Lab 1

Introduction to ENVI

Remote sensing for agricultural applications: principles and methods (2013-2014)

Instructor: Prof. Tao Cheng (tcheng@njau.edu.cn)

Nanjing Agricultural University

April 1st, 2014

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

ENVI versions

Latest version: ENVI 5.1 + IDL 8.3

ENVI 5.1

• 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.

• A significant change in the interface • Changes in ENVI and IDL programming

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

Test data: a Landsat Scene

• Collected by the OLI on Landsat-8

• Time of acquisition: 2014-03-16, 05:07:50

• 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 =

"LC81190382014075LGN00"

• Data download:

http://earthexplorer.usgs.gov/

• Data files

▫ 8 TIFF files and one txt file

• We only use the 7 multispectral bands.

A Landsat-8 false color composite (4-5-3)

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 0.43 - 0.45 0.433 30

Band 2 - Blue 0.45 - 0.51 0.482 30

Band 3 - Green 0.53 - 0.59 0.562 30

Band 4 - Red 0.64 - 0.67 0.655 30

Band 5 - Near Infrared (NIR)

0.85 - 0.88 0.865 30

Band 6 - SWIR 1 1.57 - 1.65 1.610 30

Band 7 - SWIR 2 2.11 - 2.29 2.200 30

Band 8 - Panchromatic 0.50 - 0.68 0.590 15

Band 9 - Cirrus 1.36 - 1.38 1.375 30

Band 10 - Thermal Infrared (TIRS) 1

10.60 - 11.19 10.800 100

Band 11 - Thermal Infrared (TIRS) 2

11.50 - 12.51 12.000 100

Landsat -8 band designations

What to do with this Landsat scene in

ENVI?

1. Image loading

2. Image displaying

3. Image stacking

4. Image cropping

5. Image calibration

• 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.

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?

Interactive

displays Image window

Scroll window

Scroll window

Zoom control

1. Be familiar with the display group. Navigate on your image and focus on your target area.

Image window

Scroll window

Scroll window

Zoom control

2. Double click in the Image window to display cursor info.

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

Image window

Scroll window

Scroll window

Zoom control

3. Right click in any window to bring up a box with commonly used functions.

Right click with your mouse:

How to locate the pixel for Ruigao (32.26598º N, 120.76102º E)?

Save data as ENVI

standard format

Load Landsat-8 bands 1~7 and stack them up to generate a multispectral image file.

Step 2

Step 1

Step 3

Step 4

Edit band

names

Assign meaningful band names.

Step 2

Step 1

Step 3

Step 4

• Find this file in your working directory.

• Check if the file size

matches the number you calculated in slide 8.

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).

The subset image is in display.

Conversion of DN to TOA (Top

of Atmosphere) radiance

• 𝐿𝜆 = TOA spectral radiance (W/m2*srad*um)

• 𝑀𝐿 = the band-specific multiplicative factor (RADIANCE_MULT_BAND_x)

• 𝐴𝐿 = the band-specific multiplicative factor (RADIANCE_ADD_BAND_x)

• DN = Landsat-8 level-1 product pixel values

𝐿 = 𝑀𝐿 × 𝐷𝑁 + 𝐴𝐿

Step 1 Step 2

Input band-specific

gain and offset values

• Gain = RADIANCE_MULT_BAND_x (in the *_MTL.txt file) • Offset = RADIANCE_ADD_BAND_x

Compare DN data to radiance data

Radiance data Unit: W/m2*srad*um

DN data Unit: dimensionless

Resources

• ENVI help documentation • http://www.exelisvis.com/Company/PressRoom/Blogs/ImagerySp

eaks.aspx - Imagery Speaks Blog (a good blog site) • http://www.exelisvis.com/Learn/EventsTraining/Webinars.aspx -

ENVI/IDL webinars

• http://www.exelisvis.com/ProductsServices/IDL.aspx - Exelis VIS, the developer of IDL.

• http://www.exelisvis.com/Company/PressRoom/Blogs/IDLDataPoint.aspx - IDL data point (a good Blog site)

• LearnIDL.com – Dr. Ken Bowman’s web site • http://www.idlcoyote.com/ - Coyote’s guide to IDL programming

(IDL tips, training, consulting) • http://gumley.com/ - Practical IDL programming (IDL book) • http://michaelgalloy.com/ - resources for IDL developers

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.