introduction:
After getting a satellite image, the first thing we should do is to perform radiometric correction and atmospheric correction . These two corrections are the most basic and important, and this is also the prerequisite for the next step of image processing.
As for what is radiation correction and atmospheric correction, I think there are a thousand ways to figure it out. The focus of this blog post is the specific steps of radiation correction and atmospheric correction , so I won’t go into details here. Let's get straight to the point.
Radiation Correction Procedure
1. Preparation: Landsat8 satellite data.
As for how to obtain the data, the recommended website: http://www.gscloud.cn/
2. Open the file: we open the MTL, txt header file
as shown in the figure
below 2. Search in the toolbox on the right, "radiomatric Calibration", double-click to open
3. Click to open, as shown below, select the first one, multi-band image. Why are there five images to choose from? Here we need to understand the characteristics of the Landsat8 satellite. The satellite has two sensors, the OLI Land Imager (Operational Land Imager) and the TIRS Thermal Infrared Sensor (Thermal Infrared Sensor). Among them, the TIRS sensor contains two infrared bands. You can clearly see the band information by opening the data manager of the image, and it is clear at a glance. Quality is the quality assessment band.
4. Click OK, as shown in the figure below, select Radiance as the calibration type, and select BIL as the output format to prepare for the next atmospheric correction. Select float as the output type, then click "Apply FLAASH Settings" directly, and the scale factor will automatically match. Choose the output location, pay attention here,The path should not be too long, and Chinese characters should not appear in the path.Otherwise, it is easy to make mistakes.
Click OK, and you're done!
Note:After the radiometric calibration, there is no major visual change in the image, but the parameters have changed. Don’t be surprised.
image cropping
Why introduce image cropping here? Because we need to perform atmospheric correction after the radiometric calibration is completed, and the process of atmospheric correction is relatively long, so in order to save time, we can cut out the area we want to study first, and then perform atmospheric correction on the area, which can save a lot of time and do not make unnecessary sacrifices.
Then the question comes again, why not crop it first before radiation calibration? Because radiation calibration requires the header file of the image, and the header file is gone after cropping, and the header file is not required for atmospheric correction, so...
let's start!
Preparation: 1. Administrative boundary line of the research area 2. Landsat8 image
Note: As for how to obtain the vector file of the administrative boundary line, I have a very detailed introduction in the previous blog, and students who need it can take a look.Administrative boundary vector data download tutorial
1. Let's continue with the above image after the radiation calibration is completed. Open the right amount of files, files with shp suffix. Here we take Haidian District of Beijing as an example.
2. Load it in. After loading it in, search for "Subset Data from ROIs" in the toolbox on the right, as shown in the figure below, double-click to open it.
3. Select the image to be cropped. Note here that you must select the image after our radiation calibration, and don't make a mistake. Click OK.
4. Then enter the following interface, here is to select the ROI area, that is, the cropping area, because my appropriate amount of data is composed of two files, so there are two, select both. "Mask pixel outside of ROI?" Select yes for this place. After selecting yes, it means to cut according to your irregular boundary area. If it is no, it will cut according to the rectangular frame of ROI, and the cut out is a rectangle. The "Mask Background" background value is set to zero, meaning black.
5. OK, this is the effect picture after cutting out.
atmospheric correction
Reminder: The steps of atmospheric correction seem complicated here, but they are actually very simple, just fill in the information in any form. How to obtain the parameter information in the table is the key, so I hope you can read the way to obtain information in each step, nanny level.
Come on, let's continue, take the above results and find this thing.
2. Next, we just fill in this form, as shown in Figure 2.1, and then we can perform atmospheric correction.
Figure 2.1
Figure 2.2
(1) First of all, Input Radiance Image is to load the cropped image in, select the second use single... (as shown in the above picture) in the pop-up dialog box, and then select the output path for the following two Outputs.
(2) Sensor Type sensor type, select the type corresponding to your image.
(3) Flight Date time, year, month, day, minute, second, how to get this information? You can find it in the right-click layer-view metadata-time. (I believe everyone, there should be no need to put pictures here)
(4) The average altitude of Ground Elevation, how to obtain this data? There are two ways, 1. Direct Baidu, the average altitude of the area. 2. More professional means.(This method is placed in the appendix after the text due to the length of the article)
(5) How to choose the Atmospheric Model atmospheric model? Come on, give everyone a list, just choose according to this list. As shown in Figure 2.3.
Figure 2.3 Atmospheric model correspondence table
(6) aerosol model aerosol model, choose urban or rural according to the situation.
(7) Multispectral Settings multi-band settings (in the bottom line), after clicking on it, the following interface will appear, and you can select it according to the method shown in the figure. Select "GUI", click "Defaults", click to open and select the first one.
After finishing the work, click OK, then Apply, and the atmospheric correction will start.
appendix:
Let's introduce the above-mentioned, how to count the average elevation of the area.
1. File-open, find this file under the ENVI installation path. This is the global 900m DEM data that comes with ENVI.Look directly at the picture, this is the file.
2. Open this tool and make a statistics.
3. Select the file that comes with it, look at the picture, and click "Stats Subset".
4. Then click file, see the picture,
6. Select the area you are studying, click OK, and then follow the screenshot below.
7. This place, the statistics come out, Mean, is the average altitude, the unit here is meters. OK, so the average altitude of this place can be found out.
Note: When filling in the altitude, the unit is km. Be sure to check the unit before filling in. It is easy to make mistakes in this place.
At this point, radiation correction, atmospheric correction, irregular cropping, the content is all over