BOREAS TE-09 in situ Understory Spectral Reflectance within the NSA

Summary

Spectral reflectance of the forest understory at the ground level, in three 
boreal forest sites of Northern Manitoba (56 degrees N latitude and 98 degrees W 
longitude), were obtained and analyzed. In particular, angular variation of the 
reflectance in the Old Jack Pine and Young Jack Pine forests, as well as nadir 
reflectance in the Young Aspen forest, were investigated. The complexity of 
understory composition and the light patterns limited quantitative conclusions; 
however, a number of interesting trends in the behavior of the measured values 
can be inferred. In particular, the unique spectral profiles of lichens show 
very strongly in the Old Jack Pine understory, yet are definitely less 
conspicuous for Young Jack Pine, and virtually absent in the Aspen forest. The 
angular variation of the reflectance by the Young Pine understory seems to be 
significantly toned down by fine-structured branches and their shadows. Our 
study also indicates how difficult the ground reflectance problem in a forest 
is, compared to certain previously investigated areas that have a  more uniform 
in appearance, such as prairie grassland, bare soil, or agricultural crops. This 
is owing to several factors, generally typical of a forest environment, that may 
influence the overall understory reflectance, and include: (i) a strong 
diversity of the forest floor due to the presence of dead tree trunks, holes in 
the ground, patches of different types of vegetation or litter, etc. (ii) 
pronounced three dimensional structures at the ground level, such as shrubs, 
bushes and young trees, and (iii) an irregular shadow mosaic, which not only 
varies with the time of the day, causing intensity variations, but likely also 
effectively modifies the spectrum of the illuminating light and hence the 
reflectance signal as well.  The data are stored in tabular ASCII files.

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

1.1 Data Set Identification

BOREAS TE-09 in situ Understory Spectral Reflectance within the NSA

1.2 Data Set

Introduction

The Data Set attempts to evaluate the characteristics of forest understory 
reflectance at the ground level. Whenever possible, this goal was pursued in a 
reasonably selective manner by avoiding or at least minimizing the direct 
contribution of mature trees to the recorded spectra. The field work was 
designed to concentrate on randomly selected fragments of the ground landscape 
rather than its individual components, which gives a distinctly statistical 
character to the data.

1.3 Objective/Purpose

The long term objective of the study within which these data were acquired was 
the following: To estimate light levels inside the forest canopy and to link 
these estimates with airborne images taken above the canopy, in order to tie the 
photosynthetic experiments and models with the remotely sensed measurements.  
The TE-9 project contained several sub-projects designed to work together to 
meet this goal: a high-resolution canopy modeling component, extensive 
measurements of canopy architecture and structure, photometric measurements 
inside the canopy, and spectral measurements of both the canopy and the 
understory. 

During the planning discussions, it was determined that the weakest link in this 
effort was the characterization of the subcanopy at a level sufficiently 
systematic to support the analysis of high resolution airborne imagery. This 
seemed even more important at the Northern Study Area (NSA) than at the Southern 
Study Area (SSA). Appropriate understory measurements would provide good support 
for the canopy modeling effort by providing much needed additional data for this 
work. Furthermore, the understory, especially in the northern sites, contributes 
a serious "noise" component to the canopy spectral properties, as measured above 
the canopy. Because of this, appropriate measurements of the understory are 
essential to understanding both the angular light response of the canopy and 
scaling issues. 

Furthermore, the understory contains information about the boreal environment 
which should not be ignored. We therefore participated in canopy structure 
measurements and in some basic photometric measurements, but focused most of our 
attention on laboratory and in situ spectral measurements of the understory in 
the NSA.  

Within this framework, the following specific objectives were set:

1) To study the spatial variability of in situ spectral reflectance measurements 
of the understory at accessible sites in the NSA;

2) To study the seasonal change of the in situ spectral measurements during 
leaf-up;

3) To study the angular dependence of spectral measurements of the understory; 
and

4) To obtain laboratory spectral measurements of canopy and understory 
components.

1.4 Summary of Parameters

The parameters measured include the following:  

visible & near infrared spectra at selected gridded plot sample locations in 
three NSA sites [Old Jack Pine (OJP), Young Jack Pine (YJP) and Young Aspen 
(YA)], taken at seven different angles; 

simultaneous Photosynthetically-Active Radiation (PAR) measurements;

simultaneous reflectance panel measurements; 

spectral measurements of selected vegetative and non-vegetative components; 

35-mm photographs taken at the time of the measurements; 

notes on sun and view angles, time of day and date; 

and a detailed physiological characterization of plant species present at each 
plot sample location.

1.5 Discussion

Two types of spectral reflectance measurements were obtained. One consisted of 
sequences of angular measurements, while the second consisted of a larger sample 
of nadir measurements. For each type of measurement, a different instrument 
mounting was designed and built. For the angular measurements, a tripod mounting 
was built which permitted the spectrometer to be oriented in seven directions (-
45 degrees, -30 degrees, -15 degrees, 0 degrees, 15 degrees, 30 degrees and 45 
degrees). The average height of the spectrometer head was 1.5 meters, but it 
could be adjusted between 1.3 and 1.6 meters. A spectrometer head with Field of 
View (FOV) of 18 degrees was used, giving a ground footprint of 50 centimeters 
at nadir.

A swivel bracket for the reflectance panel was added, and a camera  mount was 
also installed. For each sample plot location, the mount was set up, leveled and 
adjusted for height. Reflectance panel measurements were taken only at nadir and 
only with the panel fully sunlit (non-sunlit panel measurements are marked as 
suspect in the data set).   

Photographs were also only taken at nadir. To the extent possible, repeat 
measurements of sample plots were taken at the same time of day and a similar 
azimuth orientation, so that shadowing on the sample plot would be similar. The 
nadir mount was simpler, but also contained a means to level it and adjust the 
height. The same spectrometer head was used at a height of 1.4 meters, giving a 
similar, although slightly smaller footprint.  Data were acquired only on clear 
days, or through large holes in the clouds on partially cloudy days. The 
simultaneous PAR measurements and frequent reflectance panel measurements 
allowed for control over poor light measurements. Measurements were not taken at 
either the Old Black Spruce (OBS) or the Old Aspen (OA) site. Access to the 
first was too difficult since the instrument mounts were somewhat fragile, and a 
45 minute ride on an Argo in each direction would have ruined the instruments. 
The OA site was characterized by an understory well above 2 meters in height, 
and hence could not be properly characterized by the instrumentation set-up we 
had.

