CN113945927B - Forest canopy height inversion method through volume scattering optimization - Google Patents
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Abstract
The invention relates to the technical field of forest canopy height inversion, in particular to a method for inverting forest canopy height by a three-stage algorithm optimized through volume scattering, which comprises the following steps: step one, carrying out polarized complex phase dry line fitting based on an RVoG model; step two, the gamma is measured PDlow Estimating a ground phase phi g ；γ PDlow And gamma PDhigh The polarization interference complex coherence with the maximum polarization space phase difference; step three, determining volume scattering; screening out a volume scattering complex coherence observation value from each polarized complex phase trunk and carrying out volume scattering estimation by adopting a mode value invariant projection method; step four, passing the thickness h of the vegetation v And estimating the height of the canopy by using a lookup table LUT constructed by the extinction coefficient sigma. According to the forest canopy height estimation method, the ground phase and the volume scattering in the classic three-stage algorithm are optimized, and the accuracy of forest canopy height estimation can be improved.
Description
Technical Field
The invention relates to the technical field of forest canopy height inversion, in particular to a forest canopy height inversion method through volume scattering optimization.
Background
The forest occupies about 30% of the land area, is the most complete and largest land ecosystem on the earth, and is the most important maintainer of the land ecosystem. The forest height is one of basic structural parameters of the forest and is an important forest stand arbor measuring factor. However, due to the restriction of factors such as forest distribution, complex terrain conditions, weather conditions and the like, large-scale continuous forest height measurement is always a technical problem of forest investigation. Synthetic Aperture Radar (SAR) is widely used for inversion of forest structures and biophysical parameters due to its ability to acquire vegetation surface polarization and interference mode data. InSAR (Interferometric Synthetic Aperture radar) and PolInSAR (polar and Interferometric Synthetic Aperture radar) are sensitive to the shape, direction and vertical structure of forest body scattering, and can obtain different polarization interference complex coherence under different vegetation heights for monitoring forest structures.
The PolInSAR technology integrates the measurement of the InSAR technology on the vertical structure of the scatterer and the sensitivity of the PolSAR technology on the shape and the orientation of the scatterer, can generate a complex phase dry image under any polarization scattering mechanism, and various polarization scattering mechanisms correspond to forest structure characteristics, thereby laying a physical foundation for extracting forest structure information. In 1998, cloude estimates forest height for the first time by using different scattering mechanism interference optimization algorithms. The RVoG model proposed by Treuhaft et al is the basis of a model for estimating the height mechanism of a forest canopy by the current PolInSAR technology. In 2001, the Papathanassiou combines a polarization interference coherence optimization algorithm with an RVoG model, provides a single-baseline RVoG complex phase dry model tree height inversion 6-parameter method, and adopts nonlinear iterative optimization solution. In 2003, Cloude and Papathanassou propose a classic three-stage algorithm of RVoG complex coherence model based on the geometrical distribution characteristics of complex coherence, and simplify the complexity of forest canopy height estimation. The classical three-stage algorithm is influenced by terrain phase estimation errors and bulk coherence estimation errors, and the phenomenon of underestimation occurs in inversion of vegetation height. Khati adopts a classical three-stage inversion algorithm and TanDEM-X data to invert the height of forest canopy in tropical forest region of India, and underestimation phenomenon (-7 to-10 m) occurs. The solution is subjected to SAR data simulation, and an underestimation phenomenon exists in the classical three stages when no terrain influence exists; in the forest with larger canopy density, the phase centers of all polarization channels are relatively concentrated, the ground phase center is higher, and the vegetation height estimation value is lower. The RVoG model is a mechanism model for estimating the height of the forest canopy by PolInSAR, and the model estimates the height of the forest canopy by solving the distance between a ground phase center and a bulk scattering phase center and the bulk scattering amplitude.
