Comparison and evaluation of several methods for estimating the average density of total forest volume in forest farm
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摘要:目的 以林场或县森林资源总体为调查对象,及时、准确地调查监测总体平均每公顷蓄积量,对上级(如市、省)部门开展森林资源宏观管理、生态保护价值评价、森林碳储量计量、领导干部任期绩效考核等工作都有重要支撑作用。将卫星、无人机等多源遥感数据作为辅助数据,采用较少抽样调查样地数据,实现总体参数有效估测的新方法,已成为国内外重要的研究方向,但目前,国内尚无多种现有估计方法的比较评价研究。为了促进新一代遥感技术在森林资源调查业务中的应用,提高我国森林资源天空地一体化调查监测技术水平,亟需对现有林场或县总体参数估测方法进行比较评价研究。方法 以内蒙古旺业甸实验林场主要人工林树种为总体,基于2019年在林场获取的无人机激光雷达(LiDAR)抽样数据、Sentinel-2A多光谱数据(全覆盖)和少量样地数据,针对样地(p)、样地−卫星(ps)、样地−抽样无人机LiDAR(pl)以及样地−抽样无人机LiDAR-卫星(pls)4种模式,利用适合这4种模式的概率抽样法(DB)、模型辅助法(MA)、模型法(MD)和混合法(HY)4类共5种估测方法(DBp、MDps、MAps、HYpl以及MDpls)对总体森林蓄积量密度均值(MSVD)进行估计与对比分析。结果 (1)DBp、MDps、MAps、HYpl、MDpls 5种方法估测的MSVD分别为212.54、202.09、202.38、210.07以及208.96 m3/hm2,精度分别为90.44%、91.54%、91.69%、96.35%和96.45%,方差依次减小。(2)其他4种方法相对于MDpls方法的估计效率(RE)均大于1(REDBp,MDpls = 5.39,REMDps,MDpls = 3.82,REMAps,MDpls = 3.69,REHYpl,MDpls = 1.07);HYpl相对于MDpls的RE略大于1,但几倍于其他3种方法(REDBp,HYpl = 5.02,REMAps,HYpl = 3.43,REMDps,HYpl = 3.56)。(3)包含LiDAR数据的HYpl与MDpls方法相对于包含Sentinel-2A数据的MDps与MAps方法均为正效率(REMAps,HYpl = 3.43,REMDps,HYpl = 3.56,REMDps,MDpls = 3.82,REMAps,MDpls = 3.69);MDps与MAps方法之间的RE接近1,但MAps的效率微高于MDps(REMDps,MAps = 1.04)。结论 和只利用样地数据的估计方法相比,将遥感数据作为辅助变量建立估测模型,无论是采用对蓄积量不够敏感的林场全覆盖Sentinel-2A多光谱遥感数据,还是采用对蓄积量很敏感的抽样式获取的LiDAR数据,都可有效提高林场总体MSVD的估测精度。涉及遥感数据应用的4种方法,估计精度最高的为MDpls,其次为HYpl,这2种方法都包含了LiDAR遥感抽样观测数据的应用,都是适应于林场总体MSVD估计的年度监测方法。可实现天空地3种观测数据协同应用的MDpls估测精度和相对效率最高,可作为林场森林蓄积量年度监测的首选方法。Abstract:Objective Taking the overall forest resources of forest farms or counties as the object of investigation, timely and accurately investigating and monitoring of the mean stock volume density(MSVD)will play an important supporting role in the macro management of forest resources, the evaluation of ecological protection value, the measurement of forest carbon reserves, and the performance evaluation of the tenure of leading cadres by the superior departments (such as cities and provinces). It has become an important research direction at home and abroad using remote sensing data, such as satellites and unmanned aerial vehicles, combining with less sampling plot to effectively estimate the overall parameters. At present, there is no comparative validation of several estimation methods for mean stock volume based on multi-source data in domestic. In order to promote the application of remote sensing technology in the forest resource survey, there is an urgent need to compare and evaluate the methods for estimating overall parameters of forest farms or counties.Method The main plantation tree species of Wangyedian Forest Farm in Inner Mongolia of northern China were taken as the research object. Based on the sampling UAV LiDAR (herringbone system distribution), Sentinel-2A data (full coverage) and a small amount of sample plot data obtained in 2019, four patterns of sample plot (p), sample plot-satellite (ps), sample plot-sampling LiDAR (pl), and sample plot-sampling LiDAR-full coverage satellite (pls) were estimated and compared with five methods, including DBp, MDps, MDpls, HYpl and MAps, which were suitable for these four patterns and belong to design-based method (DB), model-assisted