1.6 Related Data Sets

BOREAS RSS-19 1994 Seasonal Understory Reflectance Data

2 Investigators

2.1 Investigator(s) Name and Title

Dr. Geoffrey Edwards, Associate Professor

Dr. Keith P. B. Thomson, Full Professor Dr.

Alain P. Viau, Assistant Professor 

2.2 Title of Investigation

Relationship Between Measures of Absorbed and Reflected Radiation and the 
Photosynthetic Capacity of Boreal Forest Canopies and Understories 

2.3 Contact Information 

Contact 1
------------------------------
Jan Supronowicz
Centre de Recherche en Gomatique
Pavillon Casault, Universit Laval
Sainte-Foy, Qubec
Tel: (418) 656-2653
Fax: (418) 656-7411
suprono@crg.ulaval.ca

Contact 2
------------------------------
Dr. Geoffrey Edwards
Centre de Recherche en Gomatique
Pavillon Casault, Universit Laval
Sainte-Foy, Qubec
Tel: (418) 656-2653
Fax: (418) 656-7411
Geoffrey.Edwards@scg.ulaval.ca

Contact 3
------------------------------
Shelaine Curd
Raytheon STX Corporation
NASA/GSFC
Greenbelt, MD 
(301) 286-2447
(301) 286-0239  fax
shelaine.curd@gsfc.nasa.gov

3 Theory of Measurements

The value of the reflectance function for a reasonably flat surface, in a 
direction characterized by angles phi and theta (see Fig. 1), can be expressed 
as a ratio of the luminance of a fragment of that area (perceived from the 
defined direction), to the luminance of an identical in size and shape fragment 
of a fully diffuse reflecting (Lambertian) plane, seen from the same direction 
and distance, and under the same illumination conditions: Frame for Fig. 1.

Rlambda(phi, theta) = llambda,s(phi, theta)/llambda,l(phi, theta)           (1)

where llambda,s and llambda,l are luminances of the investigated surface and the 
lambertian (reference) plane, respectively. The angle phi is introduced to 
account for the possible anisotropy in the azimuth distribution of the 
reflectance factors, which a priori may be either due to anisotropic 
illumination or to specific asymmetrical structure of the examined area itself. 
The above formula is, of course, an idealization of a real physical situation, a 
fact which has important implications in our case. To begin with, it is 
rigorously valid only in the limit of an infinitesimally small solid angle of 
view, subtended by the dimensions of the observed patch as compared with the 
viewing range. Secondly, in the case of anisotropic illumination, e. g., direct 
sunlight, the variables in Eqn. (1) may also comprise the sun angular 
coordinates: the solar azimuth phisol and the solar zenith angle thetasol. 
Finally, Eqn. (1) is strictly wavelength selective, which is indicated by the 
subscript "lambda". In practice, both the solid angle of view and the spectral 
range of luminance intensity measurements, are finite. The assumption of the 
reference surface as having Lambertian properties means, that the intensity of 
light reflected into the direction (phi, theta), is directly proportional to the 
cosine of the polar angle, cos(theta). However, for small solid angles of view, 
the area seen by a detector aimed at the surface at angle theta, is proportional 
with a very good approximation to 1/(cos(theta)). Moreover, in the case of the 
conical field of view (FOV) that area has the shape of an ideal ellipse. In 
consequence, since the detector always sees the same solid angle, determined by 
its own geometry, those two effects will cancel one another. Hence, the amount 
of radiative energy per unit solid angle or, in other words, the perceived 
luminance intensity of the observed surface, will vary only very weakly with the 
polar angle theta. It is, therefore, perfectly reasonable to make use of the 
reference nadir (theta = 0 degrees) measurements in place of angular (theta 
different from 0 degrees) measurements also, as both are expected to yield 
virtually the same result.  The idea of the angular reflectance measurements was 
to acquire values of forest understory reflectance at nadir, i.e., vertical 
direction with respect to the ground, as well as at directions 15 degrees, 30 
degrees, and 45 degrees with respect to the vertical on both sides of the 
vertical in the same plane, perpendicular to the ground. Each set of data 
constitutes a spectral series, in which the reflectance in each narrow spectral 
increment is represented by a number, denoting the fraction of radiative energy 
reflected by the observed spot on the ground, as compared with the amount of 
energy in the same spectral region, reflected by a "white" reference standard 
(barium sulfate panel) (BaSO4). The basic concept of carrying out the angular 
measurements was to have a detector attached to a rigid arm that was allowed to 
swing in a plane perpendicular to the ground, around a pivot placed at the 
ground level. In this way, the arm was able to assume and be locked in any 
required position within the desired interval, while the detector could "look" 
along the direction of the arm - from the same distance at the same point of the 
ground irrespective of the angle between the arm and the vertical.

4 Equipment

4.1 Sensor/Instrument Description

a) portable spectrometer PS II, from the company Analytical Spectral Devices 
(ASD):

- spectral range: 342 nm - 1068 nm

- number of spectral channels - 512

- resolution: 10 nm (in the recent communication with an ASD representative, a 
resolution of 3.5 nm was quoted) - accuracy: 1 nm at 24 degrees C and a maximum 
of 4 nm between 0 degrees C and 50 degrees C

- integration time: 44 ms - 5.6 s

- spectrometer heads with FOV of 18 degrees, 10 degrees, 5 degrees and 1 degree

- a mounting for the ASD PS II spectrometer was provided by ASD 

b) portable personal computer POQET, which served as a controller for the PS II

c) 35 mm camera:

- during measurements attached to the tilting arm, next to the spectrometer head

4.1.1 Collection Environment

All the field measurements were carried out at various points within the BOREAS 
NSA, an 80 by 100 km rectangle whose geometrical center is located approximately 
at 98 degrees10 W longitude and 55 degrees 52 N latitude, about 25 km bearing W-
NW from the town of Thompson, Manitoba. The terrain is rather flat, with local 
relief generally not exceeding 15 m, while its elevation above sea level varies 
between 220 and 350 m. The climate of the region is moderately dry continental. 
The Jack pine (Pinus banksiana) sites can be divided into two distinct types: 
old (mature) jack pine and young (regeneration) jack pine, subsequently referred 
to as OJP and YJP. In the mature forest, the mean tree height is close to 10 m, 
while the mean age is approximately 60 years. Apart from the trees themselves, 
the understory is mostly free of natural three dimensional structures such as 
plants taller than 10-15 cm or branches at the ground level. It is dominated by 
various species of the lichen family (e.g., Cladina stellaris, Cladina mitis, 
Cladina rangiferina), usually pastel bluish, pinkish or whitish in color, with 
the addition of rusty brownish litter and/or miscellaneous types of ordinary 
green colored forest herbs in places, as well as moss (Pleurozium schreberi) at 
random locations.

