Guoliang Tian

Modeling directional brightness temperature over a maize canopy in row structure

A study on modeling the variations of directional brightness temperature (DBT) for row-structure crops was carried out with the images captured by a large-aperture thermal infrared camera over a maize canopy. The model assumes that the DBT is a function of target component brightness temperatures and their directional fractions. The canopy has three brightness temperature components: the sunlit soil, the shaded soil, and the vegetation. Their fractions in the scene depend on the sun-view geometry and the distributions of gaps within and between plant rows. To describe canopy geometrical features, a series of porous hedgerows with a rectangular cross section is used. The directional variations of gap fractions are described by the Kuusk function. The model demonstrated how the features of DBT depend on the sun-view geometry, canopy geometrical structure, and component brightness temperatures. In the simulation of DBT over a middle-density canopy near the local noontime, the results revealed an evident row-direction-oriented hot stripe in DBT polar maps, where the hot spot appeared along the sun direction. The sensitivities of the model to the input parameters were tested. Further validation demonstrated a close correlation between predicted DBT and field observations.

Combining vegetation index and remotely sensed temperature for estimation of soil moisture in China

The relationship between remotely sensed surface temperature (T) and normalized difference vegetation index (NDVI) was studied over China with 10‐day composite Advanced Very High Resolution Radiometer (AVHRR) data at 8‐km spatial resolution. Results showed that the slope of the relationship between T and NDVI (T/NDVI slope) could be determined effectively during the growing season with a certain range of NDVI. The derived T/NDVI slope from image windows was significantly correlated to in situ soil moisture (r = 0.77, p<0.01). Based on determination of wet and dry edges of T/NDVI space and the relationship between soil moisture and the T/NDVI slope, a new approach was proposed to estimate distributed soil moisture. Using the approach, the derived soil moisture approximated the conditions recorded on the ground.

Study on thermal infrared emission directionality over crop canopies with TIR camera imagery

In order to investigate directionality of thermal infrared emission from crop canopies, a wide-angle thermal video camera (INFRAMETRICS) equipped with an 80° FOV lens was mounted on a small aircraft and used to acquire thermal imagery along several different flight traces. Accordingly, multi-angle directional brightness temperatures were acquired at different view angles for individual pixel. The flight experiment was carried out from January 1997 to October 1997 over a 5 km×5 km flat agricultural area, located near Avignon, southeastern France.This paper presents results from analyses performed using these data including instrument calibration, radiometric correction, atmospheric correction, temperature temporal adjustment, geometric matching and registration of images. Results are presented for different thermal infrared emission patterns of different surface types including bare soil, wheat, maize and sunflower at different growth stages.

Classification of brightness temperature components for a maize canopy

In order to modeling directional thermal radiation and energy balance for a partially covered canopy, surface brightness temperature is usually classified into several components. This paper researches the methodology for brightness temperature component classification and temporal variations of component number and values by an in situ experiment, dedicated to analyze maize canopy brightness temperature distribution. The measurement was carried out by using a TIR camera and a visible camera mounted on an industrial crane, the experiment lasted 3 months throughout a maize growth cycle. In the analysis of the brightness temperature, a Gaussian distribution has been assumed. Results show the number of components and their brightness temperature values vary with time of day and biomass density. Three brightness temperature components of vegetation, sunlit and shaded soil could be identified at midday during the measurement period. In the daytime, temperature variability of sunlit soil is much larger than the other two components when the canopys density is not high. When the canopy is fully covered, vegetation brightness temperature has a wider range.

Study on the law of radiant directionality of row crops

The style of crops planting is frequently in row-structure, the row-structure style may result in big difference among the sunlit, shaded soil surface and foliage temperatures and cause pixel component to vary in azimuth orientation, these further lead to the change of radiant directionlity of row crops in the zenith and azimuth orientations. Since the row crops are often tackled as isotropic in the azimuth orientation based on continuous vegetation assumption, big errors will be brought about. In order to eliminate the errors, it is necessary to study the law of radiant directionality of the row crops. In this paper, Monte Carlo method has been employed to simulate the angular effects on radiation caused by row architecture parameters. The simulated results show that the parameters, for example, row height, row width, row interval between the neighbor rows and the leaf area index have significant influences on the radiant directionality, but the azimuth orientation ranks the first among the parameters.

