生物多样性 ›› 2018, Vol. 26 ›› Issue (8): 892-904.doi: 10.17520/biods.2018039

• 研究报告 • 上一篇    下一篇

轻小型无人机航摄技术辅助的热带森林样地测量精度问题探讨

邓云1, 3, 4, *(), 王彬2, 李强2, 张志明2, 邓晓保1, 3, 曹敏1, 林露湘1, 3   

  1. 1 中国科学院西双版纳热带植物园热带森林生态学重点实验室, 云南勐腊 666303
    2 云南大学生态学与环境科学学院, 昆明 650091
    3 中国科学院西双版纳热带植物园云南西双版纳森林生态系统国家野外科学观测研究站, 云南勐腊 666303
    4 中国科学院大学, 北京 100049
  • 收稿日期:2018-02-06 接受日期:2018-08-05 出版日期:2018-08-20
  • 通讯作者: 邓云 E-mail:dy@xtbg.org.cn
  • 基金项目:
    国家重点研发专项(2016YFC0500202)

An analysis of lightweight-drone-assisted mapping accuracy in tropical forest plot

Deng Yun1, 3, 4, *(), Wang Bin2, Li Qiang2, Zhang Zhiming2, Deng Xiaobao1, 3, Cao Min1, Lin Luxiang1, 3   

  1. 1 CAS Key Laboratory of Tropical Forest Ecology, Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences, Mengla, Yunnan 666303
    2 School of Ecology and Environmental Sciences, Yunnan University, Kunming 650091
    3 National Forest Ecosystem Research Station at Xishuangbanna, Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences, Mengla, Yunnan 666303
    4 University of Chinese Academy of Sciences, Beijing 100049
  • Received:2018-02-06 Accepted:2018-08-05 Online:2018-08-20
  • Contact: Deng Yun E-mail:dy@xtbg.org.cn

准确的样地坐标位置是无人机航摄数据与地面调查数据融合使用的必要前提, 但是在森林样地的具体实践中, 会有许多因素制约着样地位置的测量精度, 这有可能影响后期的数据融合过程甚至得出错误的结论, 研究者们需要对此予以足够的重视。本文通过对比西双版纳地区10个热带森林样地及周围区域无人机航摄过程中的地面控制点测量精度、Photoscan摄影测量软件所得点云解算精度和照片曝光点重投影精度, 发现: (1)即使使用性能相对较好的实时差分(real time kinematic, RTK)式GNSS系统进行定位, 在林内也很难获得很好的定位精度, 林窗处的地面控制点均方根误差(root mean square error, RMSE)在水平和垂直方向分别为0.167 ± 0.158 m和0.297 ± 0.170 m, 林下样地顶点桩处分别为0.392 ± 0.368 m和0.657 ± 0.412 m; (2)软件的全局解算精度主要受控制点地面测量精度和控制点数量的影响; (3)若仅依托普通的单站式GPS对无人机位置进行定位, 则照片曝光点的重投影坐标位置可能存在较大误差(RMSE在水平和垂直方向上分别为18.434 ± 5.252 m和34.042 ± 6.920 m); (4)估测地形与实测地形间的高差标准差与林冠平均高度正相关(r = 0.713, P < 0.05), 估测地形模型在20 ha样地尺度下的验证结果优于1 ha样地。基于以上结果, 我们建议: (1)在对热带森林进行无人机航摄的过程中, 必须有足够数量和质量的分布相对均匀的地面控制点对测量误差进行控制; (2)摄影测量法的优势在于能够以相对简单的前端设备建立数字表面模型, 但该方法可能很难在森林样地中建立准确的数字地形模型。在使用无人机获取数据之前, 研究者应预先考虑到适合自己的恰当方法以应对以上的精度控制问题。

