CN113065230A - High-spectrum inversion model for establishing rice leaf SPAD based on optimized spectral index - Google Patents

High-spectrum inversion model for establishing rice leaf SPAD based on optimized spectral index Download PDF

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CN113065230A
CN113065230A CN202110279004.8A CN202110279004A CN113065230A CN 113065230 A CN113065230 A CN 113065230A CN 202110279004 A CN202110279004 A CN 202110279004A CN 113065230 A CN113065230 A CN 113065230A
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于海业
于跃
隋媛媛
王洪健
李晓凯
张郡赫
周海根
郭晶晶
张蕾
张昕
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Abstract

The invention provides a rice leaf SPAD hyperspectral inversion model established based on an optimized spectral index, which comprises the following steps: s1, collecting the spectrum and chlorophyll content data of the target rice leaf: s2, preprocessing the spectral data: noise elimination and signal local characteristic refinement are realized by scaling translation operation of wavelet mother functions, and spike signals and mutation signals in original signals are effectively protected. The invention optimizes the integral calculation defined by the NAOC spectral index by simplifying the integral calculation based on integral limits (a, b), namely a mode of dual-wavelength combined operation, and screens out characteristic band combinations with higher correlation coefficients with rice leaf SPAD in the original spectrum and three mathematical transformation spectra by utilizing a correlation analysis method.

Description

High-spectrum inversion model for establishing rice leaf SPAD based on optimized spectral index
Technical Field
The invention relates to the technical field of a hyperspectral inversion model of rice leaf SPAD, in particular to a hyperspectral inversion model of rice leaf SPAD established based on optimized spectral indexes.
Background
Chlorophyll is an important pigment for plant photosynthesis, and the content, distribution and change of the chlorophyll can directly or indirectly reflect the photosynthetic capacity, nutrient condition and growth health condition of plants. The accurate diagnosis of the chlorophyll content has important significance on scientific management means such as growth monitoring, nutrition diagnosis, crop yield estimation, pest and disease early warning and the like of crops. The rice is an important grain crop in China, the gold belt is produced in the rice in Jilin province, the main variety is japonica rice, the tillering period of the rice is ended in one season of one year and in the middle of 7 months, and the rice is successively put into the stage of jointing and booting, the growth period is a key turning period of the rice from the vegetative growth period with tillering as the center, such as long root, long leaf, long tiller and the like, to the reproductive growth period with young ear development as the center, and the chlorophyll content in the rice leaf directly influences the photosynthetic strength and the accumulation amount of organic matters, so that the growth of the young ear and the yield of the rice are influenced. Therefore, the method has certain practical significance for researching the chlorophyll content of the rice in the jointing and booting stage.
At present, the inversion of the chlorophyll content of vegetation by using the spectral index is a very effective method. The spectral index is constructed to extract information that can form a high correlation with the chlorophyll content in the spectrum, and generally, the reflectance corresponding to a specific wavelength band is combined and calculated, and the selection of the specific wavelength band generally needs to refer to a certain physical basis. Initially, Horler et al defined the maximum value of the first derivative of a characteristic curve in the range of 670-780 nm of green vegetation as the "red edge", and the corresponding physical meaning is the peak position of the strong absorption of chlorophyll by the red light band, and thought that there may be a close relationship between the "red edge" position of the spectrum of the vegetation canopy and the content of chlorophyll. Curran et al, corroborated this conclusion by studying the "red edge" position and the sensitivity of the total chlorophyll content in the canopy, and then some scholars established a series of spectral indices such as Ratio Vegetation Index (RVI), Difference Vegetation Index (DVI), Normalized Vegetation Index (NDVI), etc. using the combined operation of some key bands within this band range, and amplified the relationship between spectral reflectance and chlorophyll content of the Vegetation. With the progress of the inversion research on the chlorophyll content, some researchers find that the spectral index obtained based on the key waveband combination in the 'red edge' position range only contains two or three wavelengths of operation, and the effective information contained in the spectral index is very limited relative to a complete spectral reflectivity characteristic curve, so that when the spectral index is applied to crops under the influence of factors such as different varieties, growth periods, places and the like, the calculated result may have a large deviation from the real situation. Therefore, some scholars propose the concept of integral index, integrating more sensitive wavelengths to increase the applicability of spectral index. The method comprises the steps that Oppelt, Mauser and the like calculate the ratio of the area of two-point integral operation in the range of 600-735 nm of a corn reflectivity spectrum to an envelope line, namely a Chlorophyll Absorption integral index (CAI), the prediction capability of the index on the corn leaf area index and the canopy Chlorophyll density is researched, and the sensitivity of the index on a canopy structure, illumination geometry, soil background reflectivity and atmospheric conditions is analyzed; delegido et al established a reflectance Curve Normalized Area index (NAOC) for studying the spatial distribution of chlorophyll of urban vegetation in a complex environment, and used the index to invert the chlorophyll content of various green vegetation such as phoenix tree, date palm and the like, and found that the NAOC index has strong inversion capability for the chlorophyll content of the green vegetation, and provided a simple and convenient method for remote sensing estimation of chlorophyll of vegetation through linear fitting between the NAOC index and the chlorophyll content of the chlorophyll.
