CN111238416B - Mountain land leaf area index measuring method based on radiation transmission path length correction - Google Patents
Mountain land leaf area index measuring method based on radiation transmission path length correction Download PDFInfo
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Abstract
The invention discloses a mountain land leaf area index measuring method based on radiation transmission path length correction, which comprises the following steps of: s1, obtaining mountain vegetation porosity data by using a measuring instrument, and collecting mountain slope, mountain slope direction, observation zenith angle and observation azimuth angle data; s2, calculating a conversion coefficient between the porosity of the mountain vegetation and the porosity of the flat land vegetation; s3, converting the mountain vegetation porosity data into flat land vegetation porosity data through the conversion coefficient; and S4, calculating the leaf area index according to the porosity data of the flat vegetation. According to the method, firstly, the mountain porosity is converted into equivalent flat land porosity, and then the mountain leaf area index is estimated through the equivalent flat land porosity, so that the defects that the existing method violates vegetation direction ground growth and is inconvenient for actual operation are overcome, and the measurement precision of the leaf area index under the mountain condition is remarkably improved.
Description
Technical Field
The invention relates to the field of ground measurement, in particular to a mountain land leaf area index measuring method based on radiation transmission path length correction.
Background
The ground measurement of the leaf area index is a precondition for the resource environment survey in mountainous areas and the construction of a leaf area index remote sensing inversion algorithm, and is of great importance to the general survey of land and soil resource information, the calibration and verification of remote sensing products and the like. These measurement methods have high accuracy already in case of even terrain, but direct application to terrain with complex terrain features, such as mountains, can cause large uncertainties.
The uncertainty of the mountain leaf area index measurement is rooted in the modulation of the topographic vegetation cover macrostructure characteristics, and is visually represented that under the condition of the same observation altitude angle, the vegetation cover in the uphill direction is thicker than the vegetation cover in the downhill direction in visual effect, so that the rotation invariance failure of the porosity hidden in the leaf area index ground measurement method in the azimuth direction is caused. The failure of the rotational invariance of porosity in the azimuthal direction is essentially determined by the distortion of the geometric distance (i.e., path length) traveled by the light. In order to inhibit uncertainty brought to the ground measurement of the leaf area index by the terrain, the existing processing method is to place an instrument in parallel to an inclined ground surface, although the processing method reduces the asymmetry of the porosity in the azimuth angle direction to a certain extent, because the leaf inclination angle space distribution characteristics of the vegetation are mainly influenced by gravity and the rotation invariance of the vegetation is expressed in a global coordinate system rather than a local coordinate system, the instrument placed in parallel to the slope surface seriously violates the geotropic growth of the vegetation, and finally causes the measurement error of the leaf area index; and the instrument is difficult to be arranged into a posture strictly parallel to the slope surface during the actual operation in the field; therefore, it is necessary to research a method for measuring the mountain leaf area index to improve the measurement accuracy of the mountain leaf area index.
Disclosure of Invention
The invention aims to solve the problems and provides a mountain land leaf area index measuring method based on radiation transmission path length correction, which can effectively improve the measurement accuracy.
In order to achieve the purpose, the technical scheme of the invention is as follows:
a mountain land leaf area index measuring method based on radiation transmission path length correction comprises the following steps:
s1, obtaining mountain vegetation porosity data by using a measuring instrument, and collecting data of a slope, a sloping direction, an observation zenith angle and an observation azimuth angle;
s2, calculating a conversion coefficient between the porosity of the mountain vegetation and the porosity of the flat land vegetation;
s3, converting the mountain vegetation porosity data into flat land vegetation porosity data through the conversion coefficient;
and S4, calculating the leaf area index according to the porosity data of the flat vegetation.
Further, in step S2, a calculation formula of a conversion coefficient between the porosity of the mountain vegetation and the porosity of the flat vegetation is:
wherein, lambda is the conversion coefficient between the porosity of the flat vegetation and the porosity of the mountain vegetation, theta is the observation zenith angle, phi is the observation azimuth angle, alpha is the slope, and beta is the slope.