Modeling field of view effect on the ground observations of directional brightness temperature over a maize canopy

Composite scene of row crops induced an unavoidable error in ground measurements of directional brightness temperature (DBT) due to the use of wide field of view (FOV). The measurement results vary with sample size and position, detector height and view direction, and bias due to project principle. This is called FOV effect. The study focuses on the estimation of FOV effect on the measurements of maize canopy using a computational geometric 2D model. The model was developed to simulate the fractional variations of canopy brightness temperature components. The simulation results revealed that the errors caused by FOV effect have a complex feature. Generally, vegetation fraction is always over counted in the nadir view, errors increase dramatically with the decrease of detector height as well as the enlargement of sample size, the deviation of the error corresponding to detect position is small; in oblique view, the errors are limited to a low level due to an effect called compensation effect. However, the deviation of the error keeps large when the sample size is small. Nevertheless, the best approach to reduce FOV effect in ground observation is levering the detector to a higher altitude as the model suggested.

Retrieve component temperature for wheat field with ASTER image

In order to retrieve land surface component temperature from multi-spectral remote sensing images, such as ASTER, we choose a component equivalent emissivity model, and iterative linear regression inversion algorithm. The method is tested with ASTER image of VNIR and TIR channels, as well as supplementary and validation data acquired from ground experiment. The atmospheric effect is corrected with the dark-object method; surface structural information is derived from ASTER VNIR observations; component emissivity is measured in situ with the BOMEN MR-154 spectrometer; validation data are also measured in ground experiments. Finally, the accuracy of the results and sources of error are analyzed.

Experimental study on directionality in thermal infrared observations of corn canopy

In order to investigate the directional distribution of thermal infrared radiance from corn canopy, a field campaign was elaborately schemed in North-China-Plain from July to August, 2000. Corn was designed to be row planted and uniformly planted, to simulate the typical structures of agricultural canopy. One TIR radiometers was set at an automatic multi-angular observation bracket while another was fixed nadir-view over the crop canopy. Directional TIR radiance distributions were measured every two hours at several typical weather conditions during different corn growth stages. Canopy structural parameters and meteorological parameters were also measured simultaneously. After temporally normalized, the directional TIR radiance distributions were extracted, diurnal and seasonal variation of the directionality is analyzed.

Vicarious calibration of MODIS visible and near infrared bands using gongger test site

On 29 and 31 May 2006, a comprehensive vicarious calibration experiment for the Moderate Resolute Imaging Spectroradiometer(MODIS) visible and near-infrared bands was performed at Gongger test site located in Inner Mongolia, which is a flat and uniform area. The reflectance-based method was used for calibration of MODIS visible and near-infrared bands. In situ measurements of surface and atmospheric conditions were carried out. By computing the surface reflectance of the site, it was concluded that the site was appropriate for calibration because of its stable and uniform characteristics. These data were then inputted to a radiative transfer code, 6S, to compute top-of- atmosphere (TOA) radiances and TOA reflectances, which were compared with the MODIS on-board calibration results. The in situ estimated results were in good agreement with the MODIS on-board calibration results on May 31 with the variations about 2%, while vicarious calibration results on May 29 were slightly inconsistent with those of on-board calibration, whose differences were about 7%.

Using night TIR images to model the gap fraction of a dense maize canopy

In order to estimate directional variation of gap fraction over a high dense maize canopy, a geometric optical and radiative transfer (GORT) model was improved to simulate the hemispherical gap fraction, the model was validated by a crane borne experiment using a narrow FOV thermal infrared camera conducted at night. The research revealed that the path length is a function of canopy geometrical structure, view direction and position, which leads to the row effect on hemispherical gap fraction: azimuthal variation of gap fraction is insignificant except for the observations parallel to the rows. For dense canopies, the value of gap fraction declined quickly in small view zenith rather than in large view zenith range, which leads the curves to show a concave shape. The experiment was conducted at 22 h local time on July 26 in 1999 (LAI=5) for the validation. At the time, brightness temperatures of leaves and soil had Gaussian distribution, their mean values presented a significant difference (24.3/spl deg/C and 26.5/spl deg/C) comparing to their small standard deviations (0.52 degC and 0.44 degC), gap fraction could be discriminated from canopy background. Observations showed that most gaps appeared between the adjacent rows, which lead the high dense canopy still to keep row feature in thermal infrared images. As conclusion of the comparison, the model could capture the main features of the measured gap fraction. With a proper adjustment of input leaf optical parameters, the simulated gap fraction showed a fairly good agreement with observed gap fraction.