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About Imaging Spectroscopy

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IMAGING SPECTROSCOPY/SPECTROSCOPY INFORMATION

(The information described here is specifically written for USGS scientists to understand imaging spectroscopy and what it takes to analyze the data for USGS programs using our currently developed methods. However, the information should be generally useful to all those interested in imaging spectroscopy.)

EXECUTIVE SUMMARY

WHAT IS IMAGING SPECTROSCOPY?
Imaging spectroscopy is a new tool that can be used to map specific materials by detecting specific chemical bonds. As a result it is an excellent tool for environmental assessments, mineral mapping and exploration, vegetation communities/species and health studies, and general land management studies.

The premier imaging spectrometer is the NASA/JPL AVIRIS system, covering a 10.5 km swath with 17-meter pixel spacing. AVIRIS collects data at a rate of 2 square kilometers per second!

DATA ACQUISITION PLANNING
Normally, AVIRIS data acquisition must be planned well in advance, up to approximately a year. However, as paying customers some acquisitions can be planned a few months in advance.

COST
AVIRIS data cost about $10 per square kilometer, but must be funded in full flights ($64k per flight + $6k per flight hour). A full flight can cover up to about 8,000 square kilometers spread over multiple sites.

CALIBRATION
AVIRIS data require ground support information to calibrate the data to surface reflectance for mapping materials. It takes about 30 to 60 person days to calibrate an AVIRIS data set from one area (up to 8,000 sq km) for one flight-day. Calibration uses a combinations of field spectrometers and laboratory spectrometers, as well as radiative-transfer calculations. (To see our tutorial on calibration, click here).

WHAT IT TAKES TO ANALYZE IMAGING SPECTROMETER DATA
Computer analysis takes a trained person only a few days per couple of hundred square kilometers' worth of data. This can include creating maps of many minerals. Current standard analysis produces materials maps for more than 260 materials and is expected to grow to hundreds of materials in the near future (note not all materials may be found in a given area).

WHAT IT TAKES TO VERIFY IMAGING SPECTROMETER DATA
Imaging spectroscopy analysis can be done with only a few days of work per couple of hundred square kilometers of data. The analysis can produce a several-hundred-layer GIS database. The various layers can be assembled into materials maps which must begeometrically corrected and verified in the field to ensure the data were analyzed correctly and all relevant materials were included in the mapping process. It can take a couple of weeks or more to verify the results and begin to understand the complexities mapped, and depends on the scientific questions being posed. Minerals, environmental materials, man-made materials, and vegetation species/communities maps are a new paradigm and understanding this new technology and what science questions can be answered with it will take training and effort.

PUTTING IT ALL TOGETHER, FROM PLANNING TO COMPLETED ANALYSIS AND REPORTS
Complete analysis of a full flight of AVIRIS data, from flight planning, calibration, materials mapping and verification, laboratory analyses of samples, data registration, map production, and science reports takes on the order of seven person years of effort. Smaller areas take less time, depending on the science to be done.

UNIQUE FACILITIES
Specialized facilities are required to successfully map materials with imaging spectroscopy. Laboratory and field spectrometers are required to calibrate imaging spectroscopy data, measure reference samples, and verify mapping results.

TRAINING BY THE SPECTROSCOPY GROUP
The USGS Denver spectroscopy group can train about six scientists per year, covering a maximum of three new AVIRIS sites per year.

WHAT IS IMAGING SPECTROSCOPY?

