constant 1 from Table 11.5 |
L |
=   Spectral radiance in watts/(meter squared
* ster * µm) |
Table 11.5   ETM+ and TM Thermal
Band Calibration Constants |
![](images/one_pixel.gif) |
Constant 1- K1
watts/(meter squared * ster * μm) |
Constant 2 - K2
Kelvin |
Landsat 7 |
666.09 |
1282.71 |
Landsat 5 |
607.76 |
1260.56 |
11.3.4 Product Size
The same two 1R options exist for users when defining the size or spatial
extent of a Landsat level 1G product ordered from the LP-DAAC.
- Standard Worldwide Reference System (WRS) Scene. The standard
WRS scene, as defined above for the 0R product, can be ordered in 1G
form. Partial scenes that may exist at the beginning and end of subintervals
may be also be ordered.
- Partial Subinterval. A partial subinterval can also be ordered
in 1G form. Unlike the 0R product the 1G is limited to a maximum of
3 WRS scenes in size. The variably sized 1G product can float or be
positioned at any scan line starting point within a subinterval. Alternatively,
the product can be defined by up to three contiguous WRS locations.
11.3.5 Product Components
The 1G product ordered from the LP-DAAC consists of the corrected image
files and descriptive metadata. All other ancillary files delivered with
the 0R and 1R products are not included. A user may order a subset of
the available bands which affects the actual file count in a 1G product.
11.3.6 Product Format
The 1G product can be packaged into one of following user-specified output
formats:
- HDF. The HDF packaging format used for the 0R and 1R products
is also used for structuring the 1G. The design employs external elements
for the band files and metadata. These are standalone files that are
referenced via tags and pointers residing in an HDF directory. External
elements provide users with two processing options - exploit the NCSA
HDF libraries for data access or process the data files directly using
homegrown code.
The number of files comprising an HDF-formatted 1G product will
vary according to the number of bands ordered. A product with a full
band complement has 11 files - the HDF directory, a metadata file,
and a separate file for each band. The HDF directory and metadata
files are always present regardless of bands ordered. Please refer
to the Landsat 7 0R Distribution
Product Data Format Control Book, Volume 5 (PDF) for details regarding
band file specifics. The 1R metadata file description can be found
in the ESDIS Level
1 Product Generation System Output Files DFCB (PDF).
The HDF format can be specified for any type of 1G product ordered
from the LP-DAAC.
- Fast. The Fast Format was originally developed by EOSAT as
a means for quickly accessing Landsat 4 and 5 image data. Its structure
is straightforwardly simple. Each band is self contained in its own
file (i.e external element style). A header file containing three records
accompanies the image data. The three records in order of appearance
are labeled administrative, radiometric, and geometric respectively.
Sensor specific information is placed in the administrative record,
gains and biases can be found in the radiometric record while projection
information and image coordinates are stored in the geometric record.
A single header file along with the image files constitute the Fast
product.
A derivative of the Fast Format (Fast-L7) used by EOSAT for Landsat
(FAST-B) and Indian Remote Sensing products (Fast-C) was created for
Landsat 7. Several differences are worth noting. File names are now
included in the administrative record which allows for direct file
access. A separate header file now accompanies the panchromatic, thermal
and VNIR/SWIR band groups for Landsat 7. For Fast-B and Fast-C all
bands were resampled to a common grid cell size thus permitting a
single header file. In all likelihood each of the band groups for
Landsat 7 will be resampled to a common resolution (i.e. 15, 30, &
60 meters) thus requiring a distinct header file for each.
All critical fields required for product ingest were left unchanged
in the Fast L-7A Format. As a consequence Heritage Fast readers residing
on user systems can be used for the Landsat 7 Fast formatted product.
A full layout of the Fast L-7A Format can be found in the
the ESDIS Level 1 Product Generation system Output Files DFCB.
The Fast-L7 format supports all variations of the 1G product.
- GeoTIFF. Geographic tagged image file format (GeoTIFF) is based
on Adobe's TIFF - a self-describing format developed to exchange raster
images such as clipart, logotypes, and scanned images between applications
and computer platforms. Today, the TIFF image file format is used to
store and transfer digital satellite imagery, scanned aerial photos,
elevation models, and output from digital cameras. TIFF is the only
full-featured format in the public domain, capable of supporting compression,
tiling, and extension to include geographic metadata.