关键词: 轻小型无人机, 全球导航卫星系统, 定位精度, 森林样地

Accurate coordinate position is a prerequisite for combining drone-assisted remotely sensed data and ground survey data. However, in the practice of surveying forests, many factors prevent accurate measurement of coordinate position and inaccurate coordinates may lead to incorrect conclusions. Therefore, researchers must pay attention to factors effecting accuracy of position. In this study, we compared location error of ground control points (GCPs), model error of photogrammetric point cloud (estimated by Photoscan software) and reprojection error of camera exposure position. First, we found that real time kinematic (RTK) global navigation satellite system (GNSS) cannot locate position in tropical forest with high accuracy. The root mean square error (RMSE) of GCPs in canopy gaps were 0.167 ± 0.158 m and 0.297 ± 0.170 m in the horizontal and vertical axes respectively. In comparison, RMSE of GCPs within forests were 0.392 ± 0.368 m and 0.657 ± 0.412 m respectively for horizontal and vertical axes. Second, the number and measurement accuracy of GCPs influenced model error of photogrammetric point cloud. Third, reprojection error of camera exposure position (18.434 ± 5.252 m and 34.042 ± 6.920 m in horizontal and vertical axes respectively) was much greater than location error of GCPs when the drone acquired position with a single-station GPS system. Fourth, standard deviation of difference between estimated digital terrain model (DTMestimated) and measured digital terrain model (DTMmeasured) was positively correlated with mean canopy height (r = 0.713, P < 0.05). DTMestimated was better estimated at 20 ha scale than at 1 ha scale. Based on these results, we suggest that uniform distribution and sufficient numbers of GCPs can improve drone-assisted mapping accuracy. Lightweight-drone-based photogrammetry has an advantage in requiring fewer equipment and enabling creation of accurate DSM (digital surface model), but remains incapable of estimating ground elevation. Researchers should consider these factors related to accuracy before using drones for surveys.

Key words: lightweight drone, global navigation satellite system, location accuracy, forest plot

图1

研究样地分布位置。1: 补蚌; 2: 过门山; 3: 茶地头; 4: 大平掌; 5: 石灰山; 6: 新开地; 7: 茶厂后山; 8: 44公里; 9: 勐仑水库; 10: 江边站。"

表1

样地基本情况"

序号
No.
样地名称
Plot name
植被类型
Vegetation type
北纬
North
latitude
东经
East
longitude
海拔
Elevation
(m)
飞行高度
Flying altitude
(m)
地面分辨率
Ground
resolution
(cm/pix)
航测面积
Coverage
area (km2)
点云密度
Point cloud density
(points/m2)
1 补蚌
Bubeng
季节雨林
Seasonal rainforest
21.613° 101.580° 730 362 5.52 1.85 82
2 过门山
Guomenshan
山地雨林
Montane rainforest
22.246° 100.599° 1,120 300 4.47 1.87 31
3 茶地头
Chaditou
常绿阔叶林
Evergreen broad-leaved forest
22.250° 100.612° 1,284 310 4.18 0.58 143
4 大平掌
Dapingzhang
常绿阔叶林
Evergreen broad- leaved forest
22.230° 100.574° 1,750 193 2.46 0.25 103
5 石灰山
Shihuishan
石灰山季雨林
Limestone monsoon forest
21.911° 101.283° 606 258 3.36 0.90 56
6 新开地
Xinkaidi
季节雨林次生林
Secondary forest of seasonal rainforest
21.903° 101.275° 556 223 2.88 0.78 75
7 茶厂后山
Chachanghoushan
季节雨林
Seasonal rainforest
22.155° 100.675° 784 169 2.21 0.49 129
8 44公里
44 Gongli
季节雨林
Seasonal rainforest
21.971° 101.148° 806 287 3.58 1.07 49
9 勐仑水库
Menglunshuiku
季节雨林
Seasonal rainforest
21.935° 101.179° 656 361 4.56 0.72 30
10 江边站
Jiangbianzhan
季节雨林
Seasonal rainforest
22.219° 100.734° 640 407 5.24 1.03 23

图2

森林中地面控制点坐标测量(左)与标识布设(右)示例"

图3

航拍影像中的地面控制点标识示例"

图4

基于Photoscan软件的航片数据处理流程"

图5

实测数字地形模型(a)、估测数字地形模型(b)、数字表面模型(c)和林冠高度模型(d)示意图"

表2

样地控制点与样地顶点均方根误差"

样地名称
Plot name
地面控制点RMSE RMSE of ground control point 样地顶点RMSE RMSE of corner stake
数量
Number
垂直
Vertical (m)
水平
Horizontal (m)
全局
Total (m)
数量
Number
垂直
Vertical (m)
水平
Horizontal (m)
全局
Total (m)
补蚌 Bubeng 10 0.581 0.454 0.476 4 0.891 0.659 0.846
过门山 Guomenshan 6 0.247 0.057 0.076 4 0.833 0.634 0.648
茶地头 Chaditou 8 0.404 0.324 0.368 4 0.125 0.007 0.012
大平掌 Dapingzhang 10 0.333 0.202 0.260 4 0.633 0.336 0.532
石灰山 Shihuishan 7 0.086 0.005 0.007 4 0.671 0.311 0.560
新开地 Xinkaidi 7 0.205 0.090 0.091 4 0.284 0.022 0.100
茶厂后山
Chachanghoushan
7 0.104 0.006 0.009 4 0.293 0.020 0.104
44公里 44 Gongli 5 0.131 0.007 0.013 4 1.430 1.101 1.700
勐仑水库
Menglunshuiku
5 0.487 0.292 0.406 4 1.082 0.691 0.994
江边站 Jiangbianzhan 7 0.397 0.228 0.310 4 0.325 0.139 0.161
最大 Maximum 10 0.581 0.454 0.476 4 1.430 1.101 1.700
最小 Minimum 5 0.086 0.005 0.007 4 0.125 0.007 0.012
平均值 ± 标准差
Mean ± SD
7 ± 2 0.297 ± 0.170 0.167 ± 0.158 0.202 ± 0.182 4 ± 0 0.657 ± 0.412 0.392 ± 0.368 0.566 ± 0.523

