CN111507312A - Soil moisture extraction method based on hyperspectral data - Google Patents

Soil moisture extraction method based on hyperspectral data Download PDF

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CN111507312A
CN111507312A CN202010457335.1A CN202010457335A CN111507312A CN 111507312 A CN111507312 A CN 111507312A CN 202010457335 A CN202010457335 A CN 202010457335A CN 111507312 A CN111507312 A CN 111507312A
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wave band
soil moisture
sensitive
absorption rate
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徐驰
曾文治
张宏雅
宋子亨
赵树辰
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Wuhan University WHU
Changjiang Institute of Survey Planning Design and Research Co Ltd
Changjiang River Scientific Research Institute Changjiang Water Resources Commission
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Changjiang Institute of Survey Planning Design and Research Co Ltd
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Abstract

The invention discloses a soil moisture extraction method based on hyperspectral data. It comprises the following steps: preliminarily processing remote sensing hyperspectral reflectivity data, and solving an apparent absorption rate and a first derivative of the apparent absorption rate according to the reflectivity; extracting a wave band screening coefficient by using a principal component analysis method, and selecting a sensitive wave band; establishing an optimal exponential method based on the standard deviation of the wave band and the correlation coefficient, and further optimizing the sensitive wave band by combining the physical significance of the wave band; and establishing an inversion model of the soil moisture by utilizing the sensitive wave band and using a stepwise regression method. The invention overcomes the defects of excessive variables, lower efficiency, poorer model stability and the like of the existing method model; has the advantage of improving the extraction precision of soil moisture.