The classical three-stage algorithm of the RVoG model is to solve for the ground phase and the bulk scattering from the polarization scattering features of L, P long wavelengths. Most of data of the current on-orbit satellite-borne SAR system needs to be acquired in a heavy orbit mode, and the influence of time losing coherence is serious during forest height estimation. And the TanDEM-X satellite-borne data adopts a one-sending and two-receiving mode to observe the ground objects, and the time base line is zero. If the X-band data and the classical three-stage algorithm are used for jointly inverting the forest canopy height, the ground phase and the body scattering complex phase interference estimation are inaccurate, and the forest canopy estimated height is underestimated or overestimated. Therefore, the classical three-stage inversion algorithm solution of the RVoG model aiming at the X-band data needs to be optimized.
Disclosure of Invention
It is an object of the present invention to provide a method for inverting forest canopy height by volume scattering optimization that overcomes some or all of the disadvantages of the prior art.
The method for inverting the forest canopy height through the volume scattering optimization comprises the following steps:
step one, performing polarized complex phase dry line fitting based on an RVoG model;
step two, the gamma is measured PDlow Estimating a ground phase phi g ；γ PDlow And gamma PDhigh The polarization interference complex coherence with the maximum polarization space phase difference;
step three, determining volume scattering; screening out a volume scattering complex coherence observation value from each polarized complex phase trunk and carrying out volume scattering estimation by adopting a mode value invariant projection method;
step four, passing the thickness h of the vegetation v And estimating the height of the canopy by using a lookup table LUT constructed by the extinction coefficient sigma.
Preferably, in the first step, the track of each polarization complex coherence on the plurality of unit circles CUC will be in a linear form; fitting parameters of a complex phase dry fitting straight line by adopting a total least square method; and the phases of two intersection points of the complex coherent fitting straight line and the complex unit circle CUC are used as candidate ground phases.
Preferably, in step two, the catalyst is prepared from gamma PDlow The specific steps for estimating the ground phase are as follows:
2.1, calculating the phase difference between each observed complex coherent value and two intersection points, and respectively sorting according to the sequence from small to large, wherein the phase difference is delta phi i,ω The following formula is calculated:
wherein gamma (omega) is the total complex coherence of polarization interference of a ground and vegetation two-layer model and is changed along with a polarization scattering mechanism omega; i represents an intersection; j is an imaginary number unit, abs () is an absolute value function, and arg () is a phase value function;
2.2, sequencing: if gamma is PDlow When the first 3 bits are ordered in Rank1, then the ground phase is g ＝φ 1 (ii) a If gamma is PDlow When the first 3 bits are ordered in Rank2, then the ground phase is g ＝φ 2 (ii) a If neither of the two sets of ranks exceeds the first 3 bits, then φ 1 And phi 2 Then, the two estimated canopy height values are estimated as candidate ground phases; selecting the candidate ground phase with canopy height estimate within a reasonable range as phi g And (6) outputting.
Preferably, in step three, the specific steps for determining the volume scattering are as follows:
3.1 observed value of gamma (omega) from scattering observe ) Medium selective volume scattering complex coherence observation
Will gamma PDhigh 、γ opt3 And gamma PDlow Excluding from γ HH 、γ HV 、γ VH 、γ VV 、γ HH-VV 、γ HH+VV 、γ OPT1 、γ OPT2 、γ LL 、γ RR Selecting the complex phase trunk farthest from the ground as a body scattering complex coherence observation value
HH. HV, VH, VV, HH + VV, HH-VV, LL, RR, OPT1, OPT2, OPT3, PDhigh, PDlow are polarization interference complex coherence of 13 polarization scattering mechanisms;
3.2 estimating the volume scatter gamma (omega) by a mode-invariant projection method v )；
Firstly, using the origin of the coordinate system as the center of a circle and the body scattering complex phase coherence observation value
The module value of (A) is a radius of a circle; complex coherent observation of volume scattering
The vertical projection point of the complex phase trunk fitting straight line is p'; calculating the intersection point P of the circle and the complex phase dry fitting straight line 1 And P 2 (ii) a If P 1 P' is less than P 2 Distance of P', then P 1 As an estimate of the volume scatter gamma (omega) v ) (ii) a Otherwise, then P 2 As an estimate of the volume scatter gamma (omega) v ) As shown in the following formula:
wherein x is gamma (omega) v ) Y is gamma (ω) v ) K is the slope of the complex-phase-coherent fitting line, b is the intercept of the complex-phase-coherent fitting line, and m is the modulus of the observed value of the complex-phase-coherent scattering
If the circle and the complex phase interference fitting straight line have no intersection point, the vertical projection point p' is used as the estimated volume scattering gamma (omega) v )。
Preferably, in step four, the canopy height estimation method specifically includes: after constructing the LUT, according to the RVoG model, calculating a plurality of groups (h) v σ) theory γ v Comparison theory of gamma v And estimating the volume scatter gamma (omega) v ) The canopy height of the active area can be estimated using a minimization function F, as shown in the following equation:
γ v is interference complex coherence, k, caused by vegetation layer body scattering z Is the vertical effective wave number, and theta is the radar wave incidence angle.