The appearance of the YJP landscape is notably different. First of all, it is 
characterized by a far greater density of live tree stems per unit area, which 
leads to a significant overlap of the branches of adjacent young pines, as the 
average distance between trees is considerably less than 1 meter. Secondly, the 
height of the young pines (aged ~25 years in our case) reaches only up to about 
5 m, but many of them are much shorter and may be considered bushes for all 
practical purposes. Finally, the forest floor alone also differs from that of 
the OJP.  Other than the live tree branches, the understory does not exhibit 
much vivid green color. It seems predominantly brownish with numerous dead tree 
trunks of diameter in the 5-10 cm range, devoid of bark and sometimes partially 
decomposed. The trunks have a silvery-grayish tint, while the pale reddish brown 
hue of the floor itself is owing to an abundance of dried pine needles spread 
over bare soil. Lichens and mosses of varying light hues also occur in places, 
but in a considerably smaller amount than in the OJP forest. The aspen (Populus 
tremuloides) stand understory was characterized at the time of the field 
campaigns (June July 1994) by the abundance of litter, i.e., dried leaves on the 
ground. Those leaves, which vary in color from pale gold to dark grayish-brown, 
determine the appearance of the floor cover in certain places. In other 
locations, however, the dead leaves can be overlapped to a considerable degree 
by live, broadleaf vegetation (herbs and grasses), in which case a bright green 
color dominates the understory. There are also 
some isolated dead tree trunks and branches of an ashen grayish tint, but the 
occurrence of lichens or mosses is less than sporadic in the aspen forest. Table 
I gives an account of the basic parameters of the three types of forest.

                           TABLE I.

Basic characterization of the investigated forest types

OLD JACK PINE :

Stem density :  ~1900/ha
Age :   55-60 years
Height :        ~10 m
Ground cover :  lichen-dominated + mosses + vascular herbs

YOUNG JACK PINE :

Stem density :  ~23000/ha (even denser including young regenerating trees)
Age :   25 years
Height :        3-5 m (even shorter including regenerating trees)
Ground cover :  young pines, needle litter, bare soil, dead wood, lichens, herbs

YOUNG ASPEN :

Stem density :  ~2400/ha
Age :   14-24 years     Data for BOREAS Southern Study Area
Height :        6-12 m
Ground cover :  mostly leaf litter, green herbs, dead wood

4.1.2 Source/Platform

Source: plant species, forest litter and bare soil present at each plot sample 
location.
Platform: portable tripod mounting;
Angular mounting - a pivot mounting allows the spectrometer head to be tilted, 
while ensuring that the target remains constant;
Nadir mounting - fixed;
Device is operated by a person on the ground.

4.1.3 Source/Platform Mission Objectives

To perform in situ measurements of the forest understory reflectance.

4.1.4 Key Variables

spectral distribution of reflected radiation intensity angular distribution of 
reflected radiation intensity 

4.1.5  Principles of Operation

The main device used for measurements is a holographic grating spectrometer, 
designed to disperse the captured light and project the respective components 
onto an array of silicon photodiodes. Each illuminated photodiode generates an 
electric current, which is proportional to the number of photons of light 
striking it from the corresponding spectral interval. The photodiode output 
signal is registered by a microcomputer and stored in its memory. The photo 
camera plays only an auxiliary role to obtain the pictures (from the nadir angle 
of view) of the investigated sites.

4.1.6 Sensor/Instrument Measurement Geometry

Angular mounting:
sensor height = 1.5 - 0.2 m at nadir; conical FOV with cone angle of 18 degrees
angular settings = -45 degrees, -30 degrees, -15 degrees, 0 degrees, 15 degrees, 
30 degrees, 45 degrees 

Nadir mounting:
height = 0.75 - 1.5 m, depending on plot
conical FOV = 18 degrees

4.1.7 Manufacturer of Instrument

Analytical Spectral Devices
4760 Walnut Street, Suite 105
Boulder, Colorado  80301
USA
(303) 444-6522
(303) 444-6825 (fax)

4.2 Calibration

Dark count subtraction occurs at the time of measurement;  
Integration time was varied to maintain similar signal to noise ratio;  
Each recorded spectrum is an average of five "snapshot" spectra; 
Three spectra were recorded at each location; Uses a 4-inch barium sulfate 
reflectance panel

4.2.1 Specifications

The calibration of the spectral measurements with the reflectance panel was 
carried out by plotting the reflectance panels throughout the day. Panel 
reflectance's were acquired before and after each sequence of spectral 
measurements. For each data set, the appropriate panel reflectance was then 
interpreted along these curves. Linearity of spectral response was also 
evaluated during the calibration procedure by ratioing reflectance panel 
spectra, to ensure that the spectra could be reliably scaled in intensity. The 
instrument was calibrated at the ASD headquarters before the field campaign. 
This included calibration of the wavelengths and intensity response of the 
individual diode elements.

4.2.1.1 Tolerance

Each of the 512 photodiode channels is capable of registering 4095 counts 
(photons), after which it becomes saturated.  Hence, care has to be taken that 
for each measurement the signal amplitude in the largest count channel does not 
exceed that number while, on the other hand, it is reasonably large to make the 
best use of the dynamic range of the instrument.  This is done by varying the 
accumulation (integration) time, depending on the luminosity of the scene.

4.2.2 Frequency of Calibration

The calibration with respect to the standard panel was carried out before and 
after the series of measurements at each site.  We relied upon the company 
wavelength calibration. 

4.2.3 Other

Calibration Information
The factory calibration:
First channel corresponds to 342 nm
Step 1.4209 nm over the next 511 channels
Last channel 1068.08 nm
Reproducibility 1 nm at constant temperature 24 degrees C; resolution 3.5 nm

5 Data Acquisition Methods

The mounting constructed for angular measurements was placed in a designated 
spot. Three "raw" spectra were recorded for each of the 7 positions of the 
movable arm, corresponding to angles -45 degrees, -30 degrees, -15 degrees, 0 
degrees, 15 degrees, 30 degrees and 45 degrees between the arm and the vertical.  
Both before and after taking these data, also the spectra of the reference panel 
were recorded (3 before and 3 after). At the nadir arm position, a photo picture 
of the site was taken by the camera, mounted on the arm. In the case of nadir 
measurements, the reference spectra also preceded and followed three recordings 
of the understory signal. The spectra in digitized form were subsequently 
transferred to floppy disks.