Imaging spectroscopy is the application of reflectance/emittance spectroscopy to every pixel in a spatial image. Spectroscopy can be used to detect individual absorption features due to specific chemical bonds in a solid, liquid, or gas. Solids can be either crystalline (i.e. minerals) or amorphous (like glasses). Every material is formed by chemical bonds, and has the potential for detection with spectroscopy. Actual detection is dependent on the spectral coverage, spectral resolution, and signal-to-noise of the spectrometer, the abundance of the material and the strength of absorption features for that material in the wavelength region measured. In remote sensing situations, the surface materials mapped must be exposed in the optical surface (e.g., to map surface mineralogy it must not be covered with vegetation), and the diagnostic absorption features must be in regions of the spectrum that are reasonably transparent to the atmosphere (the atmosphere can be corrected for all but the strongest absorptions). The optical surface is the same as what the geologist sees in the field with his or her eyes. Spectroscopy can be used in laboratories on hand samples, in the field with portable field spectrometers (spatial resolution in the millimeter to several meter range), from aircraft, and in the future from satellites. The aircraft systems now operational can image large areas in short time (~2 sq. km per second!), producing spectra for each pixel that can be analyzed for specific absorption bands and thus specific materials.

The premier sensor is the NASA/JPL AVIRIS system (Airborne Visual and Infra-Red Imaging Spectrometer). AVIRIS currently covers the wavelength region from 0.38 to 2.50 microns with 17-meter pixel spacing (20-meter spot) and a 10.5-km swath. AVIRIS is flown on an ER-2 (U2) aircraft at 65,000 feet. Next year AVIRIS should have capability to fly on a C130 aircraft and have 5-meter pixel spacing and a 2.5 km swath. While AVIRIS data can be used to make any scale maps, the data from ER-2 altitudes makes excellent 1:24,000-scale maps.

The AVIRIS spectral range is excellent for detecting electronic transitions in minerals (e.g., iron oxides, Fe2+ bearing minerals, etc.), vegetation (vegetation species, health, green leaf water content), and vibrational absorptions due to lighter elements (OH, SO4, CO3, CH, etc., so OH-bearing minerals, carbonates, sulfates and organics are mappable). Diagnostic absorptions also exist due to other processes. For example, Rare Earth ions involve deep-lying electrons and are not diagnostic of mineralogy but of the presence of the ions in the mineral, thus specific Rare Earth Elements are detectable with the AVIRIS spectral range. Vibrational absorptions from heavier elements such as Si-O (quartz) occur in the mid-infrared and are not covered by AVIRIS. Currently, only broad-band sensors (e.g., TIMS) are available in the mid-IR, although new mid-IR systems are under development.

The NAVY/Civilian HYDICE imaging spectrometer has AVIRIS like capabilities but 1-meter to 3-meter spatial resolution, is currently flying in a testing mode, and should become available for use in the next year.

DATA ACQUISITION PLANNING

A NASA ER-2 flight is like launching a rocket through commercial airspace and requires significant advance planning on their part. Thus, NASA plans the whole spring/summer/fall flight season the previous year. The ER-2 is only deployed from several (continental United States) places: Moffet Field near San Francisco, Whollops Island on the east coast, and occasionally Topeka, Kansas, or Spokane, Washington. Depending on the data requests, the aircraft may be deployed out of one of these locations for a several week period to acquire data near that base.

As paying customers, we do not need to give exact sites a year in advance, but NASA would like our best ideas so they may plan accordingly (e.g., what bases they need to deploy from). Each year, AVIRIS acquires data equivalent to an area greater than the size of the State of Nevada to provide data for numerous research programs, ranging from geology, ecosystems, water/ocean, to clouds/atmosphere. Sites generally are found all over the United States. One year AVIRIS was deployed in Europe; this year it went to Alaska and South America.

Consequently, as soon as we can after a project's budget is approved, we must give NASA an idea of how much data we might want in the coming year. For example, we may want 3 sites in the western United States (deployment from Moffet Field) covering X sq km in states A, B, and C. Then approximately next March, at the latest, we would need to finalize the sites and give NASA coordinates of specific flight lines.

As part of the data-acquisition planning stage, it is also necessary to plan for ground calibration. When NASA flies the requested sites (usually the May-July time frame), a team must obtain ground calibration data as near as possible to the day of the overflight. Depending on the ER-2 schedule and weather, the ground crew could be sitting in the field for days waiting for the flight, or do a last-minute scramble to get to another site. (See CALIBRATION below.)