However, the strong prediction capability of the spectral index on the chlorophyll content of the rice in the jointing and booting stage under the farmland environment is still to be researched.
Disclosure of Invention
The invention aims to provide a rice leaf SPAD hyperspectral inversion model established based on an optimized spectral index so as to solve the problems in the background technology.
In order to achieve the purpose, the invention provides the following technical scheme:
the method for establishing the rice leaf SPAD hyperspectral inversion model based on the optimized spectral index comprises the following steps:
s1, collecting the spectrum and chlorophyll content data of the target rice leaf:
s2, preprocessing the spectral data: noise elimination and signal local characteristic refinement are realized by scaling translation operation of wavelet mother functions, and spike signals and mutation signals in original signals are effectively protected;
s3, optimized spectral index NAOC: the NAOC index is an integrated spectral index comprising a plurality of spectral bands, the NAOC index being defined as:
Figure BDA0002977720950000031
simplifying the spectral index NAOC based on integral operation to obtain the optimized spectral index based on dual-wavelength simplified operation, which is defined as:
Figure BDA0002977720950000032
s4, performing hyperspectral pretreatment on the rice leaves based on wavelet analysis: selecting 4-order Daubechies wavelets, namely db4 wavelets, to perform 1-6 layers of wavelet decomposition on the original reflectivity spectrum, and determining L4 layers of decomposed and reconstructed spectra as a result of wavelet analysis denoising processing of the original spectral reflectivity curve;
s5, optimizing the spectral index and waterCorrelation analysis of rice leaf SPAD: respectively carrying out three mathematical transformation processes of logarithm, reciprocal and square opening on the denoised L4 layer spectral reflectivity curve R to obtain transformed spectra LgR, 1/R and
Figure BDA0002977720950000033
and are respectively substituted into
Figure BDA0002977720950000034
Obtaining four groups of optimized spectrums and conversion spectrum indexes;
s6, performing statistical analysis on correlation coefficients corresponding to the optimized spectrum and the transformed spectrum index in the four forms, wherein 3 integral wave bands with the correlation coefficients higher than 0.87 are combined and are respectively R (641, 790) (0.8726),
Figure BDA0002977720950000035
(653, 767) (0.8717) and R (644, 774) (0.8716), calculating 60 optimized spectral index values corresponding to 3 integration band combinations with correlation coefficients higher than 0.87 in 20 original samples, according to a ratio of 2: 1 into a modeling set and a verification set, and determining samples by adopting an equal-interval sampling method.
Preferably, the data acquisition of the target blade spectrum is carried out by using a Hand-Held type ground object spectrometer of Hand Held 2 produced by analytical spectroscopy instrument company, the spectral band range of the instrument is 325-1075nm, the sampling interval is 1.4nm, the resolution is 3nm @700nm, the instrument is preheated for 20min before use, each blade acquires 3 spectral curves, and a white board is used for correction after each measurement.
Preferably, the collection of the chlorophyll content selects a Japanese SPAD-502 chlorophyll meter to synchronously obtain the relative chlorophyll content of the corresponding rice leaf, the SPAD value is used for replacing the chlorophyll content of the rice leaf, a main leaf vein is avoided during measurement, five points are marked at intervals from the leaf tip to the leaf tail, each point is measured once, and the average value of the points is taken as the measurement result of the chlorophyll content of the leaf after 5 times of repeated measurement.
Preferably, in S6, 40 modeling set samples and 20 validation set samples are determined by equal-interval sampling, and partial least squares regression is usedThree rice leaf SPAD inversion models are established by a regression model (PLSR), a Support Vector Machine (SVM) and a BP neural network so as to determine a Coefficient (R)2) And Root Mean Square Error (RMSE) as a model evaluation index.
