Research On Canopy Parameters Measurement Algorithm Of Typical Vegetation Based On Photography

Canopy parameters are important parameters for evaluating or studying plant growth status,studying vegetation carbon absorption,light radiation absorption,radiation energy exchange and other capabilities.The main canopy parameters include Leaf Area Index(LAI),Clumping Index(CI),Fractional Vegetation Coverage(FVC).Among the many measurement methods,the measurement method which based on hemispheric photography is widely used,but the actual use of this method is affected by many factors.Based on hemispheric photography and 3D software modeling,this research studies the potential influencing factors in the process of canopy parameters measurement,as follows:(1)With the help of two typical vegetation experimental scenes of corn and woodland built by 3D modeling,the potential influencing factors in the process of LAI measurement based on single-angle hemisphere photography were studied.After measurement and analysis,it is found that the projection model has a great influence on the LAI measurements,and the wrong use of the projection model can lead to a maximum error of nearly 45%.The effect of the camera's field of view on the LAI measurements is negligible and can be ignored.The camera resolution has a great influence on the LAI measurements.In this study,the 500 w pixel resolution is the best.Under the premise of ensuring the accuracy,the processing time and memory overhead of 500w's are within the acceptable range.The height of the camera sampling point has a certain influence on the LAI measurement.As the height increases,the LAI measurements will decrease.(2)A method for measuring the true LAI was proposed,using the circular quadratic area as the representative area measured by hemispheric photography.After simulation experiments,this method was closer to the measured value of hemispheric photography.Aiming at the problem of uneven brightness in vegetation canopy images,an image brightness unevenness correction algorithm is introduced,which greatly reduces the problem of underestimation of LAI measurements caused by uneven brightness.The influence of vegetation density,wood composition,solar altitude angle and other factors on the LAI measurements is studied.The deviation between the LAI measurements and the true value increases with the increase of vegetation density,and the increase of vegetation density will reduce the influence of wood components,and the solar altitude angle has a great influence on the LAI measurements,and the maximum error can exceed 50%.(3)Evaluate two algorithms in the existing CI measurement: LX method and LXG method.It is found that the two algorithms have their own scope of application.When the LAIe is below 4,the LX method is better,and when the LAIe is greater than 5,the LXG method is relatively better,effectively reducing clumping effects.The top-view method in the existing vegetation coverage measurement was evaluated.After comparative analysis,it was found that the error between the top-view method using the standard lens and the true value was small,and the measured value of the top-view method could be approximately regarded as the true value.The method of obtaining FVC by the bottom-view method of the fisheye camera is found through simulation experiments.It is found that this method has a good correlation with the true value,but there is always a 7% error between the measured value and the true value.Therefore,a correction coefficient is introduced.After the correction,many experiments have proved that the method is a feasible measurement method,which provides the possibility for the two-in-one measurement of LAI and FVC.