AVIRIS data are recorded on a tape that vholds 70 minutes of data. The ER-2 aircraft travels at 734 km/hr (12.233 km/minute). Pre-calibration plus post calibration for each flight takes up about 5 minutes of data tape, leaving about 65 minutes of flight data (about 795 km in length). Some tape movement occurs at the beginning and end of each line of data, so about 1.25 minutes are lost for each flight line (equivalent to 15.3 km). Thus the achievable flight data lengths = 795 - lines*15.3 km.

Thus the following table shows in practice what can be covered per flight:

Line Segments Total km of data
1 780
2 764
3 749
4 734
5 718
6 703
7 688
8 673
9 657
10 642
11 626
12 611
13 596
14 581
15 565

Finally, a flight can be no longer than 6.5 hours without special permission. So you must plan how long it takes to get to and from your site, and plan on about 15 minutes per turn.

COST

AVIRIS data cost about $10.00 per sq. km when large areas are covered. AVIRIS covers a 10.5 km swath and about 800 km of data can be flown per day (~8,000 sq km). No cost has been set for HYDICE data.

In practice, NASA requires whole flights funded (the cost for a flight is mostly fixed, regardless of how much data are collected). Cost sharing with other researchers is possible if multiple requests are made to fit together into a single flight. One full flight of data costs $64,000 plus $6,000 per flight hour of the ER-2. Fortunately, at a speed of 734 km/hr, only 1.1 hours of data collection time are needed to acquire a full data tape (10 Gbytes; ~8,000 sq km). Additional flight time involves getting to and from the site, and in making turns for each flight line.

Note: one full AVIRIS tape is 10 gigabytes of raw data, but when converted to 16-bit calibrated data for delivery, this becomes about 14 GBytes!

An additional requirement to getting a full flight's worth of data is that the sites covered must be near each other (like a few hundred miles) so that there is reasonable time for the plane to get to each one. For example, when launching from Moffet Field, a site in Nevada, Utah and Colorado might be feasible because they could be on a general line from the base. An example that would not work would be something like the New World Mine near Yellowstone and a site in Arizona (we would be charged for 2 flights: $64k each + flight hours).

CALIBRATION

Imaging spectroscopy data come in radiance, and thus have the solar flux spectrum, the earth's atmospheric absorption bands (you can map atmospheric gas abundances), and surface reflectance in the data. The surface reflectance is what we need for land-management studies. The data are calibrated by a combination radiative transfer atmospheric transmission model and by measuring the reflectance of relatively large homogeneous areas on the ground with a field spectrometer (a playa, rock outcrop or grassy field have all been used; playas work best). The effort to calibrate one site (one day's worth of data in one region up to ~8,000 sq km) requires a minimum of 2 people to visit the calibration site(s) near the time of the overflight (within a couple of weeks; before any rainstorm). The field spectrometer data are obtained by walking over the site collecting a few hundred spectra averaging the site as much as possible. Samples are obtained and measured on a laboratory spectrometer to confirm the field data. Spectra from the imaging spectrometer data are extracted over the calibration site(s), and compared with the field and lab data. A set of correction multipliers and offsets are derived and applied to the data. (To see our tutorial on calibration, click here).

The entire calibration process takes about ~30 PERSON DAYS OF WORK (1.5 person-months), for each day/site of imaging spectrometer data, whether the site is a small area of only a hundred sq. km, or large ~8,000 sq km). If the calibration is complex with no uniform areas, it could take longer.

WHAT IT TAKES TO ANALYZE IMAGING SPECTROMETER DATA

Although many groups around the country are developing methods for analyzing imaging spectrometer data, the USGS imaging spectroscopy group is currently leading the world in capability. The USGS Tetracorder algorithm can analyze for hundreds (or thousands) of materials simultaneously solving for what is present on the surface, whether it be minerals, environmental materials, land, water, or ecosystems related. Note that not all materials may be found in a given area, but that too can be important information, especially environmental contamination.

