西南地区破碎地表空间异质性刻画与空间尺度关系初探
Characterize the spatial heterogeneity of fragmented land-surface and its relationship with spatial scale in southwest china
- 2023年27卷第3期 页码:802-809
纸质出版日期: 2023-03-07
DOI: 10.11834/jrs.20232134
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纸质出版日期: 2023-03-07
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尺度效应是定量遥感领域的经典且重要问题之一,其中地表异质性的判断和明确对地表真实性检验和场站优化布设问题的前置工作。并且地表异质性的判断和计算,一般是通过一景低分辨率待检验产品与同步获取的地面测量结果或者高分辨率产品进行尺度转换实现间接表达。然而,由于待检低分辨率与高分辨率遥感影像之间几乎很难做到完全同步。那么如何在缺乏同步产品的基础上,仅利用高分辨率产品去刻画地表异质性,是为下一步对空间异质性进行进一步探索和分析的前提条件。本文针对该问题使用优于0.2 m空间分辨率的无人机光谱反射率数据,计算得到归一化差值植被指数NDVI(Normalized Difference Vegetation Index)数据,通过三次卷积升尺度算法计算获取了0.2—30 m共39个不同空间分辨率结果,通过目视解译获得土地利用/覆盖变化LULC(Land Use and Land Cover change)数据,结合地理探测器对1 km×1 km图幅内的空间异质性行评价。结果表明:3个地形破碎的喀斯特槽谷区,其空间异质性评价值q的阈值存在差异,但总体上q值都随着空间分辨率的提高(30—0.2 m)由震荡趋于平稳; Mann-Kendall突变检测发现,柑橘研究所和虎头村的空间异质性突变点和q值震荡曲线的稳点阈值基本一致。
Scale effect is one of the classical and important problems in the field of quantitative remote sensing especially in surface validation field, in which the judgment of surface heterogeneity is a precursor to the problems of surface validation and station optimized layout, and is also one of the important error sources of surface parameters validation. The first way is to calculate the accuracy evaluation of the scale transformation results between the medium resolution remote sensing products and the ground measurement results or the very high resolution products acquired at the same time to express spatial heterogeneity indirectly, and a series of errors, such as different sensors optical parameters, different measurement angles, spatial and temporal scale inconsistency, geometric mismatching etc., they all affect the results directly or jointly, and the error contributions are difficult to quantitatively, it means that is difficult to describe the spatial heterogeneity clearly. The second way is to use geostatistical methods to describe the images for evaluation the spatial heterogeneity directly. Then how to express the surface heterogeneity with only very high resolution remote sensing measurement image based on the lack of moderation satellite retrieval products is a workable way to describe to spatial heterogeneity for further exploration and analysis of spatial heterogeneity in the next step. Therefore, this paper uses a typical algorithm to portray spatial heterogeneity and discusses the relationship between spatial resolution and spatial heterogeneity in the absence of a reference base of medium-resolution data, with a view to reflecting the relationship between resolution and spatial heterogeneity and conducting a preliminary analysis. Specifically, this paper calculates Normalized Difference Vegetation Index (NDVI) data using Unmanned Aerial Vehicle(UAV) spectral reflectance data with spatial resolution better than 0.2 m that has been Radiation calibration by reflector plates, and obtains results for 39 different spatial resolutions from 0.2 m to 30 m by cubic convolution upscaling algorithm, and obtains land use and land cover change (LULC) by visual interpretation. The spatial heterogeneity of the 1km×1km map area was evaluated with GeoDetector algorithm, and then the regional spatial heterogeneity was described to explore the relationship between resolution and spatial heterogeneity. The results showed that the thresholds of spatial heterogeneity evaluation q value were different in three regions with fragmented land-surface, but the overall q value tended were oscillate to stable with the increase of spatial resolution (30 m to 0.2 m), and the minimum threshold from oscillation to stability was 2 m resolution; then the change curve of q value with spatial resolution and done M-K mutation detection found that the thresholds and q values of spatial heterogeneity mutation points in Ganyansuo and Hutou Village oscillation curve existed for the oscillation to stable points basically matched, but there were multiple mutation points and mismatched in the Caoshang. There were pass the 5% significance test of M-K test for all three areas, which tested the relationship between q value and spatial resolution in the aforementioned in statistical significance. In conclusion, all this classification system was now regionally stable when the resolution was lower than 2 m, i.e., when the resolution was higher than 2 m, its spatial heterogeneity tends to stable, and its could provide some reference for the sampling of ground and space-based platforms.