Further, in the step S3, the conversion formula for converting the mountain vegetation porosity data into the flat land vegetation porosity data by the conversion coefficient is as follows:
Pf＝(Ps)λ；
wherein, PfPorosity of vegetation on level ground, PsThe porosity of the mountain vegetation is shown, and the lambda is the conversion coefficient between the porosity of the flat land vegetation and the porosity of the mountain vegetation.
Further, in step S4, the leaf area index is calculated by reversely deriving beer 'S law, where the beer' S law calculation formula is:
wherein, P is the porosity of the vegetation, e is an exponential function, namely, an exponential function, theta is an observation zenith angle, phi is an observation azimuth angle, l is a path length, G is a blade projection function, and rho is the density of the blade surface integrated body; the leaf area index is the product of the leaf surface volume density and the path length in the zenith direction (the path length in the zenith direction is the same for flat ground and mountain land due to the geotropic growth of vegetation).
Compared with the prior art, the invention has the advantages and positive effects that:
the invention provides a mountain land leaf area index measuring method based on radiation transmission path length correction, which is based on a vegetation radiation transmission basic mechanism, considers the distortion of the light transmission path length caused by the ground surface inclination, firstly converts the mountain land porosity into equivalent flat land porosity, and then estimates the mountain land leaf area index through the equivalent flat land porosity, overcomes the defects that the existing method violates the vegetation ground growth performance and is inconvenient for actual operation, and obviously improves the measuring precision of the leaf area index under the mountain land condition.
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In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to these drawings without creative efforts.
FIG. 1 is a block diagram of the framework of the present invention;
FIG. 2 is a schematic diagram comparing the true plano porosity, mountain porosity and equivalent plano porosity;
FIG. 3 is a comparison graph of calculation accuracy of different mountain leaf area index estimation methods.
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 from the embodiments of the present invention by a person skilled in the art without any creative effort, should be included in the protection scope of the present invention.
As shown in fig. 1, fig. 2 and fig. 3, the invention starts from the basic mechanism of vegetation radiation transmission, realizes the conversion from mountain porosity to equivalent flat porosity by the topographic correction of path length, and finally estimates the mountain leaf area index based on the equivalent flat porosity.
For ease of understanding, the key words in the technical solution are first described:
leaf area index: half of the surface area of all the vegetation leaves on the surface area of a unit level represents the effective cross section of energy and substance interaction between the vegetation and the atmosphere, and is closely related to photosynthesis, respiration, transpiration and the like of the vegetation.
Radiation transmission path length: the geometrical distance that the light travels when passing from the top of the vegetation canopy to the bottom of the canopy.
Porosity: the probability that light rays are not intercepted by the blades when reaching the bottom of the canopy from the top of the canopy along a certain direction is closely related to a vegetation structure and is the basis of a leaf area index ground measurement method.
Observing an azimuth angle: and observing the included angle between the projection of the direction on the horizontal plane and the magnetic north direction.
Observing a zenith angle: the angle between the observation direction and the horizontal normal direction.
Theoretical derivation
Vegetation has a directional growth and therefore satisfies the uniform assumption of horizontal orientation, then the porosity of the vegetation canopy base can be described by beer's law, namely:
in the formula, theta and phi are respectively an observation zenith angle and an azimuth angle, e is an exponential function, l is a path length, and G is a blade projection function, and depends on blade inclination angle distribution and is independent of terrain. ρ is the leaf surface bulk density (m)2/m3). The leaf area index can be expressed as the product of the leaf surface volume density and the zenith direction path length.
The path length of vegetation growing on a horizontal surface can be expressed as:
l(θ)＝1/cos(θ)； (2)
the path length of the mountain vegetation can be expressed as:
in the formula, alpha and beta are respectively the slope and the sloping direction of the mountain land. It can be seen that the path length of mountain vegetation is influenced by terrain in addition to the observation geometry (altitude, azimuth).
Substituting the formulas (2) and (3) into the formula (1), and removing the terrain-independent term to obtain mountain land porosity (P)s) And porosity of flat ground (P)f) The following conversion relationships exist:
Pf＝(Ps)λ； (4)
the two transformation coefficients λ can be expressed as:
therefore, the mountain land area index ground measurement can firstly convert mountain land porosity into equivalent flat land porosity by using the formula (4), and then use the existing flat land estimation method to obtain the final leaf area index.