We group AVIRIS data in 10.5-km wide by 17.5-km long segments (614x1024 pixels), covering 184 sq. km. Called a double segment, it takes less than 2 hours to analyze for 250 materials on a 140 MFLOP workstation (like our HP 9000/K250). The analysis is fairly routine in that command files exist to automatically do such analyses. One double segment of imaging spectrometer data takes up 598 megabytes of disk space (282 Mbytes of radiance data, 282 Mbytes of calibrated data, and ~34 Mbytes of ancillary data = 598 Mbytes).

The output products comprise 3 files per material mapped, so if 300 materials were mapped, 566 Mbytes of results (614*1024*300*3) are obtained (we compress them, so the output is 3 to 10 times less space). Results of mapping for 300 materials in essence makes a 300 layer GIS data base. Note that current GIS systems only use uncompressed data (to our knowledge) and we are not aware of any that can handle this many layers.

The biggest bottleneck in producing final products is the scientist evaluating and understanding what was mapped and not data processing. After all, these mineral/material maps are a new paradigm providing unprecedented new information. For each data set, images of all the materials must be reviewed. We have clustering analysis tools and the system builds a set of image display commands to quickly go through such a huge amount of images. Typically, once you have experience in an area with the data, you can setup, analyze, and review the data for about 1 double segment per day (184 sq. km per day).

Another day is required to construct color composite map images, and yet another is required to register the data to a map base. Thus about 3 days of work by a trained, experienced person are required per double segment to analyze. This does not not include detailed evaluation or verification of results. That depends on the area complexity and the science questions being asked.

WHAT IT TAKES TO VERIFY IMAGING SPECTROMETER DATA

Verification is the most time intensive aspect. Such large areas are covered (even one double segment of 184 sq. km) that field checking can require a significant amount of time. The materials maps steer you to the interesting areas, and with the help of a field spectrometer, the mapped minerals can, in most cases, be verified real time in the field. A final verification will sometimes require study of hand samples from the field using laboratory X-ray diffraction analysis or other methods as appropriate. Verification depends on the complexity of the scene, remoteness and topography. Basic verification can take a couple of weeks, or more, depending on how much area needs to be checked.

Verification may result in additional minerals being identified which were not included in the original imaging spectroscopy analysis. Thus, a second round of analysis and verification is required (this again depends on the site complexity, what is already in the standard Tetracorder mapping, and what science questions are being asked).

PUTTING IT ALL TOGETHER, FROM PLANNING TO COMPLETED ANALYSIS AND REPORTS

Complete analysis of a full flight of AVIRIS data, from flight planning, calibration, materials mapping and verification, laboratory analyses of samples, data registration, map production, and science reports takes on the order of seven person years of effort. Smaller areas take less time, depending on the science to be done. Below is a list of the generalized steps. Verification steps take the majority of the time.

WHAT IS POSSIBLE

Small Area (~100 to 400 sq. km)

A team of 3 trained in imaging spectroscopy could calibrate the data in about 1.5 person-months and then map the surface mineralogy, vegetation, and environmental materials for a small area (100-400 sq km) for about 200-400 materials in about 0.5 person-month. Verification and field studies would take an additional couple of months.

Data volume for 2 double segments (368 sq km) would be 564 Mbytes of radiance data, 564 Mbytes of calibrated data, about 68 Mbytes of ancillary data, and about 761 Mbytes of uncompressed data products (for 400 materials). All data are stored on read/write optical disks, each with a 1.3 Gbyte capacity. Three optical disks would be needed ($200 data storage costs). Data acquisition costs would be a few thousand dollars, depending on cost sharing other sites covered by the AVIRIS flight. Note: a full AVIRIS flight ($64k + flight hours) must still be funded.

An example of a study of this size is the one we completed at Leadville, the California Gulch Superfund Site, where we mapped mine drainage, finding the most hazardous waste piles, saving time and dollars in the remediation process. Leadville took a couple of person-years of effort.