6 Observations

6.1 Data Notes

The orientation of the plane of measurements, i.e., the plane in which the arm 
of the mount was able to swing, relative to the solar plane, is a piece of 
information usually important for the overall interpretation of the results. 
That orientation (angle between the planes) is known in all but one case of the 
angular data. The angular solar elevation above the horizon is also known for 
all cases, although it does not vary much from site to site: all the angles are 
included within the 43.7 degrees - 55.2 degrees interval. See Section 11.4 for 
more precise information. In certain cases, a tree branch would be very close to 
the instrument's sensor, blocking its FOV. Generally, such a situation should be 
regarded as nothing else but an event of recording the spectrum of a randomly 
chosen fragment of the landscape. If, however, the measuring device itself 
projected a shadow upon the investigated element, then the outcome of the 
experiment ought to be considered as somewhat distorted by the very act of 
measurement, and hence treated with caution.

6.2 Field Notes

Some of the YA data originating from 31-JUL-1994 were recorded in the presence 
of a certain amount of smoke in the air. The smoke most likely originated from a 
forest fire burning  nearby.

7 Data Description

The data are submitted as a collection of spectral series, each one of which was 
recorded at a pre-defined location in one of the three types of boreal forests: 
YJP, OJP and YA, and at a specific time.

7.1 Spatial Characteristics

7.1.1 Spatial Coverage

The data pertaining to the Old Jack Pine forest were collected at 7 pre-selected 
sites within a radius of about 40 m, in the area adjacent to the NSA OJP Flux 
Tower (site ID: T7Q8T, BORIS Grid coordinates: X=768.5, Y=617.1; longitude 
98.624 degrees W, latitude 55.927 degrees N). For the Young Jack Pine forest, 8 
sites were chosen in a similar-size area around the NSA YJP Flux Tower (site ID: 
T8S9X, BORIS Grid coordinates: X=789.6, Y=618.2; longitude 98.288 degrees W, 
latitude 55.903 degrees N). In the Young Aspen forest, the data were recorded 
twice at the same 14 spots in mid June and at the end of July. The data 
collection area was approximately of a size of 50 m x 50 m. It was located at 
one of the auxiliary sites, W0Y5A (BORIS Grid coordinates: X=845.9, Y=640.5; 
longitude 97.336 degrees W, latitude 56.004 degrees N). 

7.1.2 Spatial Coverage Map

A hard copy of the NSA with the collection sites marked will be sent by mail

7.1.3 Spatial Resolution

The "footprint" of the instrument collecting signal in the nadir position was a 
circle of a diameter of approximately 44 cm. At non-nadir positions, the 
footprint would become an ellipse having its major axis increasing in the 
direction of displacement from the nadir. At 45 degrees, that axis was about 64 
cm. The minor axis stays practically unchanged vs. displacement. This implies 
that also the area of the observed ellipse increases for non-nadir viewing 
angles. 

7.1.4 Projection

Not applicable to this data set

7.1.5 Grid Description

In all cases, the actual investigated spots were selected near the corners of a 
local rectangular coordinate system (grid).  In the OJP forest it was a 10m x 
10m grid, and the examined sites had the following coordinates:  (all numbers 
are given in meters), where the first of the coordinates denotes bearing south-
east of the T7Q8T flux tower (130 degrees relative to the north), and the second 
one northeast (40 degrees relative to the north). 

(90, 12)

(100.8, 9.8)

(110.8, 8.5)

(120.8, -0.8)

(81, -11)

(90.6, -20)

(110.7, -19.8)

The YJP site was covered by a 5m x 5m grid, anchored at the T8S9X tower. The 
spot coordinates were:

(160, 28.2)

(130, 6.3)

(121.6, 5)

(120.8, 15)

(126.4, 20)

(130.9, 25)

(136.3, 30.2) and

(146.9, 35)

with the angular orientation of the axes identical as in the OJP case. 

The YA grid was also 10 m x 10 m, yet its (0, 0) origin was chosen arbitrarily 
in the forest, as there were no discriminated points or objects at the W0Y5A 
site, to which such a grid could be anchored. The first and second coordinates 
were measured along the axes directed towards east and north, respectively. On 
13-JUN-1994 they were: 

(-20, -19)

(-20, -8.8)

(-20, 1)

(-20, 10.9)

(-20, 21)

(-20, 31.5)

(-20, 42.4)

(18.9, -20) (no photo available from 13-JUN-1994) (19.2, -10)

(18.5, 0)

(19, 10)

(18.9, 20)

(8.9, 10) and

(-1.1, 0)

On 31-JUL-1994 all the above sites underwent investigation one more time. The 
actual positions of the instrument footprint were in most cases reproduced with 
the accuracy better than 0.1 m. In two instances, however, namely for sites (-
20, 21) and (-20, 42.4), it can be inferred from the appropriate photos that the 
difference between the coordinates on those two dates may have been considerably 
larger, and that in fact signals from different patches of the ground may have 
been recorded. In addition, data from a site having coordinates (-11.1, -10) 
were also acquired on that day.

 
7.2 Temporal Characteristics

7.2.1 Temporal Coverage

The data pertaining to the OJP forest were recorded on 2 different days: the 
(81, -11), (90.6, -20) and (110.7, -19.8) sites were investigated on 16-JUL-
1994, whereas the remaining OJP sites on 21-JUL-1994.  All the data pertaining 
to the YJP forest were recorded on 17-JUL-1994. The YA data were recorded twice 
at the same sites: on 13-JUN-1994 and on 31-JUL-1994.

7.2.2 Temporal Coverage Map

The times of measurements [all in Greenwich Mean Time (GMT)]

For OJP sites (on 16-JUL-1994):

(81, -11)       - 15:48

(90.6, -20)     - 16:10

(110.7, -19.8) - 16:38

and on 21-JUL-1994:

(90, 12)        - 16:22

(100.8, 9.8)    - 16:40

(120.8, -0.8)  - 19:12

(110.8, 8.5)    - 19:34

For YJP sites:

(130, 6.3)      - 15:45

(121.6, 5)      - 16:09

(120.8, 15)     - 16:25

(126.4, 20)    - 16:42

(130.9, 25)    - 17:00

(136.3, 30.2)  - 18:15

(146.9, 35)    - 18:58 and

(160, 28.2)    - 19:15

Table II shows the times of the YA measurements.

