遥感图像处理的基础知识

一、数字图像基础

(一)数字图像的含义

  1、模拟图像

     灰度及颜色连续变化的图像。

  2、数字图像

    能够被计算机储存、处理和使用,将模拟图像分成同样形状的小单元,将各个小单元的亮度值进行数字化。

(二)图像直方图

用平面直角坐标系表示一幅灰度范围为0-n的数字图像像元灰度分布状态,横轴表示灰度级,纵轴表示某一灰度级的像元个数占像元总数的百分比。

通过灰度直方图可以直观地了解到图像特征。

 

、图像校正(预处理):

(一)辐射校正

  1、概念:由于传感器的响应特性和大气的吸收、散射及其他随机因素的影响,导致图像模糊失真,分辨率和对比度下降。需要辐射校正。

包括系统辐射校正、大气校正

2、系统辐射校正(光学摄影机内部辐射误差,光电扫描仪内部辐射误差)

3、大气校正:

消除主要由大气散射(大气分子和颗粒)、吸收(主要是气溶胶)引起的辐射误差。

 其目的是:

    去除大气效应引起的辐射误差;

转换TOA辐射值为地表反射率。

(二)几何校正

指从具有几何变形的图像中消除变形的过程。

1、误差来源:

    内部误差:由于传感器自身的性能、结构等因素造成

    外部误差:传感器以外的因素造成,如地球曲率、地形起伏、地球旋转等

2、校正原理

(1)数字校正:通过计算机对图像的每个像元逐个解析纠正处理完成

(2)但通常利用由地理坐标的地形图或图像作为参考,通过选择地面控制点,对变形图像进行纠正:

 第一步,像元坐标转换

    由于校正前后像元大小可能变化,像元点位置的相对变化等,不能简单的用原图像的像元灰度值代替输出像元灰度值。

因此,需要插值(重采样)。

第二步,像元灰度值重新计算(重采样)

    最临近法、双线性法、三次卷积法

(三)图像的镶嵌:

将多个具有重叠部分的图像制作成一个没有重叠的新图像。

 

三、增强处理(反差增强、线性拉升、色彩增强、PCA变换等):

(一)☆反差增强(对比度增强)

    通过改变像元的亮度值来改变图像的对比度,从而改善图像质量的图像处理方法。

主要通过调整直方图来实现,使直方图分布均匀,没有大量亮暗像元集中分布。

(二)彩色增强

    人眼对彩色的分辨力较高。

  • 彩色合成:

   真彩色:颜色对应相应的波段,是自然物本来的颜色

假彩色:颜色不对应相应的波段,不是物体本来的颜色

2、密度分割

 将连续的灰度值转为少量的灰度区间,并用不同的颜色表示。

伪彩色:单波段灰度图像的彩色显示或表示

(三)图像滤波(邻域增强):

    抑制噪声,增强某些特征,根据像元点周围的关系作邻域运算。

1、平滑(低通滤波):滤掉孤立的单点噪声和一些细节,突出图像的基本结构。

分为均值滤波(窗口中心的象元被赋予窗口内所有象元的均值)和中位数滤波。

2、锐化(高通滤波):提高边缘灰度值的变化率,使边缘更清晰。

3、中值滤波:窗口中心的象元被赋予窗口内灰度值的中位数

(四)图像变换

涉及不同图像之间的运算:影响代数。

1、算术运算(加减乘除运算):是重要的信息提取方法

例:植被指数的计算。

植被指数是基于植物叶绿素在红色波段的强烈吸收以及近红外波段的强烈反射,通过红和近红外波段的比值或线性组合实现对植被信息状态的表达。

公式:NDVI=(NIR-R)/(NIR+R)即(近红外-红)/(近红外+红)

用途:识别植被、检测植被健康状况等

2、主成分分析

3、图像融合

用一定的算法综合两个或两个以上不同的图像形成一个新的图像。

 

四、图像信息提取:图像分类、空间信息提取、光谱信息提取

(一)图像分类

  1、基本原理:不同的地物具有不同的光谱特征,同类地物具有相同或相似的光谱特性

  2、目标:将图像中所有的像元自动地进行地表覆盖类型的分类。

3、存在问题:

      同物异谱:同类地物具有不同的光谱特征

      同谱异物:不同的地物可能具有相似的光谱特征

  4、一些概念:

(1)监督分类:通过选择代表各类已知样本的像元光谱特征,事先取得各类别的参数,确定判别函数,从而进行分类。

(2)非监督分类:根据事先指定的某一准则,而进行计算机自动判别归类,无需人为干预。

(3)训练区:已知覆盖类型的代表样区;用于描述主要特征类型的光谱属性;其精度直接影响分类结果。

(4)检验区:用于评价分类精度的代表样区

(二)图像分类评价

    通常用分类图与标准数据进行比较,以正确的百分比来表示精度

1、非位置精度:以面积、像元数目等表示分类精度,未考虑位置因素,所得精度偏高。

2、位置精度:将分类类别与其所在空间位置进行统一检查。目前普遍采用混淆矩阵方法

 

五、图像处理的步骤

原始图像==>系统辐射校正==>大气辐射校正==>几何校正==>(图形镶嵌)==>增强处理==>指标/分类/变化检测/定量反演==>专题信息提取

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吾爱地理 » 遥感图像处理的基础知识
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