Medium area: Lessons from Summitville (12 AVIRIS double segments)

A team of 3 trained in imaging spectroscopy could map the surface mineralogy for a Summitville class area (~2,000 sq km) including calibration and first mapping pass of 200 to 400 materials in about 2.5 months (5 person-months). Verification and field studies would take several months.

Data volume would be about 7.2 Gbytes (12 double scenes). Data acquisition costs would be about $20-40k assuming cost sharing with another study in the area. Cost per sq. km: $10 to $20, depending on the cost sharing for the flight. Note: a full AVIRIS flight ($64k + flight hours) must still be funded.

Large area: 1x2-degree sheet (~19,600 sq km, 115 AVIRIS double segments)

A team of 6 trained people could map a 1x2-degree area with 2 to 2.5 AVIRIS flights, and about 6 weeks of processing on a dedicated computer. A 1x2-degree data base would comprise a data set of about 68 Gbytes for data (32 Gbytes for raw, 32 Gbytes for calibrated, and 4 Gbytes of ancillary) PLUS 87 Gbytes for 400 [uncompressed] materials, for a total of approximately 155 GBytes. Compression of results should bring products down to about 30 Gbytes, for a total of 98 Gbytes. Approximately 85 1.3 GB optical disks would be required (storage cost about $5k). AVIRIS data acquisition costs would be about $240k. Cost per sq. km: ~$12 (in this scenario, there is about 0.5 AVIRIS flight of capacity that could be used for additional areas).

Such a large area, complete with registered materials maps, verified and scientific papers written would involve about 20 person-years of effort.

Very large area (several 1x2-degree sheets up to a state or more)

Studies of this size would require special negotiation with NASA and JPL. A large study of this size would be a significant portion of the AVIRIS program. The AVIRIS group is interested in doing such large projects, even if spread over a couple of years. Such a large-scale effort could add enough money into the AVIRIS system that components like ground computers and airplane tape recorders could be upgraded so the cost per sq. km could be substantially less, perhaps as low as about $5 per sq. km.

Total time for field verification and registered maps and scientific reports would be substantial but possible within the scope of the USGS.

LIMITS TO WHAT PRESENT STAFF CAN DO

The spectroscopy group staff is currently limited and believes a maximum of 3 new AVIRIS sites (in one AVIRIS flight) can be calibrated, mapped and verified per year without additional staff. See training issues section below. Laboratory and field spectrometers are another limitation; see unique facilities section below.

UNIQUE FACILITIES

Successful imaging spectroscopy analysis requires high quality spectra of reference materials. Development of the reference database requires laboratory spectrometers that can match (or better) the spectral resolution and signal to noise of the flight instruments. In addition, field spectrometers are needed for calibration of the data, and for verification of the results. The USGS has these facilities, but the project load is very high.

TRAINING BY THE SPECTROSCOPY GROUP

The central region spectroscopy group could probably train about 6 scientists per year in imaging spectroscopy if the trainees worked on cooperative projects where all were interested in the results. Our best estimates indicate that minimum training would take about 6 weeks, including fundamentals of spectroscopy, familiarization with computer software, calibration, and field verification. The training would be broken up with 3 to 4 periods in 1- to 1.5-week segments. Training to become an expert in imaging spectroscopy, such that you can lead projects and research can take 2 or more years.

The best way to make training a success is as follows. Choose 3 sites for which the USGS programs would like imaging spectroscopy mapping each year. Have up to six scientists plus support people who are interested in the 3 areas agree to learn imaging spectroscopy. We believe that scientists will learn more and be able to apply imaging spectroscopy to their problems if they can work directly with the imaging spectroscopy group on sites all are interested in and on problems that benefit all those involved. Scientists in the MRS Program seem to have a lot of common interests so cooperative research should not be a problem.

If suitable sites and scientific problems can be addressed, then the imaging spectroscopy group and students will learn together how to solve interesting scientific problems with these new tools.

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Last modified Sept 25, 2002.