Compared with the prior art, the invention has the beneficial effects that
The method optimizes integration calculation defined by the NAOC spectral index by simplifying the integration calculation based on integration limits (a, b), namely a dual-wavelength combined operation mode, screens out characteristic band combinations with higher correlation coefficients with rice leaf SPAD in an original spectrum and three mathematical transformation spectrums by using a correlation analysis method, uses the optimized spectrum and the transformation spectrum index obtained by calculation to construct an inversion model of the rice leaf SPAD in the heading and booting stage, and finally contrasts and analyzes the precision and stability of the three modeling methods to determine the optimal rice leaf SPAD inversion model established based on the optimized spectrum and the transformation spectrum index.
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FIG. 1 is a schematic of the spectra of the present invention;
FIG. 2 is a decomposition reconstruction spectrum of the L4 layer according to the present invention;
FIG. 3 is a scatter plot of the prediction of PLSR;
FIG. 4 is a scatter plot of the predicted results of the SVM;
FIG. 5 is a scatter plot of the prediction results of the BP neural network.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
Referring to fig. 1-5, in the embodiment of the present invention, the method for establishing a hyperspectral inversion model of rice leaf SPAD based on an optimized spectral index includes the following steps:
s1, collecting the spectrum and chlorophyll content data of the target rice leaf:
s2, preprocessing the spectral data: noise elimination and signal local characteristic refinement are realized by scaling translation operation of wavelet mother functions, and spike signals and mutation signals in original signals are effectively protected;
s3, optimized spectral index NAOC: the NAOC index is an integrated spectral index comprising a plurality of spectral bands, the NAOC index being defined as:
Figure BDA0002977720950000051
simplifying the spectral index NAOC based on integral operation to obtain the optimized spectral index based on dual-wavelength simplified operation, which is defined as:
Figure BDA0002977720950000052
s4, performing hyperspectral pretreatment on the rice leaves based on wavelet analysis: selecting 4-order Daubechies wavelets, namely db4 wavelets, to perform 1-6 layers of wavelet decomposition on the original reflectivity spectrum, and determining L4 layers of decomposed and reconstructed spectra as a result of wavelet analysis denoising processing of the original spectral reflectivity curve;
s5, optimizing the correlation analysis of the spectral index and rice leaf SPAD: respectively carrying out three mathematical transformation processes of logarithm, reciprocal and square opening on the denoised L4 layer spectral reflectivity curve R to obtain transformed spectra LgR, 1/R and
Figure BDA0002977720950000061
and are respectively substituted into
Figure BDA0002977720950000062
Obtaining four groups of optimized spectrums and conversion spectrum indexes;
s6, performing statistical analysis on correlation coefficients corresponding to the optimized spectrum and the transformed spectrum index in the four forms, wherein 3 integral wave bands with the correlation coefficients higher than 0.87 are combined and are respectively R (641, 790) (0.8726),
Figure BDA0002977720950000063
(653, 767) (0.8717) and R (644, 774) (0.8716), calculating 60 optimized spectral index values corresponding to 3 integration band combinations with correlation coefficients higher than 0.87 in 20 original samples, according to a ratio of 2: 1 into a modeling set and a verification set, and determining samples by adopting an equal-interval sampling method.
Respectively carrying out three mathematical transformation processes of logarithm, reciprocal and square opening on the denoised L4 layer spectral reflectivity curve R to obtain transformed spectra LgR, 1/R and
Figure BDA0002977720950000064
and are respectively substituted into
Figure BDA0002977720950000065
And obtaining four groups of optimized spectrums and conversion spectrum indexes. As the effective range defined by the NAOC index is 600-800 nm, 201 effective wave bands are respectively intercepted from an original spectrum curve and three transformed spectrum curves, correlation analysis is carried out on four groups of optimized spectra and transformed spectrum indexes and rice leaf SPAD by a correlation analysis method in the form of (a, b) two-wave band combination, the wavelength positions of the integration limits (a, b) corresponding to four groups of maximum correlation coefficients are respectively R (641, 790), LgR (628, 722), 1/R (620, 736),
Figure BDA0002977720950000066
(653, 767), the optimized spectral index based on the original spectral curve R has the maximum correlation coefficient with rice leaf SPAD, and reaches 0.8726; secondly, the
Figure BDA0002977720950000067
Transformed spectral curve of the form with maximum correlation coefficient 0.8717; the maximum correlation coefficient of the transformation spectrum curves in the logarithmic form and the reciprocal form and the rice leaf SPAD is less than 0.8, and is 0.5426 and 0.7165 respectively. The wavelength positions (641, 628, 620 and 653nm) of the lower integration limit a corresponding to the maximum correlation coefficients of the four groups of spectral indexes are all positioned on the left side of 670nm, and the wavelength positions (790, 722 and 736) of the upper integration limit b767nm) is located to the right of 670nm, which is consistent with the explanation of Delegido et al for the upper and lower limits of the integral of the NAOC spectral indices, indicating that the wavelength combinations corresponding to the four sets of maximum correlation coefficients all have corresponding physical meanings.