method (MA), model-dependent method (MD) and mixed or hybrid method (HY).Result (1)The mean stock volume densities of DBp, MDps, MAps, HYpl, and MDpls were 212.54, 202.09, 202.38, 210.07 and 208.96 m3/ha, respectively. The accuracy (P) was 90.44%, 91.54%, 91.69%, 96.35%, and 96.45%, respectively, with variances decreasing in turn. (2) The relative efficiency (RE) of other methods compared with MDpls was greater than 1 (REDBp,MDpls = 5.39, REMDps,MDpls = 3.82, REMAps,MDpls = 3.69, REHYpl,MDpls = 1.07), and the RE of HYpl compared with MDpls method was greater than 1, but close to 1. Compared with the other three methods, HYpl was more efficient (REDBp,HYpl = 5.02, REMAps,HYpl = 3.43, REMDps,HYpl = 3.56). (3) Both HYpl and MDpls methods containing LiDAR data had positive efficiency compared with MDps and MAps methods containing Sentinel-2A data (REMAps,HYpl = 3.43, REMDps,HYpl = 3.56, REMDps,MDpls = 3.82, REMAps,MDpls = 3.69). The RE of MDps and MAps was close to 1, but the efficiency of MAps was slightly higher than that of MDps (REMDps,MAps = 1.04).Conclusion Compared with the estimation method that only used the sample plot data, when the remote sensing data was used as an auxiliary variable to establish the estimation model, it can effectively improve the estimation accuracy of the MSVD of the forest farm, whether the Sentinel-2A multispectral remote sensing data, which is fully covered the forest farm but not sensitive enough to the stock volume, or the sampling LiDAR data which are sensitive to stock volume. Among four methods involving the application of remote sensing data, MDpls has the highest estimation accuracy, followed by HYpl. Both of these two methods including the application of LiDAR remote sensing sampling observation data are suitable for the MSVD estimation of forest farms. The MDpls method has the highest estimation accuracy and relative efficiency, which can realize the synergistic application of the Space-Air-Earth multi-source observation data, and can be used as the preferred method for annual monitoring of forest stock in forest farms.
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表 1 S-2A卫星数据特征
Table 1 S-2A satellite data characteristics
特征名称
Feature name特征符号
Feature symbol计算公式
Calculation formula特征名称
Feature name特征符号
Feature symbol计算公式
Calculation formula光谱特征
Spectral featureB2、B3、B4、B5、B6、B7、B8a、B11、B12 角二阶矩
Angular second momentAN k∑i,j=1P2i,j 差值植被指数
Difference vegetation indexDVI B8−B4 熵
EntropyEN k∑i,j=1Pi,j(−lnPi,j) 增强植被指数
Enhanced vegetation indexEVI 2.5 × (B8 − B4)/(B8 + 6 × B4 − 7.5 × B2 + 1) 对比度
ContrastCON k∑i,j=1(i−j)2×Pi,j 归一化植被指数
Normalized difference vegetation indexNDVI (B8−B4)/(B8+B4) 均值
MeanME 1k2k∑i,j=1Pi,j 比值植被指数
Ratio vegetation indexSR B8/B4 方差
VarianceVAR k∑i,j=1Pi,j(i,j−ME) 转换归一化植被指数
Transformed normalized difference vegetation indexTNDVI √(B8−B4)/(B8+B4)+0.5 同质性
HomogeneityHOM k∑i,j=1Pi,j1+(i−j)2 叶绿素指数
Chlorophyll indexCIg B8/B3−1 相关性
CorrelationCOR k∑i,j=1P2i,j[(i−ui)−(j−uj)√σ2iσ2j] 反向红边叶绿素指数
Inverted red edge chlorophyll indexIRECI (B7−B4)/(B5/B6) 相异性
DissimilarityDIS k∑i,j=1Pi,j|i−j| 色素简单比值指数
Pigment specific simple ratioPSSRA B7/B4 修正叶绿素吸收反射指数