Description

Soil moisture extraction method based on hyperspectral data
Technical Field
The invention relates to the fields of hyperspectral remote sensing and irrigation and water conservancy, in particular to a soil moisture extraction method based on hyperspectral data.
Background
The accurate acquisition of soil moisture plays an important role in agricultural development, ecological protection and the like in arid and semiarid regions; the traditional method for extracting soil moisture is to collect a sample in the field and bring the sample back to a laboratory for testing, and the method has high precision, but has high labor cost and long time consumption; the method for extracting the large-area soil moisture information by utilizing the hyperspectral remote sensing data is an important scientific and technological support.
The currently used satellite hyperspectral data or the hyperspectral data acquired by a ground object spectrometer are mostly expressed as reflectivity data; the reflectivity data of visible light, near infrared and short-wave infrared spectrums are closely related to various properties such as soil moisture, salt, organic matters, color, ferric oxide, texture and the like; however, the amount of information provided by the reflectivity data is limited, and the reflectivity data cannot be directly reflected due to other spectral characteristics such as absorption characteristics of a spectral curve, such as absorption peaks and absorption valleys, shape characteristics of the spectral curve, such as inflection points, convex points and concave points, and the influence of a baseline effect on the spectral curve; therefore, a transform process, such as an absorptance transform, a derivative transform, etc., is required on the hyperspectral reflectance data.
The existing regression methods for hyperspectral inversion of soil moisture models comprise a partial least square method, a neural network method, a stepwise regression method, a multiple linear regression method and the like; the linear partial least square method and the nonlinear neural network method can overcome the problem of multiple collinearity of high spectral variables and establish a high-dimensional regression model; however, because the variables of the model are too many, the efficiency of model calibration is low, the stability of the model is poor, and the popularization is not strong; the stepwise regression method, the multiple linear regression method and the like have high requirements on the number of variables, and high-dimensional hyperspectral data are difficult to directly use.
Therefore, it is very important to develop a soil moisture extraction method of hyperspectral data with good model stability and high model calibration efficiency.
Disclosure of Invention
The invention aims to provide a soil moisture extraction method based on hyperspectral data, which improves the extraction efficiency of soil moisture and has good model stability.
In order to achieve the purpose, the technical scheme of the invention is as follows: the soil moisture extraction method based on hyperspectral data is characterized by comprising the following steps of: the method comprises the following steps:
step 1: the method comprises the following steps of (1) preliminarily processing hyperspectral remote sensing reflectivity data of soil, and solving an apparent absorption rate and a first derivative of the apparent absorption rate according to the reflectivity;
step 2: extracting a wave band screening coefficient by using a principal component analysis method, and primarily selecting a sensitive wave band;
on the basis of the hyperspectral data of the apparent absorption rate and the first-order derivative of the apparent absorption rate obtained in the step 1, solving a wave band screening coefficient by using a principal component analysis method, and selecting a sensitive wave band;
and step 3: extracting the optimal index of each wave band based on the standard deviation and the correlation coefficient of the selected wave band, and further optimizing the sensitive wave band by combining the physical significance of the wave band;
and 4, step 4: and establishing an inversion model of the soil moisture content by using a stepwise regression method.
The invention has the following advantages: (1) carrying out absorption rate transformation and derivative transformation on the hyperspectral reflectivity data, extracting apparent absorption rates and first-order derivatives of the apparent absorption rates, and highlighting soil spectral characteristics to carry out soil moisture inversion; (2) solving a wave band screening coefficient by using a principal component analysis method, and extracting sensitive wave bands distributed in visible light, near infrared and short wave infrared ranges; the sensitive wave bands comprise 435, 520, 575, 1400, 1890, 1930, 1955, 1995 and 2410 nm; the sensitive wave band covers effective information in the hyperspectral data, the variable number of spectral analysis is reduced, the complexity in a soil moisture extraction equation is reduced, and the calculation efficiency and precision are improved; (3) on the basis of selecting 9 sensitive wave bands, based on standard deviation and correlation coefficient, an optimal index of the wave band is established, and four sensitive wave bands of 435nm, 1400nm, 1930 nm and 2410nm are further optimized by combining the physical significance of the wave bands; (4) the extraction efficiency of soil moisture is improved, the stability of the model is good, and the adaptability of the selected sensitive wave band is strong (the method is not only suitable for the area of the embodiment of the invention, but also suitable for other areas).
Drawings
FIG. 1 is a schematic flow chart of an embodiment of the present invention.
FIG. 2 is an apparent absorbance of an embodiment of the invention.
FIG. 3 is a first derivative plot of the apparent absorbance of an embodiment of the invention.
FIG. 4 is a graph of band-pass filter coefficients as a function of band in accordance with an embodiment of the present invention.
Detailed Description
The embodiments of the present invention will be described in detail with reference to the accompanying drawings, which are not intended to limit the present invention, but are merely exemplary. While the advantages of the invention will be clear and readily understood by the description.
The soil moisture extraction method based on hyperspectral data is characterized by comprising the following steps of: the method comprises the following steps:
step 1: the method comprises the following steps of (1) preliminarily processing hyperspectral remote sensing reflectivity data of soil, and solving an apparent absorption rate and a first derivative of the apparent absorption rate according to the reflectivity;
step 2: extracting a wave band screening coefficient by using a principal component analysis method, and primarily selecting a sensitive wave band;
extracting a wave band screening coefficient by using a principal component analysis method on the basis of the hyperspectral data of the apparent absorption rate and the first-order derivative of the apparent absorption rate obtained in the step 1, and selecting a sensitive wave band;
and step 3: extracting the optimal index of each wave band based on the standard deviation and the correlation coefficient of the selected wave band, and further optimizing the sensitive wave band by combining the physical significance of the wave band;
and 4, step 4: establishing an inversion model of the soil moisture content by using a stepwise regression method;
in the step 2, the selected sensitive wave bands are distributed in the visible light, near infrared and short wave infrared ranges, and the sensitive wave bands comprise 435, 520, 575, 1400, 1890, 1930, 1955, 1995 and 2410 nm;
