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SE-590 Field-Meas. Reflectances (OTTER)


Spectral reflectance measurements made by Spectron SE590 instruments in the context of validation of geometric-optical BRDF models.

Table of Contents:

1. Data Set Overview 2. Investigator(s) 3. Theory of Measurements 4. Equipment 5. Data Acquisition Methods 6. Observations 7. Data Description 8. Data Organization 9. Data Manipulations 10. Errors 11. Notes 12. Application of the Data Set 13. Future Modifications and Plans 14. Software 15. Data Access 16. Output Products and Availability 17. References 18. Glossary of Terms 19. List of Acronyms 20. Document Information

1. Data Set Overview:

Data Set Identification:

SE-590 Field-Meas. Reflectances (OTTER)

Data Set Introduction:

The Oregon Transect Ecosystem Research (OTTER) Project was a cooperative effort between NASA and several universities to discern the ecology of western coniferous forests using remote sensing technology supported by gound observations. OTTER is an interdisciplinary project that tested a model that estimated the major fluxes of carbon, nitrogen, and water through a temperate coniferous forest ecosystem.

Six Oregon sites across an elevational and climatic gradient were intensively studied. The transect began at the Pacific coast at the site called Cascade Head, passed through the outskirts of Corvallis, through a dense Douglas fir forest at Scio, through a mountain hemlock/subalpine fir community at Santiam Pass, through a Ponderosa pine community near Metolius, and ended at a site east of Sisters called Juniper. In all, the transect stretched some 300 kilometers west to east.

Goals of the project were to simulate and predict ecosystem processes such as photosynthesis, transpiration, above-ground production, nitrogen transformation, respiration, decomposition, and hydrologic processes; combine field, lab, and remote sensing techniques to estimate key vegetaion and environmental parameters; construct a "geo-referenced" database for extrapolation and testing of principles, techniques, and prediction; and verify the predictions through direct measurements of process rates or controls on processes.


OTTER was designed to study the ability of remote sensing to detect biophysical characteristics of plant canopies. The data sets correlating to this document contain data that has been remotely sensed by a spectroradiometer for the six different sites.

Summary of Parameters:

Two parameters were investigated: Emitted radiation and reflected radiation.


These measurements were made in the context of validation of several types of geometric-optical BRDF models that the PI and coworkers have derived over the years. These are based on the simple premise that may regard the radiance (or reflectance) measurement made by a radiometer as a linear compound of four component radiances (reflectances) associated with sunlit tree crown, sunlit background, shaded tree crown, and shaded background. That is,

R = ( Ac C + Ag G + At T + Az Z ) / A

where R is the radiance (reflectance) observed; Ac, Ag, At, and Az are the areas in the sensor's field of view associated with sunlit crown, sunlit background, shaded crown, and shaded background, respectively; C, G, T, and Z are the radiances (reflectances) of each component ("component signatures"); and A is the area of the radiometer's field of view. Our models concentrate on predicting the proportions (A's/A) acurately for different view and illumination positions, different canopy shapes and densities, etc. But information on the component radiances (reflectances) G-Z is needed if the geometric-optical models are to be validated properly. Thus these SE-590 measurements were made as a part of the OTTER project.

In addition to component signatures, directional measurements of radiance (reflectance) are also required. For these, ASAS data is used, collected from the OTTER sites as part of the OTTER data collection effort. Since the component signatures will obviously vary according to the time of day, it was attempted to collect the ground signature data close to the time of the overpass of the C-130 aircraft that collects ASAS.

Related DataSets:

Canopy Chemistry Forest-BGC Model Leaf Area Index Data Leaf Reflectances: LICOR Leaf Reflectances: Perkin-Elmer Meteorology Optical Thickness Data: Aircraft Optical Thickness Data: Ground Reflectance Reference Targets SE-590 Lab-Measured Reflectances SE-590 Landscape Reflectances SE-590 Low Altitude Reflectances Timber Measurements

2. Investigator(s):

Investigator(s) Name and Title:

Name: Alan H. Strahler, Professor Department of Geography and Center for Remote Sensing

Addresses: Boston University U.S.A.

Telephone Numbers: 1-(617)-353-5984

Electronic Mail Address:

Title of Investigation:

OTTER Reflectance Measurements Based on Geometric-optical BRDF Models.

