Retrieval of the ground surface reflectance along coast zone and island with MODIS image

doi:10.4236/ns.2011.33025

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Vol.3, No.3, 193-198 (2011) Natural Science

http://dx.doi.org/10.4236/ns.2011.33025

Retrieval of the ground surface reflectance along coast

zone and island with MODIS image*

Jinji Ma#, Xiaofan Li

College of Physics and Electronic Information, Anhui Normal University, Wuhu, China;

#Corresponding Author: [email protected]

Received 26 December 2010; revised 27 January 2011; accepted 29 January 2011.

ABSTRACT

A new method based on lookup tables (LUTs)

for retrieval of the ground surface reflectance

along coastal zones and islands with MODIS

(Moderate-resolution imaging spectroradiome-

ter) image was described. Through simulation

of the AHMAD radiative transfer model, we can

retrieve the aerosol optical character with water

pixels of MODIS image. Postulating the back-

ground is cloudless and the atmosphere on the

water is the same as that on the island, we can

use the 6S radiative transfer model to compute

the LUT about the ground surface reflectance,

then use the interpolate method to get the re-

flectance of the ground surface along coastal

zones and islands through the reflectance of the

land pixels of MODIS image, the geometric

condition and the aerosol optical thickness. The

LUT method is applied to determine the ground

surface reflectance in Xiamen’s zone from the

MODIS image. At last, the results were analyzed

and its expectation errors were reported.

Keywords: MODIS Image; Reflectance;

Optical Thickness; Aerosol; Remote Sensing

1. INTRODUCTION

How to get the ground surface reflectance is the key

technology for remote sensing because it is important to

use the image of satellite to retrieve ration and the

ground information. Because the image of satellite in-

cludes not only the ground information but also the at-

mosphere information, so we must correct the errors

from the atmosphere effect to the image. There are many

kinds of methods for correcting error, but the most valid

method is the direct correction from the image of satel-

lite. In the beginning, people wanted to evade atmos-

phere correction [1] through calculating the Normalized

Difference Vegetation Index (NDVI) and synthesizing

the DVI by the biggest value of the row of time [2]. Now,

we mainly adopt the radiative transfer measurement ac-

cording as atmospheric parameter to correct atmosphere

[3]. For example, we usually use the dark object method

[4] and the dark object subtraction method [5] to retrieve

the optical character of the aerosol. The aim of these

methods is to minimize the error due to the retrieval of

the ground surface reflectance.

Since the ocean surface is simpler than the land sur-

face, and the atmospheric signal is almost 90% of the

apparent reflectance over the ocean [6], so we think the

atmospheric optical character cannot be significantly

changed by the variation of ocean surface reflectance.

Based on this property, and using the AHMAD radiative

transfer model [7], we determine the aerosol optical

character over the ocean through water pixels of MODIS

image based on the LUT method [8]. If we think the

atmospheric condition over the ocean is the same as that

on the ground under cloudless condition, we can deter-

mine the aerosol optical character, using the 6S model [9]

on computing of the LUT about the ground surface re-

flectance. Using the interpolation method to retrieve of

the ground surface reflectance along coastal zones and

islands with the LUT, aerosol optical character can be

determined from the MODIS image.

2. METHOD

2.1. Retrieval of the Aerosol Optical

Character from Pixel of MODIS Image

There are different methods to retrieve of the aerosol

optical character on the land and over the ocean from

pixel of MODIS image. The aerosol optical character on

the land is retrieved based on the dark method, and the

aerosol optical character over the ocean is retrieved

based on the LUT method.

*National Natural Science Foundation, No. 40701132; Anhui Provincial

atural Science Founda

ion, No.070412042.

J. J. Ma et al. / Natural Science 3 (2011) 193-198

194

2.1.1. Calculation of Lookup Tables

Using the LUT method to retrieve the aerosol optical

character over the ocean with the MODIS image has

been proposed by Tanré et al. [10]. The LUT is calcu-

lated by the AHMAD radiative transfer model based on

the size of different particles. This paper use the aerosol

model proposed by the Leavy et al. [11], which have

five coarse particles and four fine particles. Errors in the

retrieved aerosol optical character over the ocean derive

chiefly from the size distribution of particles and spectral

distribution [12], so choosing seemly aerosol models is

of great importance. Leavy et al. have discussed the

large variation of about the radiation of coast [11]. For

simplicity, we choose the Cox model to compute the

reflectance of the ocean surface [13]. We take nine parti-

cles as nine kinds of aerosol models. For each kind of

aerosol model, we use AHMAD’s radiative transfer code

to calculate the LUT that have five parameters [10].

