Lab 1 Introduction to ENVI

Transcription

1 Remote sensing for agricultural applications: principles and methods ( ) Instructor: Prof. Tao Cheng Nanjing Agricultural University Lab 1 Introduction to ENVI April 1 st, 2014

2 Why ENVI? One of the best remote sensing software packages in the world Very popular in academic institutions Famous for hyperspectral analysis Developed using the programming language IDL and therefore easy to be extended with IDL

3 ENVI versions Latest version: ENVI IDL 8.3 ENVI 5.1 A significant change in the interface Changes in ENVI and IDL programming ENVI 5.1 Classic has the ENVI heritage interface. If you are a long time ENVI user since ENVI 4.0, you may want to use the Classic one. Run your old IDL scripts in ENVI Classic.

4 ENVI Start( 开始 ) -> Programs( 程序 )-> ENVI 5.1 -> Tools -> ENVI Classic + IDL After ENVI Classic is started, you should see a menu as below: Interactive Displays File management Basic Tools Classification Tools Transform Tools Filter Tools Spectral Tools Map Tools Vector Tools Topographic Tools Radar Tools

5 Test data: a Landsat Scene Collected by the OLI on Landsat-8 Time of acquisition: , 05:07:50 A Landsat-8 false color composite (4-5-3) Data_type = L1T (Landsat-8 Level 1 Terrain corrected product) Satellite orbit location at acquisition: path=119, row=238 File type: each band is in a separate GeoTIFF file Data format: 16-bit unsigned integer LANDSAT_SCENE_ID = "LC LGN00" Data download: Data files 8 TIFF files and one txt file We only use the 7 multispectral bands.

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7 Landsat -8 band designations Landsat 8 Operational Land Imager (OLI) and Thermal Infrared Sensor (TIRS) Launched February 11, 2013 Bands Wavelength (µm) Band center (µm) Resolution (m) Band 1 - Coastal Band 2 - Blue Band 3 - Green Band 4 - Red Band 5 - Near Infrared (NIR) Band 6 - SWIR Band 7 - SWIR Band 8 - Panchromatic Band 9 - Cirrus Band 10 - Thermal Infrared (TIRS) 1 Band 11 - Thermal Infrared (TIRS)

8 What to do with this Landsat scene in ENVI? 1. Image loading 2. Image displaying 3. Image stacking 4. Image cropping 5. Image calibration

9 All the file and sensor types that supported by ENVI 5.1 classic. These files can be directly opened in ENVI. We are going to open Landsat-8 image files that are downloaded in GeoTIFF format. Tips: to select multiple files, hold the Ctrl key and click.

10 Load your data Once your files are loaded into ENVI, you should see this window (Available Bands List). Display mode Gray Scale: individual bands RGB Color: band combinations (e.g. R-NIR-G) Try making different color composites Data info: Dims 7791 x 7941: # of columns x # of rows Unsigned Int: data type = Unsigned integer (16 bit) (data range?) BSQ: data storage method What is the file size of a 7-band Landsat-8 image?

11 Interactive displays Image window 1. Be familiar with the display group. Navigate on your image and focus on your target area. Scroll window Scroll window Zoom control

12 2. Double click in the Image window to display cursor info. Image window How to read these values: Disp #: (col #, row #) Scrn: screen values (0~255) Projection: coordinate system Map: coordinates in m LL: coordinates in Lat/Lon Data: DN for the RGB bands Scroll window Zoom control Scroll window

13 3. Right click in any window to bring up a box with commonly used functions. Image window Right click with your mouse: How to locate the pixel for Ruigao ( ºN, ºE)? Scroll window Scroll window Zoom control

14 Save data as ENVI standard format Step 1 Step 2 Load Landsat-8 bands 1~7 and stack them up to generate a multispectral image file. Step 3 Step 4

15 Edit band names Step 2 Step 4 Step 1 Assign meaningful band names. Find this file in your working directory. Check if the file size matches the number you calculated in slide 8. Step 3

16 Image cropping: create a subset image Step 1: draw a rectangle ROI in the Scroll window to include your study area (hold left key and drag, right click to select).

17 The subset image is in display.

18 Conversion of DN to TOA (Top of Atmosphere) radiance L = M L DN + A L L λ = TOA spectral radiance (W/m2*srad*um) M L = the band-specific multiplicative factor (RADIANCE_MULT_BAND_x) A L = the band-specific multiplicative factor (RADIANCE_ADD_BAND_x) DN = Landsat-8 level-1 product pixel values Step 1 Step 2

19 Input band-specific gain and offset values Gain = RADIANCE_MULT_BAND_x (in the *_MTL.txt file) Offset = RADIANCE_ADD_BAND_x

20 Compare DN data to radiance data Radiance data Unit: W/m2*srad*um DN data Unit: dimensionless

21 Resources ENVI help documentation eaks.aspx - Imagery Speaks Blog (a good blog site) - ENVI/IDL webinars - Exelis VIS, the developer of IDL. int.aspx - IDL data point (a good Blog site) LearnIDL.com Dr. Ken Bowman s web site - Coyote s guide to IDL programming (IDL tips, training, consulting) - Practical IDL programming (IDL book) - resources for IDL developers

22 Class project Think about what to do in your project. Make a tem of two students for each project. Start developing ideas and collecting materials (making notes is necessary) for your project. Allocate reasonable workload.