distribution of chla concentration is non-uniform. So in this
paper the other part of the lake was chosen as the study area.
13a MPU ize F Amn EK boro g
H à 2 x L
Firsoot
Med Legend
^ Samples-E
« Samples?
FIED HN
A007
7} Western
Lakeshore Central Lake
X
3 CO
IEG" 20
* Seuthern
Mo Hakexhore
o * FOUR
spero x4] - S = ]
u Perey OTE 120° 1007 E 120°2000E LODE ENUAODUE
Figure 1. Study area and the location of sample points. The
upper-right picture is the Lake drainage basin. Symbols in the
right picture are sample points which were measured in October
2010, March 2011 and September 2011, respectively
2.2 Field Measurements
In this study, we undertook field surveys to get ground data in
October 2010, March 2011 and September 2011 respectively.
Sample points are typical and evenly distributed extend from
Meiliang bay to central lake (See Figure 1.). Water sample and
water spectral were collected at each sampling site, in which
geographical coordinate was also recorded using GPS
(measuring instrument: ASD FieldSpec HandHeld, the
wavelength range 350-1050nm) (Feng et al., 2007). Meanwhile,
water sample was refrigerated, and then token back to the
laboratory to measure Chlorophyll a by means of
spectrophotometric. Finally, 34 points, 45 points and 32 points
were obtained for the following research after outliers removed.
2.3 Image Data and Pre-processing
HJ-1 CCD images and MODIS 1B (250m, 500m) were chosen
as data sources.
HJ-1A, B (Environmental Satellite) were launched in 2008 in
China. HJ-1A satellite carries CCD camera and Hyperspectral
Imager, while HJ-1B carries CCD and Infrared Multispectral
cameras. All of the CCD cameras have four bands: 0.43-0.52um,
0.52-0.60um, 0.63-0.69um, and 0.76-0.90um. And CCD
sensors have a spatial resolution of 30m and repetition cycle of
them is two days (Jin et al, 2010). HJ-1 CCD images on
October 31%, 2010, Mach 28 and September 4", 2011, were
separately downloaded from China Centre for Resources
International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B7, 2012
XXII ISPRS Congress, 25 August — 01 September 2012, Melbourne, Australia
Satellite Data and Application. (© HJ-1 CCD Image Copyright
2010, 2011)
The data of MODIS (Moderate Resolution Images
Spectroradiometer) has three different spatial resolutions: 250m,
500, and 1000m. The image has a radio resolution of 12 bit and
the sensor passes over Lake Taihu every day (Hu et al., 2004).
MODIS images (250m, 500m) on October 31“, 2010, Mach 28^
and September 4", 2011, were separately downloaded from
LAADS Web. (O MODIS Image Copyright 2010, 2011)
After obtaining those images, MODIS-Terra (250m and 500m)
were first corrected with MRT. Next measured GCP were used
to realize accurately geometric rectification of HJ-1 CCD and
MODIS data. Then radiometric calibration and FIAASH
atmospheric correction were performed on both HJ-1 CCD and
MODIS-Terra. Finally reflectance images were calculated.
3. METHODS
3.1 Estimation of Chla Concentration
In order to obtain high correlation bands of chla concentrations
and MODIS image, HJ-1 CCD image, ratio method, the
difference method, and band combination algorithms were used
to calculate the relevance (Wang et al., 2008). This article
obtained MODIS image, HJ-1 CCD image of the reflectivity or
reflectivity band combination based on latitude and longitude of
the sample points. Then the correlation coefficients were
calculated by chla and its reflectivity or reflectivity band
combination. The results showed that the MODIS RI-R2,
RI/R2, (R1 -R2) / (R1 — R2) and HJ-1 H3-H4, H3/H4 had a
high correlation with chla concentration. Therefore, these bands
combinations were used to establish chla concentration
empirical models (20 points for the model, and the rest for
validation each time).
By comparing the average relative error of seasonal empirical
model, H3-H4 of HJ-1 CCD image and RI-R2 of MODIS
images was the best. Hence, the model would be established
using H3-H4 and R1-R2.
3.2 Semi-variance function
Variograms is the quantitative description of the spatial
variables correlation. It was mainly used for studying the
correlation and spatial structure of area variables. (Zhang, 2008)
Chla concentration is interdependence and correlation among
adjacent points, being a spatial structure. It is regionalized as
random variable. Hence, variograms could be used to study the
spatial heterogeneity of chla(Liu et al., 2002). In this study,
semi-variance function was used to analyze the spatial
structures and variation law of chla. The formula is (Curran et
al., 1998):
1
=
N(h) 2
[Z(x3 Z(x v 5M (1)
where y(h) = Semi-variance
Z(x) = Regional random variable
h = Sample interval
N (h) = Numbers of the sample with the distance of h