表3

控制点数量(x1)、飞行高度(x2)和对应方向上的RTK均方根误差(x3)对点云解算模型中均方根误差的逐步回归结果"

y 模型垂直均方根
误差
Vertical RMSE in estimated model
模型水平均方根
误差
Horizontal RMSE
in estimated model
模型整体均方根
误差
Total RMSE in estimated model
截距
Intercept
-3.336E-16 4.430E-17 -1.540E-16
x1 -0.309* 0.082 -0.176
x2 -0.005 0.003*
x3 4.964* 3.157**
VIF_x1 2.022 1.026 1.237
VIF_x2 3.285 1.026
VIF_x3 3.775 1.237
R2 0.757 0.493 0.650
F-统计量
F-statistic
6.245 3.405 6.504
P 0.028 0.093 0.025

表4

各样地相机曝光点重投影误差"

样地名称 Plot name 照片数量 Number of photos RMSE
垂直 Vertical (m) 水平 Horizontal (m) 全局 Total (m)
补蚌 Bubeng 1,142 28.924 19.230 34.733
过门山 Guomenshan 1,632 21.951 12.690 25.356
茶地头 Chaditou 625 32.228 22.554 39.336
大平掌 Dapingzhang 858 36.709 13.466 39.101
石灰山 Shihuishan 1,536 39.100 23.704 45.724
新开地 Xinkaidi 1,293 37.688 25.509 45.510
茶厂后山 Chachanghoushan 2,088 47.508 18.102 50.839
44公里 44 Gongli 2,013 34.682 12.663 36.922
勐仑水库 Menglunshuiku 675 32.660 12.659 35.028
江边站 Jiangbianzhan 1,132 28.973 23.759 37.469
最大 Maximum 2,088 47.508 25.509 50.839
最小 Minimum 625 21.951 12.659 25.356
平均值 ± 标准差 Mean ± SD 1,299 ± 515 34.042 ± 6.920 18.434 ± 5.252 39.002 ± 7.108

表5

线性方程检验结果与样地结构特征"

样地名称
Plot name
面积
Area (ha)
林冠覆盖度
Canopy cover
林冠最大高度
Maximum height of
canopy (m)
林冠平均高度
Average height
of canopy (m)
估测地形与实测地形高差均值
Mean difference between DTMestimated and DTMmeasured
估测地形与实测地形高差标准差
Standard deviation between DTMestimated and DTMmeasured
估测地形模型(y)与实测地形模型(x)的检验方程参数对比
Linear equation test result of DTMestimated (y) and DTMmeasured (x)
k b R2
补蚌
Bubeng
20 0.90 54.67 22.43 10.06 6.09 0.995 14.187 0.981
过门山
Guomenshan
20 0.97 52.10 22.90 2.10 6.70 1.005 -2.782 0.992
茶地头
Chaditou
1 0.95 35.63 16.79 4.55 5.90 0.979 30.783 0.761
大平掌
Dapingzhang
1 0.93 37.66 18.37 -2.91 5.28 0.888 188.821 0.920
石灰山
Shihuishan
1 0.75 22.82 8.22 0.50 2.27 0.982 11.895 0.978
新开地
Xinkaidi
1 0.90 20.37 11.68 2.79 3.94 1.199 -108.789 0.788
茶厂后山
Chachanghoushan
1 0.98 48.98 24.44 7.62 6.37 0.839 125.731 0.834
44公里
44 Gongli
1 0.99 44.80 25.36 -3.60 8.58 1.237 -196.197 0.761
仑水库
Menglunshuiku
1 0.89 45.79 23.82 -26.48 7.42 0.973 -7.650 0.779
江边站
Jiangbianzhan
1 0.93 52.46 26.43 0.18 17.12 1.245 -159.057 0.638
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