in step 3, the selected sensitive wave bands are distributed in the visible light, near infrared and short wave infrared ranges, and the sensitive wave bands comprise 435, 1400,1930 and 2410 nm; these bands are important for soil moisture inversion, and their physical significance lies in: 435nm is a representative band of 400-600nm, and Fe3+、Fe2+The crystal field effect caused by plasma is related to charge transfer of Fe-O, B-O and the like; 1400nm is a 1300-1550nm representative band associated with the field effect of the crystal and OH-Fe, H2Molecular vibrational correlation of O; 1930,2410nm is a representative waveband of 1810-2、H2O, etc. are involved in molecular vibrations.
Example 1
The invention takes the moisture information extracted based on the hyperspectral soil data (soil samples mainly comprising clay and silty clay) in the northwest region as an embodiment for detailed description, and has guiding significance for other region soil moisture extraction methods based on the hyperspectral data.
Step 1: preliminarily processing remote sensing hyperspectral reflectivity data, and solving an apparent absorption rate and a first derivative of the apparent absorption rate according to the reflectivity;
primarily processing remote sensing hyperspectral data by utilizing a basic image processing method for resampling and a common data statistical method; resampling is a process of interpolating information of one type of pixel according to the information of another type of pixel; in remote sensing, resampling is a process of extracting a low-resolution image from a high-resolution remote sensing image;
measuring a soil sample spectrum signal by using an AgriSpec spectrometer manufactured by American ASD (analytical spectral device), wherein the spectrum range provided by the instrument is 350-2500 nm, and the resolution is 1 nm; in order to reduce the data volume and enhance the generalization of the model, the model can be conveniently applied to hyperspectral data (the resolution is 5nm) provided by the existing high-resolution five-satellite AHSI sensor, and the average value of the reflectivity of 5 continuous wave bands is calculated to replace the original reflectivity value through a spectrum smoothing technology; therefore, the reflectivity data in the range of 400-2450 nm is processed, and the spectral resolution of 1nm is resampled to 5 nm; most of the resampled information is still stored;
the soil hyperspectral data represent the reflectivity spectrum of the soil, the soil sample hyperspectral data after resampling still represent the reflectivity spectrum R of the soil, and the information content of the reflectivity data is highlighted by utilizing two spectrum pretreatment technologies:
the first spectral pretreatment method is to determine the apparent absorbance a (as shown in fig. 2):
A=log(1/R)
the second is to find the first derivative (a') of the apparent absorbance based on the apparent absorbance (as shown in fig. 3);
normalizing the two groups of data A and A 'to normalized data with the mean value of 0 and the variance of 1, so as to enhance the contrast of different forms of hyperspectral data and obtain the normalized apparent absorption rate A and the first derivative A' thereof;
step 2: on the basis of the apparent absorption rate (A) obtained in the step 1, a first derivative (A') of the apparent absorption rate and hyperspectral data, solving a wave band screening coefficient by using a Principal Component Analysis (PCA) method, and selecting a sensitive wave band; 9 sensitive bands sensitive to the moisture of the soil sample are selected, wherein the bands are 435, 520, 575, 1400, 1890, 1930, 1955, 1995 and 2410nm (the selected sensitive bands are not only suitable for the embodiment of the invention, but also suitable for other regions);
the method for solving the band screening coefficient is obtained according to a load vector and a principal component characteristic value of a Principal Component Analysis (PCA); when a Principal Component Analysis (PCA) method is used for processing data, a plurality of principal components can be extracted, and the principal components represent most of information of original measured hyperspectral data; in the PCA processing process, each principal component can obtain two groups of output variables, and the first group of variables are load vectors of the principal components; the second set of variables are eigenvalues of the principal components; it is generally considered that the greater the absolute value of the coefficient (positive or negative) of a band in the load vector, the more important the band is; the characteristic value influences the importance of the load vector, the load vector can be corrected by utilizing the characteristic value, the first three main components of A are respectively subjected to load vector square multiplication operation and the characteristic value, the load vector square multiplication operation and the characteristic value multiplication operation are added to obtain a wave band screening coefficient of A, the wave band screening coefficient of A 'is solved in the same way, and finally the wave band screening coefficient of A and the wave band screening coefficient of A' are added to obtain a final wave band screening coefficient; in the embodiment, a sensitive waveband is extracted through a waveband screening coefficient, and the waveband screening coefficient shows continuous fluctuation characteristics in an interval (generally more than 20 nm), which indicates that the waveband in the interval is relatively important;
in specific implementation, firstly, the hyperspectral data are processed by using principal component analysis, and for A, A' two groups of hyperspectral data, after PCA processing, the explained variance of the first three principal components of the data (the sum of the characteristic values of the first three principal components in each group of data) reaches 90%, and information is contained in the first three principal components; calculating A, A' two groups of hyperspectral data, adding the wave band screening coefficients, and displaying the wave band screening coefficients along with the change graph of the wave band screening coefficients along with the wavelength (as shown in figure 4); 435, 520, 575, 1400, 1890, 1930, 1955, 1995 and 2410nm 9 sensitive wavebands are selected according to the waveband screening coefficient change diagram;
and step 3: extracting the optimal index OI (the standard deviation of a certain waveband is divided by the sum of the absolute values of the correlation coefficients of the waveband and other wavebands) of each waveband based on the standard deviation and the correlation coefficient of the selected waveband, preferentially extracting wavebands with large optimal indexes and representative physical meanings of the wavebands, and further preferentially selecting sensitive wavebands; the final selected sensitive band comprises 435, 1400,1930,2410 nm;
Figure BDA0002509746830000051
Siis the standard deviation of the ith band, rijIs the correlation coefficient of the ith band and the jth band, which is k bands, in this case k is 18, i.e. the 9 sensitive bands of a plus the 9 sensitive bands of a'.
And 4, step 4: establishing an inversion model of the water content of 99 soil samples by using the sensitive wave bands in the step 3 and a stepwise regression method;
Y=-0.1837·A435+0.8016·A1930-0.6157·A2410+0.0833·A’1400-0.0424
using the decision coefficient r2The accuracy of the decision model, and the relative root mean square error rmse.
Figure BDA0002509746830000061
Figure BDA0002509746830000062
m is the soil moisture test value, y is the soil moisture prediction value, and n is the soil sample number.
Coefficient of determination r of model20.932, relative root mean square error rmse of 0.115; the model was verified using 99 actual measurement samples (model verification samples and calibration samples were selected alternately for water content), and the coefficient of determination r of the model was determined20.935 and a relative root mean square error rmse of 0.112;
other details not described are prior art.