The process of the invention is shown in figure 1, and comprises the following steps:
(1) data acquisition: and obtaining the porosity observation data of the mountain vegetation by using a common measuring instrument (such as LAI-2000, a fish eye camera and the like), and collecting the slope and the direction of the sample plot. The slope and the slope direction can be recorded in the field appearance, and can also be obtained from the existing DEM data in the operation in the industry.
(2) Equivalent conversion: mountain porosity was converted to equivalent flat porosity using equations (4), (5).
(3) Parameter estimation: and obtaining a leaf area index estimation result from equivalent flat porosity by inverting the beer law. In actual operation, the existing leaf area index calculation software can be used, and a lookup table can be manually created.
To verify the effectiveness of the method of the present invention, the difference in porosity between flat and mountain areas for the same canopy structure was analyzed using computer simulations (as shown in fig. 2). It can be seen that due to the existence of the terrain, the rotation invariance of the azimuth angle direction in the porosity of the real flat ground disappears, and the sight line is blocked by the slope surface (the right part in the figure), namely the terrain mask, when the zenith angle is too large in the upward slope reverse observation. The equivalent plano porosity obtained using equation (4) reproduces the rotational invariance in true plano porosity. It is worth pointing out that equivalent transformation of porosity cannot reconstruct information at the terrain mask, but since the average porosity of each observation altitude angle is used in the estimation of the leaf area index, the information loss does not have a great influence on the estimation of the leaf area index.
FIG. 3 is a comparison of the precision of mountain leaf area indexes estimated by different methods, and it can be seen from FIG. 3 that the leaf area index obtained by the conventional method has a significant underestimation, and the underestimation is more significant when the gradient is larger. The calculation accuracy of the technical scheme is obviously superior to the calculation accuracy of the leaf area index of the existing method, and the underestimation phenomenon of the leaf area index is basically eliminated.
The invention provides a mountain land leaf area index measuring method based on radiation transmission path length correction, which is based on a vegetation radiation transmission basic mechanism, considers the distortion of the light transmission path length caused by the ground surface inclination, firstly converts the mountain land porosity into equivalent flat land porosity, and then estimates the mountain land leaf area index through the equivalent flat land porosity, overcomes the defects that the existing method violates the vegetation ground growth performance and is inconvenient for actual operation, and obviously improves the measuring precision of the leaf area index under the mountain land condition.
Claims (3)
1. A mountain land leaf area index measuring method based on radiation transmission path length correction is characterized in that: the method comprises the following steps:
s1, obtaining mountain vegetation porosity data by using a measuring instrument, and collecting data of a slope, a sloping direction, an observation zenith angle and an observation azimuth angle;
s2, calculating a conversion coefficient between the porosity of the mountain vegetation and the porosity of the flat land vegetation;
s3, converting the mountain vegetation porosity data into flat land vegetation porosity data through the conversion coefficient;
and S4, calculating the leaf area index according to the porosity data of the flat vegetation.
2. The mountain leaf area index measurement method based on radiation transmission path length correction as claimed in claim 1, wherein: in the step S2, the calculation formula of the conversion coefficient between the porosity of the mountain vegetation and the porosity of the flat vegetation is:
wherein, lambda is the conversion coefficient between the porosity of the flat vegetation and the porosity of the mountain vegetation, theta is the observation zenith angle, phi is the observation azimuth angle, alpha is the slope, and beta is the slope.
3. The mountain leaf area index measurement method based on radiation transmission path length correction as claimed in claim 2, wherein: in step S3, the conversion formula for converting the mountain vegetation porosity data into the flat land vegetation porosity data by the conversion coefficient is:
Pf＝(Ps)λ；
wherein, PfPorosity of vegetation on level ground, PsThe porosity of the mountain vegetation is shown, and the lambda is the conversion coefficient between the porosity of the flat land vegetation and the porosity of the mountain vegetation.
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| CN103942420A (en) * | 2014-04-08 | 2014-07-23 | 北京大学 | Rapid estimation method for solar energy in construction size |
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