2.1.2. Method and Theory to Retrieval of the

Aerosol Optical Character

The simplified method proposed by Wang and Gordon

is the base for calculation of LUT and retrieval of aero-

sol optical character. The advantage of this method is

that we only need calculate 9 models not 4 × 5 × 11

models for two different absorption models.

If the total reflectance measured from the satellite is

expressed





svv svv

svv

,, ,,

1,,





  







 (1)

where



svv







is the spectral reflectance of the

fine models,



svv







is the spectral reflectance

about the coarse models, and



is the fraction of the

two models. The superscript c denotes calculation, s de-

notes the fine model and l denotes the coarse model. The

subscript



denotes central wavelength. How to choose

the aerosol model and confirm the optical thickness is

defined by minimizing the error



0.01

mcsl

sl csl



















 (2)

where n is the sum of pixels, m





is the measured

MODIS reflectance at wavelength



, and csl





is cal-

culated from the combination of modes in the LUT and

is defined by Eq.1. The small number 0.01 prevents di-

vergence for the longer wavelengths under clean condi-

tions. The best aerosol model and the optical thickness at

0.55 m



will be obtained when



has reached its

the least value.

2.2. Retrieval of the Ground Surface

Reflectance

2.2.1. RTE of 6S Model

The 6S is a radiative transfer model proposed by

Ver mo te et al. If the geometrical conditions, atmospheric

model, aerosol model, aerosol optical thickness and the

ground surface reflectance are known, we can calculate

apparent reflectance at the atmosphere top and other

characters about atmosphere, such as atmospheric trans-

mission, etc.

We can suppose that the ground surface is a Lamber-

tian, the ground surface reflectance is

  



*,,,, 1

svsva svsvsv

gass v sv



  























(3)

where





*,,

vsv





is apparent reflectance at the at-

mosphere top,





asvsv





is the intrinsic reflec-

tance of the aerosol, c



is the bottom surface reflec-

tance,







, [respectively





] is the total trans-

mission of the atmosphere between the sun and the sur-

face, (respectively between the surface and sensor),





ass v sv





is the total atmosphere transmission of

the sun to the surface and the sensor, s is the spherical

albedo of the atmosphere.

2.2.2. Using 6S Model to Calculate the LUT

The required parameters can be calculated according

to 6S-model, choosing midlatitude summer (determined

by MODIS image) as the atmospheric model, using the

best of result by the front retrieval as aerosol model. Al-

titude of target is zero and the bottom of surface is

Lambertian and its reflectance is chosen from 0.02 to

0.18 with a step of 0.02. We use the 6S model to calcu-

late the LUT for retrieval of the ground surface reflec-

tance at 550nm, for six optical thickness (a



= 0.0, 0.2,

0.5, 1.0, 2.0, 3.0), six zenith angles of sun

(00

s18.2 21.2



 step is 0.50), six relative azimuth

angels (the step is 50) and five zenith angles of view





150, 200, 250, 300, 350) in central of the front

MODIS band. Table 1 shows a part of the LUT.

2.2.3. Retrieval of the Ground Surface

Reflectance

We can use the line of interpolation method to retrieve

the ground surface reflectance base on Table 1 that have

been calculated by the 6S model, the geometrical condi-

tions (according the MODIS image, we can determine

zenith angle of sun, zenith angle of view, relative of

azimuth angle and the apparent reflectance), the best

aerosol model and the optical thickness that have been

J. J. Ma et al. / Natural Science 3 (2011) 193-198

195195

retrieved in Subsection 2.1.