                          TABLE II

Times of data acquisition in the Young Aspen forest (GMT)

DATE                  13-JUN-1994       31-JUL-1994

           SITE

        (-20, -19)      15:25           18:02

        (-20, -8.8)     16:09           18:15

        (-20, 1)        16:20           18:24

        (-20, 10.9)     16:28           18:35

        (-20, 21)       16:40           18:44

        (-20, 31.5)     16:58           19:01

        (-20, 42.4)     17:26           19:10

        (18.9, -20)     19:23           20:02

        (19.2, -10)     19:35           19:50

        (18.5, 0)       19:50           19:42

        (19, 10)        20:00           19:31

        (18.9, 20)      20:04           19:22

        (8.9, 10)       20:14           20:33

        (-1.1, 0)       20:23           20:23

        (-11.1,-10)       -             20:15

7.2.3 Temporal Resolution

At each site, carrying out the measurements, including the successive placing of 
the mount in all 7 positions, took a few minutes. However, the actual data 
recording time was much shorter: the spectral series of the reference panel were 
acquired in the time span of 88 or 175 ms each, while those of the forest 
understory - over 700 ms or 1.4 s, depending on the luminance intensity.

7.3 Data Characteristics                                                        
                                                                                
Data characteristics are defined in the companion data definition file 
(te9spref.def).

7.4  Sample Data Record

Sample data format shown in the companion data definition file (te9spref.def).
                                                                            
8 Data Organization

Data files were submitted for the OJP, YJP, and YA sites as the previously given 
coordinates (section 7.1.5).

8.1 Data Granularity

All of the In Situ Understory Spectral Reflectance Within the NSA Data are 
contained in one dataset.
 
8.2 Data Format(s)

The data files contain numerical and character fields of varying length 
separated by commas. The character fields are enclosed with  a single apostrophe 
marks. There are no spaces between the fields.  Sample data records are shown in 
the companion data definition files (te9spref.def).

9 Data manipulations

9.1 Formulae

The formulae used in processing the reflectance data are rather simple and 
straightforward in that they do not require any particular justification and/or 
derivation explanation. First of all, all the data are corrected for dark 
current signal:

Icor(lambda) = Iraw(lambda) - IDC(lambda),                   (2)

where IDC(lambda) is a value averaged over N successive measurements (in case of 
the dark current N = 5), taken over the same period of time (integration time) 
as the raw data:

IDC(lambda) = (Sum(I1DC(lambda) + ... + INDC(lambda)))/N.    (3)

Subsequently, the corrected white-reference and forest understory series are 
used to calculate the reflectance of the understory:

R(lambda, theta) = Iund(lambda, theta)/Iwr(lambda, theta=0)* tauwr/tauund,      
 (4)

   
where the understory signals are averaged over 3 (for each angle setting) 
measurements, while the white reference data over 6 measurements (3 taken before 
and 3 after the recording of all the understory signals for a given site), 
according to a formula analogous to (2).The appropriate tau-values are the 
integration times of the white reference and understory signals, and theta is 
the angle between the swing arm and the vertical.

9.1.1 Derivation Techniques and Algorithms

The conversion of the originally acquired binary spectra to the American 
Standard Code for Information Interchange (ASCII) format was carried out with 
the aid of a program "portspec.exe", provided with the Personal Spectrometer II 
software utility package designed to run on the International Business Machines 
(IBM) family of personal computers (PC`s).

9.2 Data Processing Sequence

9.2.1 Processing Steps

The dark current-corrected data pertaining to both white reference and raw 
understory measurements, were converted to the ASCII format on an IBM PC. The 
ASCII files were subsequently imported into an EXCEL worksheet. Both sets of 
data were in turn averaged over the appropriate number of records, according to 
the formula similar to (3). The following step was to apply formula (4) to 
calculate the reflectance.  In this way, the final result of the data processing 
sequence was, in case of each site, a table representing the functional 
dependence of the understory reflectance on the wavelength and additionally - in 
the cases of OJP and YJP - also on the viewing angle.

9.2.2 Processing Changes

None.

9.3 Calculations

All the necessary calculations, essentially making use of formulae (3) and (4), 
were performed using the Microsoft EXCEL 5.0 spreadsheet software.

9.3.1 Special Corrections/Adjustments

The first channel of the PS II spectrometer corresponding to lambda = 342 nm, 
yielded erroneous readings, and thus has not been included in tables and graphs.

9.3.2 Calculated Variables

The calculated variables are reflectance spectra whose functional form R(lambda, 
theta) is defined by Eqn. (4). It is a function of two arguments (or just one in 
the case of YA forest) and has dimensionless values which can be expressed as 
ordinary or percentage fractions.

9.4 Graphs and Plots

All the graphs were created with the aid of the EXCEL 5.0 spreadsheet software. 
The graphs include plots - in the rectangular coordinate system - of the 
reflectance dependence on the wavelength for various viewing angles, as well as 
plots in the polar coordinate system of the angular dependence of the signal 
intensity at selected wavelengths. Graphs of the averaged (separately over all 
the OJP, YJP and YA sites) reflectance at nadir were also produced.

10 Errors

10.1 Sources of Error

The main source of error appears to be the following fact, which found 
confirmation in an auxiliary experiment, carried out in the laboratory. In the 
off-nadir positions, the wooden rod that serves as a swinging arm, tends to bend 
towards the ground under its own weight and the weight of the attached 
equipment. Because of a considerable length of the arm, even a slight amount of 
bending results in an appreciable shift of the FOV ellipse away from nadir. The 
ellipse also becomes slightly larger, as bending of the rod effectively 
increases both the angle theta and the actual distance from the sensor to the 
viewing spot, which ideally is supposed to remain constant. It turned out that 
for the left and right 45 degrees position of the arm, the common FOV is not 
quite 50%. A similar condition, albeit less severe, also exists for smaller 
deflection angles. Hence the aim of measuring the angular reflectance of the 
same spot on the ground has been to a certain degree compromised. This effect 
becomes particularly aggravating in a highly diverse environment such as a 
forest. Irregularly and asymmetrically shaded areas, tree branches, dead tree 
trunks and other inhomogeneities cause the signal amplitude to depend on the 
"swing" angle in a very complex manner, which includes not only the a priori 
expected direct dependence of the reflectance on the reflection angle, but also 
indirectly via the change in the position of an actual spot on the ground giving 
rise to the recorded spectrum. Hence, the directional variability of the 
reflectance signal, which might be perceived with a coarser resolution from a 
much larger distance, is often effectively masked. This problem is further 
complicated in forests featuring rich undercanopy vegetation which tends to 
change perspective with the viewing angle and sometimes even completely obscures 
the view of the ground. Another, perhaps only moderately important source of 
error is the variation in the signal amplitude due to fluctuations of the 
illumination intensity and possibly also due to the movement of vegetation 
caused by wind. We try to alleviate these effects by recording a few, rather 
than one, spectral series for each case, and using the average as the result.