The data acquisition of the target blade spectrum of the embodiment adopts a Hand-Held type ground object spectrometer of Hand Held 2 produced by analytical spectroscopy instrument company to carry out measurement, the spectral band range of the instrument is 325-plus 1075nm, the sampling interval is 1.4nm, the resolution is 3nm @700nm, the instrument is preheated for 20min before use, each blade acquires 3 spectral curves, and a white board is used for correction after each measurement.
The collection of the chlorophyll content in the embodiment selects a Japanese SPAD-502 chlorophyll meter to synchronously obtain the relative chlorophyll content of the corresponding rice leaf, the SPAD value is used for replacing the chlorophyll content of the rice leaf, a main leaf vein needs to be avoided during measurement, five points are marked at intervals from the leaf tip to the leaf tail, each point is measured once, and the average value of the points is taken as the measurement result of the chlorophyll content of the leaf after 5 times of repeated measurement.
In S6 of this embodiment, 40 modeling set samples and 20 verification set samples are determined by an equidistant sampling method, three rice leaf SPAD inversion models are established by a partial least squares regression model (PLSR), a Support Vector Machine (SVM), and a BP neural network, and a decision coefficient and a root mean square error are used as model evaluation indexes.
Table 13 inversion results of modeling method
Figure BDA0002977720950000071
Table 1 shows the results of the SPAD inversion of rice leaves by 3 modeling methods. It can be seen that no matter the modeling set or the verification set, the decision coefficients of the 3 inversion models established by utilizing the optimized spectrum and the transformed spectrum index are all larger than 0.79, and the root mean square error is smaller than 1.9, which shows that the three inversion models can realize the function of accurately predicting the rice leaf SPAD. In the three inversion models, the verification set of the model established by the BP neural network method has a relatively high determination coefficient of 0.857, and the model fitting degree is relatively good; the prediction accuracy is also relatively high, and the root mean square error is only 1.7221. Through continuous attempts in the modeling test process, the optimal neural network model structure is determined to be 3 layers, the number of nodes of a hidden layer is 6, the learning rate is set to be 0.1, when the iteration times reach 900 times, the accuracy of the neural network model tends to be stable, and the model has the optimal inversion effect on rice leaf SPAD.
(1) Performing correlation analysis on the four groups of optimized spectra and transformed spectral indexes and rice leaf SPAD by using a correlation analysis method to obtain wavelength positions of integration limits (a, b) corresponding to four maximum correlation coefficients, which are respectively R (641, 790), LgR (628, 722) and 1/R (620, 736),
Figure BDA0002977720950000081
(653, 767), the optimized spectral index based on the original spectral curve R has the maximum correlation coefficient with rice leaf SPAD, and reaches 0.8726; secondly, the
Figure BDA0002977720950000082
Transformed spectral curve of the form with maximum correlation coefficient 0.8717; the maximum correlation coefficients of the transformation spectrum curves in the logarithmic and reciprocal forms and the rice leaf SPAD are both less than 0.8 and are 0.5426 and 0.7165 respectively; compared with the other 8 common spectral indexes based on dual-wavelength or three-wavelength operation, the optimized spectrum and the transformed spectral index provided by the method have obviously better effect;
(2) the combination of 3 integration bands above 0.87 in the correlation coefficient for the four forms of optimized spectra and transformed spectral indices is R (641, 790) (0.8726),
Figure BDA0002977720950000083
(653, 767) (0.8717) and R (644, 774) (0.8716), calculating 60 optimized spectral index values corresponding to the 3 integration band combinations in the 20 original samples, according to 2: 1, dividing the ratio into a modeling set and a verification set, and establishing a SPAD inversion model of the rice leaves of PLSR, SVM and BP neural network. The determination coefficients of the three inversion models are all larger than 0.79, the root mean square error is smaller than 1.9, and the results show that the three inversion models can accurately predict rice leaf SPADAnd (4) performing functions. In the three inversion models, the BP neural network has the highest validation set decision coefficient of 0.857, and the root mean square error is 1.7221. At the moment, the number of the structural layers of the neural network is 3, the number of nodes of the hidden layer is 6, the learning rate is 0.1, when the iteration times reach 900, the accuracy of the neural network model tends to be stable, and the model is determined to be the optimal model for rice leaf SPAD inversion by utilizing an optimized spectrum and a transformation spectrum index.