Modified chlorophyll absorption in reflectance indexMCARI [(B5 − B4) − 0.2 × (B5 − B3)] × (B5/B4) S-2A 红边位置指数
S-2A red edge positionS2REP 705 + 35 × [(B4 + B7)/2 − B5]/(B6 − B5) 修正窄边红边简单比值指数
Modified simple ratio red edge narrowMSRren [(B8a/B5)−1]/√(B8a/B5)+1 红边叶绿素指数
Red edge chlorophyll indexCIgre1 B5/B3−1 红边植被指数
Red edge vegetation indexNDVIre1 (B8−B5)/(B8+B5) CIgre2 B5/B3−1 NDVIre2 (B8−B6)/(B8+B6) CIgre3 B7/B3−1 NDVIre3 (B8−B7)/(B8+B7) 注:B2、B3、B4、B5、B6、B7、B8、B8a、B11、B12代表S-2A数据对应的波段,Pi,j=Di,j/∑ki,j=1Di,j,Di,j为i行j列对应的像元值,k代表计算纹理时窗口大小。Notes: B2, B3, B4, B5, B6, B7, B8, B8a, B11 and B12 represent the bands corresponding to S-2A data; Pi,j=Di,j/∑ki,j=1Di,j, Di,j is the pixel value corresponding to the i row and j column, k represents the window size when calculating the texture. 表 2 LiDAR数据特征
Table 2 Feature of LiDAR data
特征名称
Feature name特征符号
Feature symbol均值
MeanHmean, Imean 方差和标准差
Variance and standard deviationHvar, Ivar, Hsd, Isd 最大值和最小值
Maximum value and minimum valueHmax, Imax, Hmin, Imin 变异系数
Coefficient of variationHcv, Icv 四分距差
Interquartile distanceHiq, Iiq 偏斜度
SkewnessHsk, Isk 百分位数
PercentileHp05, Hp10, Hp20, Hp25, ···, Hp95, Hp99
Ip05, Ip10, Ip20, Ip25,···, Ip95, Ip99最小高度以上返回点
Count of return point above the minimum heightR1H, R2H,···, R8H, R9H 注:H代表高度,I代表强度,var代表方差,sd代表标准差,max与min分别代表最大值与最小值,cv、iq以及sk分别代表变异系数、四分距差以及偏斜度,p05—p99代表对应的百分位数。Notes: H represents height, I represents intensity, var represents variance, sd represents standard deviation, max and min represent maximum value and minimum value, respectively; cv, iq and sk represent coefficient of variation, interquartile distance and skewness, respectively; p05−p99 represent the corresponding percentiles. 表 3 材积计算公式
Table 3 Formula for volume calculation
树种 Tree species 材积计算公式 Formula for volume calculation 白桦
Betula platyphyllaV=100.000397507075980248×d2×h+3.8121380807356×10−6×d3×h−0.000843446660194932×d2−0.000290088083727618×d2×h×lgd 黑桦
Betula davuricaV=0.0199215193890509+0.00038814516708027×d2−2.05059660776977×10−5×d2×h−
0.000870310875746131×h2+8.05362136949543×10−5×d×h2落叶松
Larix gmeliniiV=10−3.49890390728004+2.75504846502564×lgd−0.394050839410844×lgd2−1.37915356404294×lgh+1.14586681477751×lgh2 樟子松
Pinus sylvestrisV=0.000445504541007861×d1.66316523786429×exp(0.0989929357004981×h−1.66681646056217/h) 油松
Pinus tabuliformisV=2.53309290763168×10−5×d2×h−8.57378841160325×10−7×d3×h−6.00033429201054×10−5×d2+
2.93720677218884×10−5×d2×h×lgd注:引自文献[40]。V代表单木材积;d代表单木胸径;h代表单木树高。Notes: cited from reference [40]. V represents single tree stock volume, d represents single tree DBH, h represents single tree height. 表 4 遥感特征优选后保留的特征
Table 4 Remotes sensing features selected after optimum feature selection
数据源
Data source特征选择结果
Result of feature selectionS-2A B2,MCARI,B2ME,B12COR LiDAR Hmean,Ip99,R2H 表 5 5种方法的总体均值估计结果和精度
Table 5 Results and accuracy of total mean estimation of the five methods
方法
Method总体均值/(m3·hm−2)
Total mean/(m3·ha−1)均值方差/(m3·hm−2)
Mean variance/(m3·ha−1)标准误/(m3·hm−2)
Standard error/(m3·ha−1)估测精度
Estimation accuracyDBp 212.54 107.51 10.37 90.44% MDps 202.09 76.18 8.72 91.54% MAps 202.38 73.55 8.58 91.69% HYpl 210.07 21.42 4.63 96.35% MDpls 208.96 19.95 4.47 96.45% -
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