10.2 Quality Assessment

10.2.1 Data Validation by Source

The data to be submitted constitute only a part of all the data recorded in the 
course of the field campaign, and is in our opinion the most reliable and error-
free. Data containing crude inaccuracies and uncertainties due to malfunctioning 
or missing parts of equipment, weak batteries, smoke or fog suddenly appearing 
in the middle of measurements etc., were eliminated. Certain submitted data, 
however, may also initially seem to make little sense, mostly in the cases of 
sites with pronounced 3 dimensional structures present, for instance low tree 
branches or undercanopy vegetation. These complexities notwithstanding, we have 
decided to include such data as well, considering that their contribution would 
render more realistic the overall representation of the understory reflectance.

10.2.2 Confidence Level/Accuracy Judgment

In spite of the above shortcomings, the collected data do carry and can yield a 
considerable amount of information, especially when carefully reviewed in the 
context of the accompanying photo pictures pertaining to each site. The angular 
variation of the signal amplitudes is usually intuitively correct for cases 
where it is governed mainly by the forward and backward scatter, that is to say 
when the forest floor is well sunlit and has an approximately homogeneous 
appearance over the region of data collection - i.e., at least 1 m x 0.5 m at 
each investigated spot. For other cases it is a priori less predictable and its 
character alters a great deal from site to site, depending on which side of the 
scene with respect to the nadir point seems to be greener, brighter, more brown, 
more beige etc., but even there the results generally tend to be in agreement 
with intuition and common sense. The already mentioned lack of ordering from the 
point of view of the solar plane versus measurement plane orientation, as well 
as a virtually random, so to speak a priori choice of the spots to investigate, 
introduces also a positive aspect to the qualitative analysis of the results, 
particularly from the remote sensing point of view. A set of data characteristic 
of the "clean" situations both in the sense of appearance and angular 
orientation - would arguably constitute a better basis for testing various 
simple theories and models. However, while it certainly would be useful to have 
such a data set, it could not really be considered as representative of the 
forest floor as a whole, since the certain amount of bias necessary to make the 
selections might discriminate either for or against all the features correlated 
with the criterion of choice, for instance different vegetation in more 
intensely sunlit places. The data also look promising from the spectral point of 
view. A relatively large number of spectral series, taken at different sites and 
at different angles, can provide material for various types of analyses, 
including correlation of the spectra with the type of ground cover and 
undercanopy vegetation, relative luminance of shadowed vs. sunlit areas, 
averaging spectra from several sites in a given forest type to produce a 
representative of the understory signature etc.. 

10.2.3 Measurement Error for Parameters

The uncertainties associated with the diversity in appearance of the forest 
floor and/or the presence of 3-dimensional structures at some of the sites, do 
not lend themselves easily to a quantitative treatment, as the signal amplitudes 
can vary rather wildly not only from site to site, but sometimes even from angle 
to angle at the same site.  The statistical error associated with signal 
intensity fluctuations during the angular and reference panel measurements was 
estimated using the standard formula for the uncertainty in the arithmetic mean 
of N samples xi:

S<x> = ((<X^2> - <X>^2)/(N-1))^1/2,                            (5)

where N was 3 for each arm position at each site, and 6 for the reference panel 
calibration runs. The above formula was evaluated independently for the signals 
in each of the 512 wavelength increments.  The relative error of the calibrated 
reflectance values at each wavelength was subsequently obtained as:

Delta R/R = ((S<wr>/<wr>)^2 + (S<und>/<und>)^2)^1/2,           (6)

where "wr" stands for the white reference and "und" for the understory signals, 
respectively. In the majority of cases, this error turned out to be not 
particularly significant, at least in the 400 - 900-nm wavelength interval, 
where it often did not exceed 2%, and very rarely reached 4%. For other 
wavelengths, however, it tended to be generally larger, peaking for the lower 
and upper limits of our scanning region (342 and 1068 nm, respectively) at 
values approximately 2 - 2.5 times larger than the middle plateau. Such a 
behavior is not unexpected in view of the very weak white reference signal for 
the limiting wavelengths, which increases the relative uncertainty. It is 
reflected in a noticeable increase in the signal-to-noise ratio for wavelengths 
longer than 900 nm, shared by virtually all the spectral curves.

10.2.4 Additional Quality Assessments

Even though derived from in situ measurements, the information contained in the 
collected data has a distinctly statistical character. First of all, the 
variable measured at the ground level is, strictly speaking, the local relative 
luminance, rather than reflectance, of the patch of ground corresponding to the 
instrument's footprint. To obtain the actual reflectance one would have to, by 
definition, normalize the understory signal to the reference signal acquired 
under identical illumination conditions, i.e., sun or shade, depending on the 
site.  Such an approach has been chosen by J. Miller's team (RSS-19, York 
University) but is not the case with our data, based on the reference panel 
routinely fully sunlit. In a broader sense, however, one might argue that these 
results constitute a very reasonable statistical representation (sample) of a 
mean reflectance averaged over a larger area and thus taking into account all 
its features, including shadows which vary with the illumination conditions. In 
the same way, for instance, the perceived reflectance of a fully grass covered 
lawn may be different from the reflectance of a single grass blade, because of 
the structural complexity of the lawn surface. Furthermore, all the submitted 
data are valid in the sense that they were acquired in the manner described in 
Section 5 while all the elements of the equipment were functioning properly to 
the best of our knowledge. However, because of the FOV being factually subject 
to variations with the viewing angle, as well as of the presence of pronounced 
3D structures in several of the investigated sites, it is not possible to know 
exactly which objects and/or area had in each case contributed to the recorded 
spectrum, especially for the off-nadir measurements. This circumstance also 
underscores the particular suitability of the data to be viewed in a statistical 
context. 

10.2.5 Data Verification by Data Center

Data was examined for general consistency and clarity.

11 Notes

11.1 Limitations of the Data

While preponderance of the submitted data have been normalized to the fully 
sunlit reference panel (see also Sections 1.5 and 10.2.4), the calibration of 
the June YA data was carried out with the panel in (partial) shadow, and 
therefore they are not entirely compatible with the series recorded in July. 
However, we still wish to present some of these data, while being particularly 
candid and clear about their limited compatibility with the main data set. 