The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. Any reference sign in a claim should not be construed as limiting the claim concerned.
Furthermore, it should be understood that although the embodiments are described herein in terms of separate embodiments, not every embodiment may comprise a single embodiment, and such descriptions are provided for clarity only, and one skilled in the art should consider the embodiments herein as a whole, and the embodiments herein may be appropriately combined to form other embodiments as will be appreciated by one skilled in the art.

Claims (4)

1. The method is characterized by comprising the following steps of:
s1, collecting the spectrum and chlorophyll content data of the target rice leaf:
s2, preprocessing the spectral data: noise elimination and signal local characteristic refinement are realized by scaling translation operation of wavelet mother functions, and spike signals and mutation signals in original signals are effectively protected;
s3, optimized spectral index NAOC: the NAOC index is an integrated spectral index comprising a plurality of spectral bands, the NAOC index being defined as:
Figure FDA0002977720940000011
simplifying the spectral index NAOC based on integral operation to obtain the optimized spectral index based on dual-wavelength simplified operation, which is defined as:
Figure FDA0002977720940000012
s4, performing hyperspectral pretreatment on the rice leaves based on wavelet analysis: selecting 4-order Daubechies wavelets, namely db4 wavelets, to perform 1-6 layers of wavelet decomposition on the original reflectivity spectrum, and determining L4 layers of decomposed and reconstructed spectra as a result of wavelet analysis denoising processing of the original spectral reflectivity curve;
s5, optimizing the correlation analysis of the spectral index and rice leaf SPAD: respectively carrying out three mathematical transformation processes of logarithm, reciprocal and square opening on the denoised L4 layer spectral reflectivity curve R to obtain transformed spectra LgR, 1/R and
Figure FDA0002977720940000013
and are respectively substituted into
Figure FDA0002977720940000014
Obtaining four groups of optimized spectrums and conversion spectrum indexes;
s6, performing statistical analysis on correlation coefficients corresponding to the optimized spectrum and the transformed spectrum index in the four forms, wherein 3 integral wave bands with the correlation coefficients higher than 0.87 are combined and are respectively R (641, 790) (0.8726),
Figure FDA0002977720940000015
(0.8717) and R (644, 774) (0.8716), calculating 60 optimized spectral index values corresponding to 3 integration band combinations with correlation coefficients higher than 0.87 in 20 original samples, according to the ratio of 2: 1 into a modeling set and a verification set, and determining samples by adopting an equal-interval sampling method.
2. The rice leaf SPAD high spectrum inversion model based on the optimized spectral index as claimed in claim 1, wherein the data collection of the target leaf spectrum is measured by Hand Held type ground object spectrometer of Hand Held 2 produced by analytical spectroscopy instrument company, the spectral band range of the instrument is 325-1075nm, the sampling interval is 1.4nm, the resolution is 3nm @700nm, the pre-heating is carried out for 20min before use, each leaf obtains 3 spectral curves, and a white board is used for correction after each measurement.
3. The rice leaf SPAD high spectrum inversion model established based on the optimized spectral index according to claim 1, characterized in that the collection of the chlorophyll content selects a Japanese SPAD-502 chlorophyll meter to synchronously obtain the relative chlorophyll content of the corresponding rice leaf, the SPAD value is used for replacing the chlorophyll content of the rice leaf, a main vein needs to be avoided during measurement, five points are marked at intervals from the leaf tip to the leaf tail, each point is measured once, and the average value of the points is taken as the measurement result of the chlorophyll content of the leaf after 5 times of repeated measurement.
4. The rice leaf SPAD high spectrum inversion model based on optimized spectral index as claimed in claim 1, wherein said S6 adopts equal interval sampling method to determine 40 modeling set samples and 20 validation set samples, and adopts partial least squares regression model (PLSR), Support Vector Machine (SVM) and BP neural network to establish three rice leaf SPAD inversion models for Coefficient of determination (R)2) And Root Mean Square Error (RMSE) as a model evaluation index.
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CN114965884A (en) * 2022-05-17 2022-08-30 南京农业大学 Method for monitoring size of brown planthopper population in rice field indoors
CN114965884B (en) * 2022-05-17 2023-07-18 南京农业大学 Method for indoor monitoring of brown planthopper population size in rice field

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