2.3. Process of the Retrieval

The method based on the LUT to retrieve the ground

surface reflectance is feasible when the ocean surface

reflectance is known and the aerosol model over the

ocean is same as that on the coast. When both conditions

are satisfied, we can retrieve the ground surface reflec-

tance. The steps are

1) Supposing there are nine kinds of particle models,

including four fine particle models and five coarse parti-

cle models.

2) Using the Cox’s model to calculate the ocean sur-

face reflectance.

3) Based on conditions 1) and 2), the LUT (A) for re-

trieval of the aerosol optical character can be calculated

by using the AHMAD’s radiative transfer code.

4) Using the Formulas (1) and (2) and the clear ocean

pixel of MODIS, the best of aerosol model and the opti-

cal thickness can be retrieved.

5) Using the best aerosol model and the 6S model, we

can get the LUT (B) for retrieval of the ground surface

reflectance.

6) Based on the LUT (B) and the step 4) retrieval

aerosol optical thickness, and the geometrical conditions

and the clear ocean of pixels that were given from

MODIS image, the ground surface reflectance can be

retrieved from the line of interpolation method.

2.4. Estimation of the Error in Retrieval of

Result

In order to verify the influence of the variation of

validate the aerosol optical thickness on the ground sur-

face reflectance, we have performed the analog compu-

tation. In the simulation calculation, we chose the zenith

angle of sun and the azimuth angle to be 19 and 95, the

zenith angle of view of sun and azimuth angle to be 280

and 900, the aerosol optical thickness to be 0.28 at 0.55

nm. The simulation is based on the LUT (B) and these

conditions. Table 2 shows the influence of increasing

5%-10% or decreasing 5%-10% of the aerosol optical

thickness on the retrieval of the result.

From Table 2, we see that the error is increasing with

the increase of relative of aerosol optical thickness in

visible band from retrieval of the ground surface reflec-

tance. While in infrared band its influence relatively

smaller. When the variation of the retrieved of aerosol

optical thickness is about ten percent, the error is less

than five percent for all band retrieved from the ground

surface reflectance. Therefore, after adding the line of

interpolation of error, the total error in the retrieved the

ground surface reflectance is less than ten percent.

Usually, the aerosol optical thickness on the land is

thicker than that over the ocean. Chlorophyll of water,

speed and direction of wind are sources of error in re-

trieval of the aerosol optical thickness. They deviates the

retrieved optical thickness from the actual value. In

some areas, the assumption that the atmospheric condi-

tion over the ocean is the same as that on the land may

make the retrieved optical thickness be less than the true

value. This is the major source error of the retrieved

ground surface reflectance.

3. APPLICATION OF THE MODIS IMAGE

AND ANALYSIS OF THE RETRIEVED

RESULT

We choose the MODIS image in south ocean of China

on May 282 003 as our research area. This research area

is 30 × 30 pixels cloudless. (Figure 1(a) shows the re-

search area.) The long-lat coordinates of the four top

points are (24.682, 118.199), (24.637, 118.563), (24.414,

118.142), (24.369, 118.505). With the dark aim’s method,

all the water pixels in Figure 1(b) are picked-up. With

these water pixels, the aerosol thickness and the aerosol

model are retrieved. The best result of the retrieval is

that the aerosol model is Multimodal Log Normal dis-

tribution, with three fine and one coarse particle in ratio

of four to one.

Figure 2(a) shows the average value of the retrieved

aerosol optical thickness. We use the retrieved average

value to retrieve the ground surface reflectance, if an

island separates the row, we choose two average values

in the row. Figure 2(b) shows the relative error in the

average of the aerosol optical thickness, the standard

value (





a550 nm0.264



) comes from the ground

measurement at the same time. Figure 2 shows that the

aerosol varies little on the space and the atmosphere is

uniform in the research of area.

Figure 3 and Figure 4 show the reflectance curve of

every pixels marked in the Figure 1(b). Figure 3 show

the ocean water reflectance curve, its value is almost

zero from 870 nm to 2100 nm. In the Figure 3, curves T,

T1 and T2 are the ocean pixel reflectance, among there

curves T1 and T2 are the retrieval results with the same

method of land, and curve of T is the calculation result

using the Cox’s model [13]. In the model, the wind

speed is 6 m/s, the chlorophyll of concentration is 0.5

mg/ m3 and the salt concentration is 35 ppt. From these

curves in the Figure 3, we find that the retrieval results

tally with the calculation result.