11.2 Known Problems with the Data

Data from the OJP site (120.8, -0.8) seem to be not fully reliable, especially 
for -30 degrees and -45 degrees. Both a field note and the photo of this site 
indicate, that at those arm positions the detector might have been very close to 
a leafy tree branch, and hence its FOV blocked (at least partially). This 
assumption is consistent with the apparent amplitudes of the appropriate 
spectral curves. The OJP site (110.7, -19.8) data are distorted by the fact, 
that the right-hand side of the scene was dominated by a deep hole in the 
ground, caused by a falling tree. As that side was predominantly viewed by the 
detector at positive swing angles, the observed angular variation of the signal 
is largely artificial, and does not reflect the actual differences in the 
forward- and backward-scattered signal, but rather the highly asymmetrical 
disposition of the scene. The results pertaining to the OJP (90, 12) site rank 
on the whole among the best of all the series of OJP data, as the forest floor 
there appeared fairly homogeneous from the optical point of view. However, 
according to a field note, at arm positions of 30 degrees and 45 degrees, the 
shadow of the camera found itself in the FOV. It can be estimated from the 
picture, that at most 8% of the FOV is occupied by the camera shadow, so it 
should not introduce too big an error. As already mentioned in Section    7.1.5, 
at sites (-20, 21) and (-20, 42.4) in the Young Aspen, not the same patches of 
ground may have undergone observation on 13-JUN-1994 and 31-JUL-1994, contrary 
to what was intended.  The reflectance spectrum of the YA site (18.9, -20) from 
13-JUN-1994 has an unusual shape, although no obvious inconsistency in the raw 
data used to produce that profile is apparent. Because of that, and also in view 
of the unavailability of the corresponding photograph, that spectrum has not 
been taken into account when calculating the average. 

11.3 Usage Guidance 

An important caution to be borne in mind when working with these data is, that 
no set of results pertaining to an individual site should be regarded as 
"typical" for a particular kind of forest since, in principle, there is no such 
thing as a typical spectral signal (meaning both amplitude and signature) due to 
a small fragment of the understory of the size of our instrument's footprint. On 
the contrary, the only typical feature of the ground appearance is its great 
diversity, at least on the scale of our measurements. This notice is 
particularly relevant to the case of the OJP data which tend to differ more from 
site to site than the YJP or YA spectra. The whole data set is "randomized" even 
further owing to the absence of constant orientation of the plane of 
measurements with respect to the solar azimuth (see the following Section), and 
partial shifting of the FOV with the angle at which the spectra were acquired.

11.4 Other Relevant Information

Table III gives the solar angles phisol and thetasol, as well as the relative 
(solar plane - swing plane) azimuth (see also Fig. 2) relevant in the OJP and 
YJP measurements.  This table is also included as a data file on the BOREAS CD.

                              TABLE III.

                                   Degrees

date       site       solar zenith   solar azimuth    relative

                        angle        angle*           azimuth

OJP

16-JUL   (81, -11)       46         121           ~50

16-JUL  (90.6, -20)      44         127            86

16-JUL  (110.7, -19.8)   41         136          ~120

21-JUL    (90, 12)       43         131          ~175

21-JUL  (100.8, 9.8)     41         137            92

21-JUL  (120.8, -0.8)    36         194           ~90

21-JUL  (110.8, 8.5)     37         203          ~100

YJP

17-JUL  (130, 6.3)       47         120          ~170

17-JUL  (121.6, 5)       44         126         ~100 

17-JUL  (120.8, 15)      42         131          ~100

17-JUL  (126.4, 20)      41         137           ~88

17-JUL  (130.9, 25)      39         143           102

17-JUL (136.3, 30.2)     35         171          ~127

17-JUL  (146.9, 35)      35         189          120

17-JUL  (160, 28.2)      35         195            12

* clockwise from the true North direction

 estimated from the direction of shadows in the photo

The solar angles have been estimated with the aid of the Multiyear Interactive 
Computer Almanac software system (MICA, see Section 14.1) taking into account 
the Julian day number, hour of the day and geographical longitudes and latitudes 
of the OJP and YJP sites, and rounded off to the nearest degree.

Frame for Fig.2.

12 Application of the Data Set

This data set was designed (1) to study the spatial variability of in situ 
spectral reflectance measurements of the understory at accessible sites in the 
NSA; (2) to study the seasonal change of the in situ spectral measurements 
during leaf-up; (3) to study the angular dependence of spectral measurements of 
the understory; and (4) to obtain laboratory spectral measurements of canopy and 
understory components.

13 Future Modifications and Plans

In addition to the results that have been interpreted and are presented in this 
report, we have a substantial amount of other data.  Those data originate 
predominantly from measurements aimed at obtaining separate spectral 
characteristics of "pure" elements of the forest landscape, in particular 
specific plant species, litter, soil etc.. They were procured either by means of 
in situ estimation at narrow (1 degrees or 5 degrees) FOV, or in the laboratory 
with the aid of an integrating sphere (LI-COR Model 1800-12 Integrating Sphere). 
However, an initial quality screening as well as analysis of that additional 
material would require considerable extra time and effort, and we are at present 
unable to plan any action along these lines in the foreseeable future. 
Prospective investigation of the various questions related to the problems of 
forest understory reflectance could be pursued along several lines. In addition 
to the issue of shadows, it would be interesting, especially in 
forests with little or no undercanopy growth, to examine more systematically the 
angular reflectance function in relation to the solar coordinates. Tracing 
changes in reflectivity due to a recent rainfall (i.e., dry vs. wet understory) 
also looks as a challenging task. Following the approach of Goward et al. (1994) 
(see Section 17.2), it would be quite useful as well to obtain some specific 
information on the spectral reflectivity of old wood, particularly in view of 
its abundant presence in certain types of the boreal forests, and likely 
appreciable contribution to the understory signal.

14 Software

14.1 Software Description

1) Program "portspec.exe", developed by Microsoft Corporation, and provided with 
the Personal Spectrometer II software utility package. The program was used to 
convert the originally acquired binary spectra to the ASCII format;

2) EXCEL 5.0 for Windows software, used to further process the ASCII data.

3) MICA for DOS 1990-1999, Version 1.0. Developed by the staff of Astronomical 
Applications Department, U.S. Naval Observatory,3450 Massachusetts Avenue, N.W., 
Washington, DC 20392-5420, USA. Used to calculate sun's angular coordinates.

14.2 Software Access

Commercially available.

15 Data Access

15.1 Contact Information

Ms. Beth McCowan
BOREAS Data Manager
NASA GSFC
Greenbelt, MD 
(301) 286-4005
(301) 286-0239 (fax)
beth@ltpmail.gsfc.nasa.gov

15.2 Data Center Identification

See 15.1.

