Using ENVI to process SPOT remote sensing images to extract water body, vegetation and impermeable surface
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foreword
简单记录遥感数字图像处理——针对SPOT4遥感影像进行地物提取的操作，方便日后回忆。
1. Obtain the Shp file of the research area
(1) Open the shp file in Arcmap, select Select Features to select the research area:
(2) Select the research area, right-click on the file and select Data——Export Data to export the shp file of the selected area:
2. Download SPOT remote sensing images
(1) First open the website for free acquisition of SPOT images:
https://regards.cnes.fr/user/swh/modules/60 , click login to register:
(2) Select the Display result over map button in the upper right corner, turn the globe, Select the area of interest to download:
(3) Select the Draw search area tool to select the image of the area to be downloaded:
(4) Select Display in table in the upper right corner:
click COLUMN to adjust the order of the displayed columns to time, satellite model, and cloud cover:
(5) Select the required module download according to the year, cloud condition, and satellite model:
(6) Here, you can also click the SEARCH button to search by condition, which is more convenient:
input satellite model, image year, cloud cover and other conditions:
提示：因为后续涉及提取物归一化指数的运算，所以这里下载spot4-spot5的多光谱遥感影像，确保影像的质量，时间最好选择4-10月。
3. Image preprocessing
(1) Open the shp file of the research area and the downloaded SPOT remote sensing image in ENVI one by one. Since the projection method is the same, you can intuitively see whether the research area is covered by the required remote sensing image: (2) Open ENVI Classic classic
version , to mosaic multiple remote sensing images:
1. Select the Map——Mosaicking——Georeferenced tool in the toolbar:
2. Select Import in Mosaic Based Mosaic, and select the remote sensing images to be mosaic:
3. Mosaicking is complete:
4. Click Flie——Apply select the path to save:
(3) Open the mosaic image and the shp file of the research area in ENVI for cropping:
1. Select the Subset Data form ROIs tool in the Toolbox to crop the region of interest:
2. Set the research area as the region of interest and save:
3. Here it is found that the background color of the cropped image has a black background, which needs to be removed:
4. Select Edit ENVI header in the Toolbox:
click Add here:
5. Select Data Ignore Value:
6. Set the Data Ignore Value below to 0:
(4) Finish cropping:
4. Ground Object Extraction
Here is the water extraction operation
（一）波段运算，打开Band math工具，填写公式：
这里要结合遥感影像的波段范围值来进行运算（植被、水体、不透水面公式依次如下）：
NDVI = (Nir - Red) / (Nir + Red)
NDWI = （Green - Nir）/（Green + Nir）
NDBI = （Mir - Nir） / （Mir + Nir）
其中Nir代表近红外波段，Mir代表中红外波段。
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SPOT-4参数：
SPOT-4卫星携带者HRVIR-1 和 HRVIR-2共计2 个仪器，其卫星的各波段详细参数如下所示：
| 名称 | 波段 | 波长范围/μm | 分辨率
|band 1 | 绿色波段 | 0.50–0.59um |20 m
|band 2 | 红色波段 | 0.61–0.68um | 20 m
|band 3 | 近红外波段 | 0.78–0.89um | 20 m
|band 4 | 中红外波段 | 1.58–1.75um | 20 m
|panchromatic | 全色波段 | 0.51-0.73um | 10 m
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SPOT-5参数：
SPOT-5卫星携带着立体视图的2 个仪器：HRG1 和 HRG2。创新性地引入了 THR 超级模式该模式，该模式能从两景5m分辨率影像中创建 2.5 m 分辨率的影像，该卫星的波段详细参数如下所示：
| 名称 | 波段 | 波长范围/μm | 分辨率
|band 1 | 绿色波段 | 0.50–0.59um |10 m
|band 2 | 红色波段 | 0.61–0.68um | 10 m
|band 3 | 近红外波段 | 0.78–0.89um | 10 m
|band 4 | 中红外波段 | 1.58–1.75um | 10 m
|panchromatic | 全色波段 | 0.51-0.73um | 5 m
|panchromatic super-mode | 超全色波段 | 0.48-0.71um | 2.5m
Fill in the formula as follows:
Select the bands in turn:
The effect after ndwi:
(2) Judging the value of the extracted water body through the histogram of the pixel value distribution:
(3) Right-click the file after ndvi and select New Raster Color Slice:
first Clear the original color segment, then create a new one, and modify the value in the following figure:
(4) Select Export Color slices-Shapeflie to export the shapefile file:
Here is a look at the final extraction effect (water body) in Arcgis:
Summarize
This article introduces the simple operation of extracting water bodies and other ground features from SPOT satellite image data through a complete remote sensing image processing platform - ENVI (The Environment for Visualizing Images).