Figure 4 shows the ground surface reflectance curves

on the coast and the island. These curves tally with the

type spectrum of vegetation that the reflectance on the

visible band is very small and the reflectance quickly

increases from 670 nm to 1240 nm with little variation

J. J. Ma et al. / Natural Science 3 (2011) 193-198

196

Table 1. Lookup table for calculating the reflectance of the ground.

Zenith of sun

angle (degree)

Zenith angle of

view (degree)

relative azimuth

angle (degree)

Aerosol optical

thickness

Apparent

reflectance

Reflectance of the

ground

… … … … … …

19.2 25.0 2.0 0.27 0.1545 0.1400

19.2 25.0 2.0 0.27 0.1713 0.1600

19.2 25.0 2.0 0.27 0.1882 0.1800

19.2 25.0 2.0 0.28 0.0557 0.0200

19.2 25.0 2.0 0.28 0. 0802 0.0500

… … … … … …

Table 2. Error in the retrieved the ground surface reflectance with the aerosol optical thickness of the relative error.

aerosol optical thickness

wavelength (550nm) Increase 10% Increase 5% Decrease 5% Decrease 10%

0.47 μm 2.60 1.10 0.90 1.86

0.55 μm 1.30 0.45 0.39 1.20

0.66 μm 1.50 0.84 0.67 1.45

0.86 μm 1.41 0.76 0.78 1.55

1.24 μm 1.20 0.47 0.59 1.02

1.64 μm 0.80 0.45 0.30 0.79

Relative Error (%)

2.13 μm 0.49 0.29 0.29 0.50

(a) (b)

Figure 1. MODIS image. (a) Region of interesting; (b) T 30 × 30 pixels at band 2.

(a) (b)

Figure 2. Result of the row of MODIS image. (a) The aerosol optical thickness; (b) The relative error.

J. J. Ma et al. / Natural Science 3 (2011) 193-198

197197

Figure 3. Reflectance curve of the ocean water marked in Fig-

ure 2(a).

on the infrared band.

The four curves in Figure 4(a) show the ground sur-

face reflectance spectrum along the coast of land, among

them curves B and F maybe mixture pixel (contain water

pixel) reflectance curve. The eight curves in Figure 4(b)

show the surface reflectance spectrum on the coast of

island, among the curve G maybe the mixture pixel of

ocean and land, curve O is the typical reflectance spec-

trum of green vegetation. The curves in Figure 4(c) and

(d) are the ground surface reflectance spectrum on the

inside of land and from island far away of the coast.

From these curves, we can see that the ground matter is

complex on the inside of land and the ground matter is

uniform on the island.

4. RESULTS AND EXPECTATION

According to the characteristic of the ground matter

on the coast near the ocean, this paper uses the retrieved

aerosol optical thickness over the ocean to retrieve the

ground surface reflectance of the land. For every pixel of

land, we use the geometrical conditions, the retrieved

aerosol optical thickness and LUT (B) from 6S model

calculation based on the retrieved aerosol model and the

interpolation method to retrieve the ground surface re-

flectance. Simulation shows that the error of retrieved

the ground surface reflectance is less than ten percent

when the error of retrieved aerosol optical thickness is

less than ten percent.

The method is fast and accurate in retrieval the ground

surface reflectance because of using the synchronized of

aerosol model. However, the method is only applicable

to near coast and Small Island; the method needs to be

further researched to the land to the ocean beyond 30

kilometer. The water near the coast is Case II water, but

there is no good model to calculate the Case II water of

reflectance. It influences the precision of the retrieved of

(a)

(b)

(c)

(d)

Figure 4. The reflectance curve of the land marked in Figure

1(b).

J. J. Ma et al. / Natural Science 3 (2011) 193-198

198

aerosol optical thickness so that the precision of the re-

trieved ground surface reflectance would be inflected.

The further works are to build a good model to calculate

the reflectance of the Case II water and to use the ASD

to measure the ground surface reflectance at real time in

comparison with the retrieved result from the MODIS

image.

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