The study of Bi-directional Reflectance Distribution
Function ( BRDF) for the Grassland
Chihiro Watanabe, Yoshiaki Honda, Koji kajiwara
Center for Environmental Remote sensing ( CEReS), Chiba University
1-33, Yayoi, Image-Ku Chiba 263, Japan
Tel(81)-43-290-3945 FAX:-43-290-3857
Abstract
As we estimate a physical quaintly of vegetation using remote sensing data, we can not ignore the effect by the sun-target-sensor geometry. Therefore, we require a method of estimation between a reflectance and a physical quantity of vegetation .for understanding bi-directional reflectance profiles, we did the ground truth for grassland in Mongolia on August 1996. and we develop a simple model based on a general layer model ( for-flux theory ) and apply the measured data to it is clear that the model is useful.
1. Introduction
When an analysis of physical quantity of vegetation using a satellite remote-sensing technology, it is very effective for monitoring of a global area. But these satellite data include many errors by an atmosphere, sun-target-sensor geometry, sensor characteristics and so on . we should understand effects by them and correct. Especially, the effect by sun-target senor geometry as measurement using a middle or a low resolution sensor is greater than others. Therefore, we carried out the ground truth for the Mongolia, grassland, and analyzed bi-directional reflectance profiles . and we develop a BRDF model and apply the measured data to it.
2. Objective
The main purpose of this are as follows.
- development of the measurement system for the directional reflectance and the ground truth.
- Development of the BRDF model for an estimation f reflectance or a radiometric correction .
3. Measurement System
A measurement system for the directional reflectance is not general. Then we develop the measurement system. Requirements of this system is as the follows.
-
The change of the solar zenith angle and the solar azimuth angle is minimum while measure.
- It is easy to setup the system.
- It can obtain the spectral reflectance and the target image at the same time.
Figure 1: Measurement System
This system consists of there parts, the main tower, the control box and the sensor mount. The height of main tower can be changed form 1.5 m to 5 m by water pressure pump. The control box collect data and control the sensor position with a personal computer. The sensor mount consists of the stepping motor for change a sensor position, the spectrometer and the CCD camera for take target images. This spectrometer have dual ports, the sample port and the reference port, so the measurement time is shortened because the measurement of reflectance is at the same time. Detail function are shown the table . 1.
Table. 1 Measurement system
| Wavelength |
350-1050nm |
| Spectral resolution |
0.342 nm/ch (2048ch) |
| Integration time |
0.008-2.04 sec |
| Gain |
1-8 |
| Accumulation count |
1.255 |
| FOV Angle |
20 deg |
| Quantizaiton Level |
16. bit |
| Sensor Height |
1.5-5 m |
| Sensor azimuth Angle |
0-360 deg |
| Sensor Zenith Angle |
0-70 deg. |