地球表面空间作为一个具有很高复杂性的系统,导致在某一尺度上观测到的现象、总结出的规律,在另一尺度上可能无效、可能相似,但是更多情况下需要修正,这就是地理学中普遍存在的尺度问题(
对于空间异质性,既往研究证明其对真实性检验有着重要的作用,例如
在前述诸多定量遥感的反演参量中NDVI作为植被生长状态及植被覆盖度的最佳指示因子,与植被覆盖度、叶面积指数、光合作用光利用效率、生物量、植被初级净生产力和景观物候学参数等密切相关,被广泛应用于环境(气候)变化和农作物估产等领域(
本文以重庆市北碚区为研究区,针对西南地区破碎地表空间异质性问题,提出破碎地表空间异质性刻画方法,探讨地表空间异质性与不同分辨率的关系。
研究区位于重庆市北碚区,具体研究区如
图1 研究区示意图
Fig. 1 Study area
实验使用的无人机载荷为5波段、空间分辨率优于0.2 m、航向重叠率高于85%、旁向重叠率高于75%的极高分辨率的反射率数据。实验设置了3个研究区,依据所属行政区划被命名为槽上生态恢复区(以下简称槽上)、虎头村通量研究区(以下简称虎头村)、西南大学柑橘研究所(以下简称柑研所),分布如
图2 3个实验区的LULC分类图
Fig. 2 LULC classification map of the three experimental areas
本文提出了在无参考条件下的基于三次卷积重采样的升尺度算法评价结果,具体过程如
图3 试验流程简图(以槽上为例)
Fig. 3 Schematic diagram of the test flow (take Caoshang as an example)
图4 3个区域的q值变化曲线
Fig. 4 q value change curve of the three experimental areas
2.3.1 升尺度算法
升尺度算法使用三次卷积重采样。公式如
G(n)=∫∞-∞f(n-x)g(x)dx+e(n) | (1) |
式中,G(n)为所求重采样结果;f(x)为原始影像数据,其中n为卷积核的大小;g(x)为所需要的求得的对于n的点扩散函数;e(n)为误差或者噪声。
2.3.2 地理探测器
地理探测器是对遥感数据的空间分异的探测,是地表异质性评价的重要方法(
q=1-L∑h=1Nh∑i=1(Yhi-ˉYh)2N∑i=1(Yi-ˉY)2 | (2) |
式中,Yhi为分区内影像辐射值,ˉYh为分区内影像辐射值均值,Yi为影像全部辐射值,ˉY为影像全部辐射值均值,具体而言,就是对影像进行分区域统计并计算区域内的均质化程度,然后用1减去该值得到整个区域内的空间分异。所以,对于求得的q值,假设分母基本保持一致,则区域内越均质,其分子越小,进而q的值越大。
2.3.3 Mann-Kendall突变检测
而对于q的变化曲线,结合既往研究,可以使用Mann-Kendall突变检测(以下简称M-K检验)对q值曲线进行分析。M-K检验,其核心为独立q值与整体曲线之间的二阶离散程度的呈现,通过对这个离散程度的展现,集合正反两个方向曲线的叠加,可以明确q值的变化趋势,即空间异质性(空间分异)的变化趋势,而当正反两条曲线叠加时,即发生了突变,即在该尺度下的空间异质性开始出现显著变化。具体的推导过程参见徐建华教授的《计量地理学》一书(
UFk=[sk-E(sk)]√Var(sk) k=1,2,…,n | (3) |
式中,E(sk)、Var(sk)为累计数sk的均值与方差,且x1,x2,…,xn相互独立,并连续分布。
依据
依据
图5 3个区域M-K分析曲线
Fig. 5 M-K analysis curve of the three experimental areas
地表真实性检验,是推动遥感进一步发展重要工作之一,也是解释地表尺度效应的重要工作之一,在地表真实性检验工作中,空间异质性是一个需要着重考虑与分析的工作,本文尝试使用地统计工具,对不同分辨率下的空间异质性进行讨论。本文讨论了一个空间异质性在不同尺度下的表现,并通过对不同分辨率下的空间异质性的模拟,说明了空间异质性的复杂表现,通过现有实验,可以发现,对于空间异质性,随着分辨率的提高(30—0.3 m),其空间异质性存在从震荡到稳定的情况,且存在一个阈值可以区分震荡与稳定的状态,可以为下一步异质地表的真实性检验与优化布设工作提供一定的参考。具体实验过程为使用分辨率优于0.2 m分辨率的NDVI数据,结合LULC结果,使用三次卷积对升尺度结果进行模拟,并使用地理探测器和突变检验对空间异质性进行评价和结果检验。结果发现随着分辨率的提升,利用地理探测器计算的q值曲线,会由震荡趋于稳定。震荡到稳定的阈值在3个地区不相同,其中柑研所为4 m,虎头村为3 m,槽上为2 m,M-K突变检测发现柑研所和虎头村通过了5%显著性检验,其突变点和q值震荡曲线的稳点阈值基本一致。因此,在西南地区的典型地表破碎区域,在1 km×1 km的空间范围内,使用本实验中的LULC得到的分区,进而对整体区域的分异评价值q可以在一定程度上表征空间异质性,且评价结果可以通过M-K突变检测进行验证通过。总结而言,本文在前人研究的基础上,抛弃了原有的需要两个尺度的异源数据来对空间异质性通过真实性检验的精度评价过程进行间接分析的方法,使用地统计方法,直接对空间异质性进行刻画,进而评价空间异质性与分辨率之间的关系,并创新性的通过M-K突变检测,检验该结论在统计学上的合理性。
本文中的空间异质性,是通过空间分异来进行讨论的,空间分异是在一个固定范围内的地表的具体分布于规律,它认为相同的地物的观测是一致的。而空间异质性指的是空间单元观测值与其他观测单元存在的结构不稳定关系引起的观测值非同质现象,它指的是不同空间分异下的地物观测的异质性。它们二者存在一定的共通性,所以,空间分异能从一个侧面对空间异质性进行刻画。同时由于地物观测存在一定的不确定性,例如系统误差和随机误差等问题,所以通过高分辨率影像通过升尺度算法对不同空间尺度的观测结果的模拟,在一定程度上将空间异质性从一定程度上表达出来。经过本文的模拟,发现不同地区的地物在不同分辨率下的“观测”的空间分异是差异巨大的。下一步需要进一步增加观测的维度,例如地基、空基与天基综合观测平台,减少观测的不确定度,进而推动地表真实性检验工作的进一步发展和深化。
致谢:此次无人机飞行数据由西南大学地理科学学院金佛山国家站无人机团队采集并处理获得,在此表示衷心的感谢!
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