Contact (for Data Production Information):

Name: ORNL DAAC User Services Office

Address: Oak Ridge National Laboratory U.S.A.

Telephone Number: 1-(865)-241-3952

Electronic Mail Address:

3. Theory of Measurements:

In collecting the radiance measurements, a number of readings were taken that were thought to be representative of the types of surfaces within the ASAS field of view. Since the directional effects were also of interest, views were collected of the target surfaces at +45, 0 and -45 degrees whenever possible. In other words, measurements were collected in the principal plane (plane of the sun's position) at +45 degrees (near the hotspot direction), 0 degrees (at nadir), and -45 degrees (in the forward-scattering direction). These angles were determined using a clinometer held against the case of the radiometer head. In the +45 direction, it was often necessary to move away from the target in an azimuthal direction to avoid the shadow of the instrument and operator. In general, measurements were made from one-half to one meter away from the target. A single measurement was made for each target, although sometimes several spectra were collected from the same type of surface.

4. Equipment:

Sensor/Instrument Description:

Spectroradiometer: In order to collect reflectance measurements of "flat fields" or pseudo-invarent targets, a "Spectron Engineering SE590" was used. This is a portable Spectroradiometer weighing about 1 kg, with interchangeable detector heads. Three measurement heads, with the following spectral ranges: 350-1100 nm, 400-800 nm and a shortwave infrared (SWIR) head 1100-2500 nm, were used during the May 1991 OTTER field campaign. Measurements were made with two heads to provide a spectral signature covering the visible, near-infrared and shortwave infrared.

Collection Environment:



Field investigation Ultralight airplane

Source/Platform Mission Objectives:

To collect information in the field by remotely sensing data with a spectroradiometer in an Ultralight, an extremely small, lightweight airplane flown at low altitudes.

Key Variables:

Surface Radiance.

Principles of Operation:

Grating spectrometer.

Sensor/Instrument Measurement Geometry:

The sensor is mounted within a hand-held box on a two-meter cable from the recording device, a data logger with casette tape recorder. The box is pointed at the target by eye, aligning the top long edge of the box so that it points toward the target.

Manufacturer of Sensor/Instrument:

Spectron Engineering, Inc. 255 Yuma Court Denver, Colorado 80223 1-(303)-733-1060



There are two parts to the calibration problem. First is the instrument itself. Second is the "Lambertian" panel against which the radiances will be ratioed. In general, the approach was to standardize the BU SE-590 and Spectralon panel to the Ames-2 instrument and Ames panel, thus providing continuinty of calibration with the OTTER experiment.

Instrument: The instrument was calibrated spectrally by Jennifer Dungan as part of the OTTER calibration plan. This was done by the use of didymium filters with narrow absorption bands. The BU instrument was found to be off by three bands with respect to the Ames-2 instrument. The range of the BU data are therefore 376.200 to 1113.700 micrometers for channels 1 to 249 (Dungan, memorandum of January 5, 1991).

For radiometric calibration, the Ames panel was measured under the same conditions by both BU and Ames-2 radiometers, and after spectral adjustment, a set of ratios were obtained to be applied to BU measurements to bring the radiometric responses into conformity with Ames-2. Since measurements were made for a single broadband brightness only, only a simple ratio providing a single multiplicative factor was possible.

To calibrate the BU panel, measurements were made of the BU panel by the Ames-2 spectrometer, and ratioed with the measurements made by the Ames-2 spectrometer of the Ames panel. As in the radiometric calibration above, only a single measurement was made, and thus only a single ratio correction was possible.


Tolerance standardized against the Ames-2 instrument and Ames panel.

Frequency of Calibration:

Information not available.

Other Calibration Information:

The SE-590 was retrofitted with the anti-aliasing filter in March, 1990. At that time, the instrument was checked out and recalibrated.

5. Data Acquistion Methods:

Each site was visited near the time of the overpass of the C-130. The first activity was to record the radiance of the BU panel. The clinometer was used to measure the solar zenith angle, and the time of day was recorded. Then the radiances of the various cover types were measured. As the data were collected, the scan number was recorded and the cover type and viewing position was noted. After the measurements were collected, the panel was measured once again, and the solar zenith angle and time of day were also recorded again.