According to the forest canopy height estimation method, the ground phase and the volume scattering in the classic three-stage algorithm are optimized, and the accuracy of forest canopy height estimation can be improved.
Drawings
FIG. 1 is a flow chart of a method for inverting forest canopy height by volume scattering optimization in example 1;
FIG. 2 is a schematic diagram of the RVoG model in example 1;
FIG. 3 is a schematic representation of the geometric representation of the internal phase trunks in a complex planar unit circle of example 1;
fig. 4 is a schematic diagram of a mode-value-invariant projection in example 1.
Detailed Description
For a further understanding of the invention, reference should be made to the following detailed description taken in conjunction with the accompanying drawings and examples. It is to be understood that the examples are illustrative of the invention and not limiting.
Example 1
RVoG model
Treuhaft proposes a forest height inversion method of a random azimuth volume layer model (RVoG) with ground echo, and a forest is a random volume particle layer which is regarded as covering the ground surface and contains a large number of randomly distributed and mutually independent volume particle particles.
In the two-layer RVoG model (see fig. 2) with radar wave incident angle θ: the earth surface is considered as a surface scattering layer which is not penetrated by radar waves and has a height Z 0 Causing the ground phase phi g Thickness of vegetation is h v The height of the forest canopy is Z 0 +h v . Assuming that the scattering energy of the vegetation layer changes exponentially with the increase of the height, after eliminating the effects of registration error, atmospheric decoherence and the like, the polarization interference complex coherence gamma (omega) based on the RVoG model is expressed as formula (1):
wherein gamma (omega) is the total complex coherence of polarization interference of a ground and vegetation two-layer model and is changed along with a polarization scattering mechanism (omega); phi is a unit of g (＝k z z 0 ) For the surface elevation z 0 The resulting ground phase. μ (ω) is the effective volume scattering amplitude ratio, see equation (2); t is g (omega) is a reflection symmetric ground scattering coherence matrix, T v And (omega) is a diagonal coherence matrix of the volume scattering. f (z) is the radar reflectivity vertical profile,
p 2 ＝p 1 +ik z (ii) a Theta is the incident angle of the radar wave; sigma is a vegetation layer extinction coefficient; gamma ray v The interference complex coherence caused by the scattering of the vegetation layer body is represented by a theoretical model of formula (3); k is a radical of z Is a vertical effective wavenumber, and in formula (4), a number of the interference conditions are monostic, and m is 2; if the interference condition is Bistatic, m is 1. Delta theta is the incident angle difference of the main image and the auxiliary image; λ is the radar wave operating wavelength.
In the RVoG complex coherence model, theta, lambda, delta theta, k z All the parameters can be obtained through the system parameter of the SAR platform; four quantities to be estimated: phi is a g 、μ(ω)、h v σ, the forest canopy height can be estimated by solving the RVoG complex coherence model.