The TIFF file consists of a number of label (tags) which describe
certain properties of the file (such as gray levels, color table,
byte format, compression size). After the initial tags comes the image
data which may be interrupted by more descriptive tags. GeoTIFF refers
to TIFF files which have geographic (or cartographic) data embedded
as tags within the TIFF file. The geographic data can then be used
to position the image in the correct location and geometry on the
screen of a geographic information display.
Baseline TIFF image types can be bilevel, greyscale, palette color,
and full color (24 bit). For simplicity's sake the grayscale model
was implemented for the Landsat 7 GeoTIFF product. Under this implementation
each ordered band is delivered as its own 8 bit greyscale GeoTIFF
image. A standard WRS scene possessing the full band complement would
thus be comprised of nine separate GeoTIFF images or files. No other
files accompany the product. For detailed information regarding the
Landsat 7 GeoTIFF implementation please refer to the
ESDIS Level 1 Product Generation system Output Files DFCB (PDF).
For GeoTIFF details, please download the
GeoTIFF Format Specification (PDF) or visit this web
site.
At the present time GeoTIFF format cannot be used for the Space
Oblique Mercator and Oblique Mercator projections. Products projected
into these reference systems must be formatted using HDF or Fast-L7.
An instrument malfunction occurred onboard Landsat 7 on May 31, 2003.
The problem was caused by failure of the Scan Line Corrector (SLC), which
compensates for the forward motion of the satellite. Subsequent efforts
to recover the SLC have not been successful, and the problem is
permanent.
The Landsat 7 Enhanced Thematic Mapper Plus (ETM+) is still capable
of acquiring useful image data with the SLC turned off, particularly
within the central portion of any given scene. Landsat 7 ETM+ will therefore
continue to acquire image data in the "SLC-off" mode.
EDC has recently released several Landsat 7 ETM+ SLC-off
data products. The first, a gap-present product became available on
October 22, 2003. This product release includes all image data acquired
by Landsat
7 in
SLC-off
mode
from July
14, 2003
to present, excluding a 2-week interval from 9/3/03 to 9/17/03.
The center of a gap-present SLC-off data product is very similar
in quality to previous Landsat 7 data. However, the scene's edges will
contain
alternating
scan lines of missing data (Level 1G) or duplicated data (Level 0Rp or
L1R). The precise location of the affected scan lines will vary from
scene to scene, and these gaps will not be visible on the browse image
preview when ordering SLC-off data. A preliminary report regarding the
utility of Landsat 7 SLC-off data is available in PDF form.
This report includes input from scientists affiliated with the USGS,
NASA, and the Landsat 7 Science Team
The gap-present SLC-off data product is available as a single scene
entity in Level 1G terrain corrected (L1T) form.
As of November, 2008, the USGS offers all archived
Landsat scenes to the public at no charge. Newly acquired Landsat 7 ETM+
SLC-off and Landsat 5 TM images with less than 40 percent cloud cover
are automatically processed and made freely available for immediate download.
SLC-off data products can be searched and ordered via the Earth
Explorer, and Global
Visualization L7 Image Browser.
The second product now being offered (as of May 10, 2004) is in 1G form
and has the gap areas filled with Landsat 7 data acquired at a similar
time of year and prior to the SLC failure. The two scenes are geometrically
registered, and a histogram matching technique is applied to the fill
pixels which provides the best-expected radiance values for the missing
data.
The USGS, in conjunction with NASA, is continuing to research other
methods of providing merged data products and will continue to provide
information resulting from this work as it becomes available. Various
methodoligies have been examined to fill the data gaps with observations
acquired during prior or later than the target scene of interest. An
exampled of a gap-filled product is illustrated in Figure 11.5. Free
stand-alone gap filling software for Windows will be made available release TBD.
![SLC Off fill example](images/SLC_off_fill_example.jpg) |
Figure 11.5 Top image: pre-SLC anomaly, middle of image. Middle
image: scene after SLC anomaly. Bottom image: scene after SLC anomaly
with data gaps filled. |
Table of Contents
Last Update: June 23, 2009
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