6. Observations:

Observation data is not available.

7. Data Description:

Spatial Characteristics:

Site 1: Cascade Head Latitude 44 03' N, Longitude 123 57' 30" W Site 1A: Cascade Head Alder Stand Latitude 44 03' N, Longitude 123 57' 30" W Site 2: Warings Woods Latitude 44 36' N, Longitude 123 16' W Site 3: Scio Control Latitude 44 40' 30" N, Longitude 123 36' 40" W Site 3F: Scio Fertilized Latitude 44 40' 30" N, Longitude 123 36' 40" W Site 4: Santiam Pass Latitude 44 025' 20" N, Longitude 121 50' 20" W Site 5: Metolius Control Latitude 44 25' N, Longitude 121 40' W Site 5: Metolius Fertilized Latitude 44 25' N, Longitude 123 40' W Site 6: Juniper Latitude 44 17' 30" N, Longitude 121 20' W

Spatial Coverage:

Information not available.

Spatial Coverage Map:

Information not available.

Spatial Resolution:

Information not available.


Information not available.

Grid Description:

Information not available.

Temporal Characteristics:

Temporal Coverage:

19 June 1990 20 June 1990 21 June 1990

Temporal Coverage Map:

Information not available.

Temporal Resolution:

Information not available.

Data Characteristics:


Emitted radiation Reflected radiation

Variable Description/Definition:

Unit of Measurement:

nm: Nanometers

Data Source:

Field investigation and an ultralight airplane.

Data Range:

Emitted radiation: 0.000, no variance Reflected radiation: Approximately 0.000 <--> 60.000

Sample Data Record:

8. Data Organization:

Data Granularity:

The data are organized into three fields of information within each file in the data set. The first field is the wavelength (nm) region in which the data have been measured. The second field is a measure of the reflected radiation, while the third field is a measure of the emitted radiation.

Data Format:

There are 156 ASCII data sets, each accompanied by an XY plot. In addition, there are two data set companion files included with the complete data set: strahler.doc and se590.doc.

9. Data Manipulations:


Derivation Techniques and Algorithms:

R = ( Ac C + Ag G + At T + Az Z ) / A

R is the radiance (reflectance) observed; Ac, Ag, At, and Az are the areas in the sensor's field of view associated with sunlit crown, sunlit background, shaded crown, and shaded background, respectively; C, G, T, and Z are the radiances (reflectances) of each component; and A is the area of the radiometer's field of view.

Data Processing Sequence:

Processing Steps:

For analysis, the data were output from the SE-590 to a laptop PC micro and then converted to Macintosh format where they were manipulated in Excel.

The data normalization carried out in two steps. First, the appropriate panel reference was established. Since the panel radiance varied between the start and end of the measurement period, a linear interpolation of panel radiances was carried out between the first and last measurement for as many measurements as were made during the measurement period. This established a separate panel reference for each measurement, which was then divided into the observed radiance. The second step was to adjust this reflectance to that of the Ames-2 radiometer and the Ames panel, which was effected by multiplying the reflectance factor by the adjustment factor as described above and shown at the conclusion of this file.

Processing Changes:

Not applicable.


Special Corrections/Adjustments:

In processing of the raw BU radiometric measurements, both ratios were combined into a single coefficient and simply adjusted every radiance by the multiplicative factor.

Calculated Variables:

Information not available.

Graphs and Plots:

There is an XY plot for each data set to show obvious discrepancies in the spectra.

10. Errors:

Sources of Error:

Surface reflectances: Since the backgrounds were quite variable, radiances for the same surface type may also vary quite a lot.

Instrument performance: An inspection of the data shows a significant amount of noise in channels 4-17 and in channels aboveabout 225 (1038 micrometers). Channels 1-3 are not present, since these are channels that the Ames-2 records but the BU radiometer does not. Measurements in channels in these wavelengths should be used only with caution.

Quality Assessment:

Data Validation by Source:

The data have been plotted and visually inspected for obvious problems. All scans included in the data set seem reasonable.

Confidence Level/Accuracy Judgement:

In general, the quality of the data is good. There is considerable variation in measurements made of the same surface cover type, which is to be expected, since these are individual, hand-held measurements.