Claims (3)

1. The soil moisture extraction method based on hyperspectral data is characterized by comprising the following steps of: the method comprises the following steps:
step 1: the method comprises the following steps of (1) preliminarily processing hyperspectral remote sensing reflectivity data of soil, and solving apparent absorption rate and first-order reciprocal of the apparent absorption rate according to the reflectivity;
step 2: extracting a wave band screening coefficient by using a principal component analysis method, and primarily selecting a sensitive wave band;
extracting a wave band screening coefficient by using a principal component analysis method on the basis of the hyperspectral data of the apparent absorption rate and the first-order derivative of the apparent absorption rate obtained in the step 1, and selecting a sensitive wave band;
and step 3: extracting the optimal index of each wave band based on the standard deviation and the correlation coefficient of the selected wave band, and further optimizing the sensitive wave band by combining the physical significance of the wave band;
and 4, step 4: and establishing an inversion model of the soil moisture content by using a stepwise regression method.
2. The hyperspectral data based soil moisture extraction method according to claim 1, characterized in that: in step 2, the selected sensitive wave bands are distributed in the visible light, near infrared and short wave infrared ranges, and the sensitive wave bands comprise 435, 520, 575, 1400, 1890, 1930, 1955, 1995 and 2410 nm.
3. The hyperspectral data based soil moisture extraction method according to claim 2, characterized in that: in step 3, the sensitivity wave bands finally selected by using the optimal exponential method comprise 435, 1400,1930 and 2410 nm.
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Application publication date: 20200807