As shown in fig. 1, the present embodiment provides a method for inverting forest canopy height by volume scattering optimization, which includes the following steps:
step one, performing polarized complex phase dry line fitting based on an RVoG model;
according to formula (1) in the RVoG model, the trajectory of each polarization Complex coherence (in this embodiment, polarization interference Complex coherence employing 13 polarization scattering mechanisms of HH, HV, VH, VV, HH + VV, HH-VV, LL, RR, OPT1, OPT2, OPT3, PDhigh, and PDlow) in the Complex Unit Circle (CUC) will be in a linear form. The first step of the present embodiment is the same as the classical three-stage algorithm, in which the parameters of a complex coherent fit straight line are fitted by using a total least square method. The phases of two intersection points of the Complex Fitting Line (CFL) and the CUC are taken as candidate ground phases, as shown in fig. 3.
Step two, the reaction is carried out by gamma PDlow Estimating a ground phase phi g ；γ PDlow And gamma PDhigh Is a space of polarizationPolarization interference complex coherence with maximum phase difference;
in the classical three-stage algorithm, the ground phase is the comparison γ HV The distance to the candidate intersection is estimated. Influenced by the uncertainty of the terrain fluctuation and the growth direction of branches and leaves, the HV polarization channel in the X wave band is not necessarily on the top of the forest crown and is possibly close to the ground; meanwhile, the polarized complex coherence distribution of the X wave band is relatively concentrated and tends to a certain intersection point. In both cases, the classical three-stage algorithm may incorrectly estimate the phase. Therefore, this embodiment is constructed from γ PDlow The ground phase is estimated. Gamma ray PDlow And gamma PDhigh Is a polarization interference complex coherence with maximum polarization space phase difference, gamma PDlow The phase center is close to the bottom of the forest, and the intersection point on the same side is the ground floor intersection point.
By gamma PDlow The specific steps for estimating the ground phase are as follows:
2.1, calculating the phase difference between each observed complex coherent value and two intersection points, and respectively sorting according to the sequence from small to large, wherein the phase difference is delta phi i,ω The following formula is calculated:
wherein gamma (omega) is the total complex coherence of polarization interference of a ground and vegetation two-layer model and is changed along with a polarization scattering mechanism omega; i represents an intersection; j is an imaginary number unit, abs () is an absolute value function, and arg () is a phase value function;
2.2, sequencing: if gamma is to be PDlow When the first 3 bits are ordered in Rank1, then the ground phase is g ＝φ 1 (ii) a If gamma is PDlow When the first 3 bits are ordered in Rank2, then the ground phase is g ＝φ 2 (ii) a If neither of the two sets of ranks exceeds the first 3 bits, then φ 1 And phi 2 Then, the two estimated canopy height values are estimated as candidate ground phases; selecting crownCandidate ground phases for which the layer height estimate is within a reasonable range are taken as phi g And (6) outputting.
Step three, determining volume scattering; screening out a volume scattering complex coherence observation value from each polarized complex phase trunk and carrying out volume scattering estimation by adopting a mode value invariant projection method;
in the classical three-stage algorithm, γ HV As the complex coherence observation of the volume scattering, the vertical projection of the complex coherence observation on the fitting straight line is used as the estimation value of the complex coherence of the volume scattering, at this time, mu HV Values close to 0, gamma HV With the ground
With the farthest distance. Due to the influence of terrain fluctuation and uncertainty of the growth direction of branches and leaves, the X-band HV polarization channel may contain other scattering contributions besides bulk scattering, and the phase of the X-band HV polarization channel is not necessarily at the top of a canopy and may be close to the ground. The classical three-stage algorithm directly selects the scattering complex coherence of the HV polarization channel as a bulk scattering complex coherence observed value. The vertical projection changes the modulus and phase of the effective observed value of the volume scattering complex phase interference, and errors are easily introduced, as shown in fig. 4. Therefore, the embodiment proposes optimized estimation of volume scattering, that is, screening out an observed value of complex coherence of volume scattering from each polarization complex coherence and performing volume scattering estimation by using a module value invariant projection method, and the specific steps are as follows:
3.1 observed value gamma (omega) from scattering complex phase coherence observe ) Medium selective volume scattering complex coherent observation value
The wavelength of the X wave band is shorter, the penetrability is poorer, and the echo of the X wave band mainly comes from branches and leaves on the upper part of a forest crown. Based on the existing principle of polarized complex phase dry algorithm and earlier research results (Chao Wan autumn, 2018), gamma is adopted PDhigh And gamma opt3 May be higher than the forest canopy. Thus, gamma will be PDhigh 、γ opt3 And gamma PDlow Excluding from γ HH 、γ HV 、γ VH 、γ VV 、γ HH-VV 、γ HH+VV 、γ OPT1 、γ OPT2 、γ LL 、γ RR Selecting the complex phase trunk with the farthest phase from the ground as the observation value of the body scattering complex coherence
HH. HV, VH, VV, HH + VV, HH-VV, LL, RR, OPT1, OPT2, OPT3, PDhigh, PDlow are polarization interference complex coherence of 13 polarization scattering mechanisms;
3.2 estimating the volume scatter gamma (omega) by a mode-invariant projection method v )；
Firstly, using the origin of the coordinate system as the center of a circle and the body scattering complex phase coherence observation value
The module value of (A) is a radius of a circle; complex coherent observation of volume scattering
The vertical projection point of the complex phase trunk fitting straight line is p'; calculating the intersection point P of the circle and the complex phase dry fitting straight line 1 And P 2 (ii) a If P 1 P' is less than P 2 Distance of P', then P 1 As an estimate of the volume scatter gamma (omega) v ) (ii) a Otherwise, then P 2 As an estimate of the volume scatter gamma (omega) v ) As shown in the following formula:
wherein x is gamma (omega) v ) Y is gamma (ω) v ) K is the slope of the complex-phase-coherent fitting line, b is the intercept of the complex-phase-coherent fitting line, and m is the modulus of the observed value of the complex-phase-coherent scattering
If the circle and the complex phase interference fitting straight line have no intersection point, the vertical projection point p' is used as the estimated volume scattering gamma (omega) v )。
Step four, passing the thickness h of the vegetation v Extinction coefficient sigmaThe constructed look-up table LUT estimates the canopy height.
The canopy height estimation method specifically comprises the following steps: after constructing the LUT, according to the RVoG model, calculating a plurality of groups (h) v σ) theory γ v Comparison theory of gamma v And estimating the volume scatter gamma (omega) v ) The canopy height of the active area can be estimated using a minimization function F, as shown in the following equation:
γ v is interference complex coherence, k, caused by vegetation layer body scattering z Is the vertical effective wave number, and theta is the radar wave incidence angle. Step four is the same as the existing classical three-stage algorithm.
In this embodiment, a fixed extinction coefficient σ is used in order to be able to examine the performance gain of the optimization method in terms of a high degree of estimation. It has been found that the extinction coefficients of tropical broad-leaved forests and coniferous forests are mainly between 0.1-0.9 dB/m. When a scholars inverts the forest height, the extinction coefficient of the conifer forest is fixed to be 0.2dB/m, and the extinction coefficient of the tropical forest is fixed to be 0.3dB/m and 0.4 dB/m. In this example, the extinction coefficients of pinocembrin in the research area are all 0.2 dB/m. In addition, h v The values are limited to 5-30 m.
The present invention and its embodiments have been described above schematically, without limitation, and what is shown in the drawings is only one of the embodiments of the present invention, and the actual structure is not limited thereto. Therefore, if the person skilled in the art receives the teaching, without departing from the spirit of the invention, the person skilled in the art shall not inventively design the similar structural modes and embodiments to the technical solution, but shall fall within the scope of the invention.