Measurement Error for Parameters:

Since the measurements are not replicated, no quantitative determination of error was possible.

Additional Quality Assessments:

Information not available.

Data Verification by Data Center:

To be determined.

11. Notes:

Notes about the data are not available.

12. Application of the Data Set:

The BRDF reflectance measurements are a key portion in the OTTER project goals. The give hard, physical data about the sites in the study. The combination of this remote sensing techniques with field study and laboratory work will help to simulate and predict ecosystem processes.

13. Future Modifications and Plans:

No future plans, the OTTER campaign is complete.

14. Software:

Software Description:

The public domain software package, Imdisp, is provided for image diplay on IBM compatibles. The popular shareware program, Stuffit, is necessary to extract the execution file for the Macintosh image display program, Image4pds.

Software Access:

Software to display most of the OTTER image data (except Aviris and Asas data) on Macintosh and IBM personal computers (and compatibles) is provided on the CD-Rom disc containing the data sets.

15. Data Access:

Contacts for Archive/Data Access Information:

Name: ORNL DAAC User Services Office

Address: ORNL DAAC User Services Office Oak Ridge National Laboratory U.S.A.

Telephone Number: 1-(865)-241-3952

Electronic Mail Address:

Data Center Identification:


Procedures for Obtaining Data:

Contact the ORNL DAAC User Services Office Oak Ridge National Laboratory U.S.A.

Telephone: 1-(865)-241-3952 FAX: 1-(865)-574-4665 Internet:

Data Center Status/Plans:

To be determined.

16. Output Products and Availability:

Available via FTP file or on CD-ROM.

Also available online via the World Wide Web at

17. References:

Strahler, A. H. and D. L. B. Jupp, 1990, Modeling bidirectional reflectance of forests and woodlands using Boolean models and geometric optics: Remote Sensing of Environment, vol. 34, pp.153-166.

Albers, B. J., A. H. Strahler, X. Li, S. Liang, and K. C. Clarke, 1990, Radiometric measurements of gap probability in conifer tree canopies: Remote Sensing of Environment, vol. 34, pp. 179-192.

Franklin, J., S. D. Prince, A. H. Strahler, N. P. Hanan, and D. S. Simonett, 1990, Reflectance and transmission properties of West African savanna trees from ground radiometer measurements: Int. J. Remote Sensing, in press.

Franklin, J. and A. H. Strahler, 1988, Invertible canopy reflectance modeling of vegetation structure in semiarid woodland: IEEE Trans. on Geosci. and Remote Sensing, vol. 26, pp. 809-825.

Li, X. and A. H. Strahler, 1986, Geometric-optical bidirectional reflectance modeling of a conifer forest canopy: IEEE Trans. on Geosci. and Remote Sensing, vol. GE-24, no. 6, pp. 906-919.

Li, X., and A. H. Strahler, 1985, Geometric-optical modeling of a conifer forest canopy: IEEE Trans. on Geosci. and Remote Sensing, vol. GE-23, no. 5, pp. 705-721.

18. Glossary of Terms:

Glossary terms can be found in the Glossary list.

19. List of Acronyms:

Additional acronyms can be found in the Acronyms list. ASAS Advanced Solid-state Array Sensor BDRF Bi-Directional Reflectance ESD Environmental Sciences Division (Oak Ridge National Laboratory) FTP File Transfer Protocol NASA National Aeronautics and Space Administration nm Nanometer ORNL Oak Ridge National Laboratories Oak Ridge, Tennessee, U.S.A. OTTER Oregon Transect Ecosystem Research

20. Document Information:

30 July 1996 (data set citation revised on 19 November 2002)

Document Review Date:

30 July 1996

Document ID:



Please cite this data set as follows (citation revised on 19 November 2002):

Strahler, A. H. 1990. SE-590 Field-Meas. Reflectances (OTTER). [SE-590 Field-Measured Reflectances (Oregon Transect Ecosystem Research)]. Data set. Available on-line [] from Oak Ridge National Laboratory Distributed Active Archive Center, Oak Ridge, Tennessee, U.S.A. doi:10.3334/ORNLDAAC/80.


DAAC Staff

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