15.3 Procedures for Obtaining Data

Users may place requests by telephone, electronic mail, or FAX.

15.4 Data Center Status/Plan

The TE-09 understory reflectance data are available from the EOSDIS ORNL DAAC 
(Earth Observing System Data and Information System) (Oak Ridge National 
Laboratory) (Distributed Active Archive Center).  The BOREAS contact  at ORNL 
is:

ORNL DAAC User Services
Oak Ridge National Laboratory
(865) 241-3952
ornldaac@ornl.gov
ornl@eos.nasa.gov

16 Output Products and Availability 

None.

16.2 Film Products

None.

16.3 Other Products

Tabular ASCII files.

17 References

17.1 Platform/Sensor/Instrument/Data Processing Documentation 

ASP Personal Spectrometer II User's Reference
ASP Personal Spectrometer II Utility Reference ASP Personal
Spectrometer II Technical Reference
All of the above published by the manufacturer (Analytical Spectral Devices, 
Inc., see Sec. 4.1.7).

MICA for DOS 1990-1999, Version 1.0, User's Guide. 
Astronomical Applications Department, U.S. Naval Observatory,3450 Massachusetts
Avenue, N.W., Washington, DC 20392-5420, USA.

17.2 Journal Articles and Study Reports

(i) B. V. Vinogradov, 1969. "Remote Sensing of the Arid Zone Vegetation in the 
Visible Spectrum for Studying the Productivity." Proceedings of the Sixth 
International symposium on Remote Sensing of Environment. Ann Arbor, MI, pp. 
1237-50. 

(ii) D. W. Deering et al., 1992. "Prairie Grassland Bi directional Reflectances 
Measured by Different Instruments at the FIFE Site." Journal of Geophysical 
Research 97, pp. 18872 903. 

(iii) E. A. Walter-Shea et al., 1992. "Biophysical Properties Affecting 
Vegetative Canopy Reflectance and Absorbed Photosynthetically Active Radiation 
at the FIFE Site." Journal of Geophysical Research 97, pp. 18925-34. 

(iv) S. N. Goward, K. F. Huemmrich, and R. H. Waring, 1994. "Visible Near 
Infrared Spectral Reflectance of Landscape Components in Western Oregon." Remote 
Sensing of Environment 47, pp. 190 203.

(v) H. P. White and J. R. Miller, 1995. "Report on Seasonal Change in the Mean 
Understory Reflectance at BOREAS Flux Tower Sites." Private Communication and to 
be published.

(vi) J. Supronowicz, G. Edwards, K. P. B. Thomson, and A. Viau, 1996. "Spectral 
Reflectance of Forest Understory in Various Types of Forests within the BOREAS 
Northern Study Area in Manitoba." Submitted for publication in the Canadian 
Journal of Remote Sensing.

17.3 Archive/DBMS Usage Documentation

None.

18 Glossary of Terms

bi-directional reflectance - in the case of strongly anisotropic source of 
illumination (e.g., the sun), the functional dependence of the surface-reflected 
radiation intensity on both the two angular coordinates of the light source 
(solar coordinates) and the direction of reflection (also defined by two 
angles).

dark count (dark current), IDC - a residual electrical signal outputted by a 
photoelectric sensor (e.g., photodiode) under its normal operating voltage but 
in the absence of any incoming optical signal.

forest litter - naturally occurring, dead but once part of a living plant, 
components of the forest floor cover; this category comprises, in the first 
instance, fallen tree leaves and needles.

forest understory - all the components present and observable at the ground 
level in the forest environment; may comprise lichens, mosses, mushrooms, low 
vascular vegetation, forest litter, soil, dead tree trunks etc..

holographic grating - a type of diffraction grating whose groove structure in 
the form of interference fringe pattern has been holographically recorded and 
permanently fixed in photographic emulsion.

instrument footprint - a surface fragment actually undergoing observation or 
giving rise to the detected signal, its shape and size determined by the 
geometry of the instrument sensor and by the surface sensor distance.

Julian day number - the interval of time in days since January 1, 4713 BC, 
Julian Calendar. For instance, 16-JUL-1994 is Julian day 2449551.

lambertian reflector - a conceptual surface reflecting 100% of the impinging 
radiation in such a way, that its radiance intensity is isotropic irrespective 
of the irradiance conditions. Hence, the perceived radiance (brightness) of such 
a reflector does not depend on the direction of view.

lichens - primitive, autotrophic plants of the order Lichenace , composed of 
parasitic fungi interwoven with algal cells. Many species of lichens with 
variously colored crusts occur, often abundantly, in certain types of forests on 
rocks, soil, tree bark etc..

solar azimuth, phisol - the angle formed by the solar plane and the plane of the 
local meridian. 

solar plane - the plane defined by the vertical upward (zenith) direction and 
the direction towards the instantaneous position of the sun in the sky as 
perceived by an observer on the ground. At the moment of the sun position 
exactly coinciding with the zenith point, the notion of solar plane loses its 
meaning.

solar zenith angle, thetasol - the angle between the zenith direction and the 
direction towards the instantaneous position of the sun in the sky. Solar zenith 
angle and solar elevation above the horizon complement each other to 90 degrees.

19 List of Acronyms

ASCII   - American Standard Code for Information Interchange
ASD     - Analytical Spectral Devices Inc.
BOREAS  - BOReal Ecosystem-Atmosphere Study 
BORIS - BOREAS Information System
DAAC    - Distributed Active Archive Center
FOV     - Field of View
GMT     - Greenwich Mean Time
GSFC    - Goddard Space Flight Center
IBM     - International Business Machines Inc.
MICA    - Multiyear Interactive Computer Almanac
NASA    - National Aeronautics and Space Administration
NSA     - BOREAS Northern Study Area
OA      - Old Aspen
OBS     - Old Black Spruce
OJP     - Old Jack Pine
ORNL    - Oak Ridge National Laboratory
PAR     - Photosynthetically-Active Radiation
PC      - Personal Computer
SSA     - BOREAS Southern Study Area
URL     - Uniform Resource Locator
YA      - Young Aspen
YJP     - Young Jack Pine

20 Document Information

20.1 Document Revision Date 

Written: 30-May-1997
Last Updated: 08-May-98

20.2 Document Review Date(s)

BORIS Review: 9-SEP-1994
Science Review:

20.3 Document ID  

20.4 Citation

20.5 Document Curator 

20.6 Document Uniform Resource Locator (URL) 

Keywords: 

Old Jack Pine, 
Young Jack Pine, 
Young Aspen, 
forest understory, 
spectral reflectance, 
angular variation, 
seasonal variability.

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