Claims (1)
1. A method for inverting forest canopy height through volume scattering optimization is characterized by comprising the following steps: the method comprises the following steps:
step one, performing polarized complex phase dry line fitting based on an RVoG model;
in the first step, the track of each polarized complex coherence on the complex unit circle CUC is in a linear form; fitting parameters of a complex phase dry fitting straight line by adopting a total least square method; the phase of two intersection points of the complex coherent fitting straight line and the complex unit circle CUC is used as a candidate ground phase;
step two, the reaction is carried out by gamma PDlow Estimating a ground phase phi g ；γ PDlow And gamma PDhigh The polarization interference complex coherence with the maximum polarization space phase difference;
in the second step, the reaction is carried out by gamma PDlow The specific steps for estimating the ground phase are as follows:
2.1, calculating the phase difference between each observed complex coherence value and two intersection points, and respectively sequencing the phase differences from small to large, wherein the phase difference is delta phi i,ω The following formula is calculated:
wherein gamma (omega) is the total complex coherence of polarization interference of a ground and vegetation two-layer model and is changed along with a polarization scattering mechanism omega; i represents an intersection; j is an imaginary number unit, abs () is an absolute value function, and arg () is a phase value function;
2.2, sequencing: if gamma is PDlow When the first 3 bits are ordered in Rank1, then the ground phase is g ＝φ 1 (ii) a If gamma is PDlow When the first 3 bits are ordered in Rank2, then the ground phase is g ＝φ 2 (ii) a If neither set of ranks exceeds the first 3 bits, then φ 1 And phi 2 Then, the two estimated canopy height values are estimated as candidate ground phases; selecting as phi a candidate ground phase for which the canopy height estimate is within a reasonable range g Outputting;
step three, determining volume scattering; screening out a volume scattering complex coherence observation value from each polarized complex phase trunk and carrying out volume scattering estimation by adopting a mode value invariant projection method;
in the third step, the specific steps for determining the volume scattering are as follows:
3.1 from the scattering Complex phase coherent observation value γ (ω) observe ) Medium selective volume scattering complex coherent observation value
Will gamma PDhigh 、γ opt3 And gamma PDlow Excluding from γ HH 、γ HV 、γ VH 、γ VV 、γ HH-VV 、γ HH+VV 、γ OPT1 、γ OPT2 、γ LL 、γ RR Selecting the complex phase trunk farthest from the ground as a body scattering complex coherence observation value
HH. HV, VH, VV, HH + VV, HH-VV, LL, RR, OPT1, OPT2, OPT3, PDhigh, PDlow are polarization interference complex coherence of 13 polarization scattering mechanisms;
3.2 estimating the volume scatter gamma (omega) by a mode-invariant projection method v )；
Firstly, using the origin of the coordinate system as the center of a circle and the body scattering complex phase coherence observation value
The module value of (A) is a radius of a circle; complex coherent observation of volume scattering
The vertical projection point of the complex phase trunk fitting straight line is p'; calculating the intersection point P of the circle and the complex phase dry fitting straight line 1 And P 2 (ii) a If P 1 P' is less than P 2 Distance of P', then P 1 As an estimate of the volume scatter gamma (omega) v ) (ii) a Otherwise, then P 2 As an estimate of the volume scatter gamma (omega) v ) As shown in the following formula:
wherein x is gamma (omega) v ) Y is gamma (ω) v ) K is the slope of the complex-phase-coherent fitting line, b is the intercept of the complex-phase-coherent fitting line, and m is the modulus of the observed value of the complex-phase-coherent scattering
If the circle and the complex phase interference fitting straight line have no intersection point, the vertical projection point p' is used as the estimated volume scattering gamma (omega) v )；
Step four, passing the thickness h of the vegetation v Estimating the height of the canopy by using a lookup table LUT constructed by the extinction coefficient sigma;
in the fourth step, the canopy height estimation method specifically comprises the following steps: after constructing the LUT, according to the RVoG model, calculating a plurality of groups (h) v σ) theory γ v Comparison theory of gamma v And estimating the volume scatter gamma (omega) v ) The canopy height of the active area can be estimated using a minimization function F, as shown in the following equation:
γ v is interference complex coherence, k, caused by vegetation layer body scattering z Is the vertical effective wave number, and theta is the radar wave incidence angle.
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