OBJECT-ORIENTED REMOTE SENSING INFORMATION EXTRACTION METHOD FOR ROCKY DESERTIFICATION IN KARST AREA-A CASE STUDY OF DAFANG COUNTY, GUIZHOU
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摘要: 针对现有基于像素的监督和非监督分类方法在地质环境复杂、地形起伏较大、阴影明显的喀斯特石漠化地区难以满足石漠化信息提取精度要求的问题,采用基于纹理特征数据和地形数据辅助面向对象方法进行喀斯特地区石漠化信息的提取。该方法首先依据石漠化分布在TM/ETM+影像面积大小不均匀的特征,利用纹理和地形因子计算最优分割参数进行多尺度分割;然后根据植被覆盖率、岩石裸露率以及坡度因子构建石漠化分级指标;最后参照石漠化分级标准、光谱信息以及纹理特征等建立的分类规则提取喀斯特地区石漠化信息。选取贵州省石漠化严重的大方县时序TM/ETM+影像进行石漠化信息提取试验,结果表明:与基于像素的监督分类和非监督分类方法相比,基于面向对象的分类可以有效地减少因复杂地形导致石漠化信息提取结果"椒盐化"现象,提取精度明显优于基于像素的监督分类和非监督分类方法。Abstract: The existing pixel-based supervised and unsupervised classification methods can't meet the requirements of rocky desertification information extraction accuracy in karst rocky desertification area under the circumstances of complicated geological environment, large topographic relief and obvious shadows. In order to improve the accuracy of remote sensing image information extraction, texture feature data and topographic data are used to assist the object oriented method in the rocky desertification information extraction in karst rocky desertification area. Firstly, based on the characteristics of rocky desertification with uneven image sizes in TM/ETM+, the optimal segmentation parameters are calculated using texture and terrain factors to conduct multi-scale segmentation. Secondly, the grading indexes of rocky desertification are established based on vegetation coverage rates, rock exposure rates and slope factors. Finally, according to the grading rules of rocky desertification, spectral information and texture features, the information of rocky desertification in Karst area is extracted. The temporal TM/ETM+images of rocky desertification areas in DaFang, Guizhou, are selected for rocky desertification information extraction. The results show that comparing with pixel-based supervised classification and unsupervised classification methods, the object-oriented classification technology can effectively reduce the "salt and pepper phenomenon" caused by complicated topography, and the extraction accuracy is much better.
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表 1 分割参数的设置
Table 1. Settings of segmentation parameters
分割层次 波段选择 分割尺度 异质性因子 光谱权重 形状权重 光滑度 紧密度 Level1 741+4纹理 20 0.7 0.3 0.5 0.4 Level2 741+4纹理 50 0.7 0.3 0.5 0.4 Level3 741+4纹理 100 0.7 0.3 0.5 0.4 强度等级 坡度/% 岩石裸露率/% 植被覆盖率/% 利用价值 无石漠化 < 15 < 10 >70 宜农宜林牧 微度石漠化 >15 10~30 50~70 宜林牧 轻度石漠化 >18 30~50 35~50 临界宜林牧 中度石漠化 >22 50~70 10~30 难利用地 重度石漠化 >25 >70 < 10 无利用价值 表 3 面向对象的喀斯特地区石漠化遥感信息提取规则
Table 3. Rules of object-oriented remote sensing information extraction method for rocky desertification in Karst area
分割层次 分割尺度 提取信息 规则(模糊分类定义的成员函数) Level1 20 水体
喀斯特地区非
喀斯特地区200≤Ratio(B4)≤800, Brightness < 180, length/width > 3.28, Mean(con) < 0.34;
Ratio(B4)≥800, Ratio(B2)≤1000, NDVI≥0.405, Shape Index > 0.62;
NDVI < 0.405, Ratio(B2)>1000, Mean(con) < 5;Level2 50 无石漠化
有石漠化NDVI≥0.525, Brightness≤1300, length/width > 1.5, Mean(slope) < 15;
NDVI < 0.525, Brightness>1300, Stdv(B2)+Stdv(B3)+Stdv(B4) < 20, 15 < Mean(slope) < 45;Level3 100 中度石漠化
轻度石漠化
重度石漠化
微度石漠化Ratio(B2) < 4.45, Stdv(B4) < 12, NDVI < -0.25, Mean(slope)>22;
200 < Brightness < 400, Mean(slope)>18, 1.5 < Mean(Ent) < 2.6;
Ratio(B3) < 2.26, Mean(slope)>25, Mean(Ent)>2.6, Mean(con)>5;
0.25 < Ratio(B7) < 0.31, Mean(slope)>15, Mean(con)>7;注:Brightness为对象平均的多光谱灰度值,length/width为对象的长宽比,Stdv标准差,Ratio为比率,Mean(con)为对比度均值,Mean(Ent)为熵均值,Mean(slope)为坡度均值,NDVI=(Nir-red)/(Nir+red),Shape Index=e/4√A(e为边长,A为面积),B2、B3、B4、B7是TM/ETM+多光谱影像的波段。 表 4 1988—2016年大方地区喀斯特石漠化面积
Table 4. Area of karst rocky desertification in Dafang from 1988 to 2016
1988年 2002年 2016年 面积/km2 比例/% 面积/km2 比例/% 面积/km2 比例/% 微度石漠化区 139.27 5.07 89.67 3.27 77.64 2.83 轻度石漠化区 78.5 2.86 135.8 4.95 144.43 5.26 中度石漠化区 74.15 2.70 66.35 2.42 61.91 2.25 重度石漠化区 37.28 1.36 30.6 1.11 19.78 0.72 水系 73.81 2.69 75.39 2.75 74.24 2.70 无石漠化区 1746.29 63.59 1751.49 63.78 1771.3 64.50 非喀斯特地层区 596.7 21.73 596.7 21.73 596.7 21.73 总计 2746 100.00 2746 100.00 2746 100.00 表 5 分类方法精度对比表
Table 5. Comparison of the accuracy of each method
方法 监督分类 非监督分类 面向对象分类 年份 1988年 2002年 2016年 1988年 2002年 2016年 1988年 2002年 2016年 总体精度 72.15% 78.81% 76.43% 62.50% 63.25% 72.90% 91.45% 93.56% 95.31% Kappa系数 0.65 0.71 0.68 0.54 0.55 0.58 0.9 0.92 0.94 -
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