Remote Sensing: Spectral Signature Analysis

Lab 8

Introduction

The purpose behind this lab is to gain experience on the measurement and interpretation of spectral reflectance (signatures) of various earth surface and near surface materials captured by satellite images. The steps behind this are as follows; collect spectral signatures from remotely sensed images, graph them, and perform analysis on them to verify whether they pass the spectral separability test. The completion of this lab will enable us remote sensing students to collect and properly analyze spectral signature curves for various earth surface and near surface features for any multispectral remotely sensed image.

Methods

The image that will be used for this lab is a Landsat ETM+ image that covers the Eau Claire area and a section of the Cities to collect and analyze spectral signatures of various earth surfaces. I measured and plotted the spectral reflectance of twelve materials and surfaces from the image eau_claire_2000.img. These twelve features are as follows;

Standing Water
Moving Water
Vegetation
Riparian Vegetation
Crops
Urban Grass
Dry Soil (uncultivated)
Moist Soil (uncultivated)
Rock
Asphalt Highway
Airport Runway
Concrete Surface (Parking Lot)

To begin the lab open up ERDAS and bring in the image eau_claire_2000.img. Zoom into the Eau Claire and Chippewa County area, then locate Lake Wissota. Click Home and then Drawing. Click on the Polygon tool. These steps are shown in the figure below.

Figure 1: This is a screenshot of the above processes of locating the Polygon tool.
Image created by: Cyril Wilson

Then proceed to digitize Lake Wissota. Next plot the spectral signature that was just collected. Click on Raster, Supervised, and then Signature Editor. After the Signature Editor is open click on Create New Signatures from AOI. This will add the signature with a default value labeled Class 1. Change the name of Class 1 to Standing Water. Then click on Display Mean Plot Window to display the spectral plot of the signature that was just collected from Lake Wissota. This process can be seen below in Figure 2.

Figure 2: This is a screenshot of the above processes from adding the signature and then displaying the spectral plot of the signature.
Image created by: Cyril Wilson

Figure 3: This is an example of what the signature plot will look like. This is the signature plat for the feature Standing Water.

Collect spectral signatures for features 2 through 12 above using the same procedure for standing water. Also change the background of the Signature Mean Plot to white.

Results

Q1: Band 1 (Blue) (0.45 – 0.52 µm) is the band that has the highest reflectance for standing water. Band 4 (Near IR) (0.76 – 0.90 µm) is the band that has the lowest reflectance for standing water.

Q2: The highest reflectance was seen with Band 1 (Blue) because water doesn’t do a good job of absorbing blue from the visible light spectrum. This blue visible light is then reflected back into the atmosphere, and this is why water appears blue to us.

The lowest reflectance for water was with the 4^th Band (Near IR). The reason this band has a lower reflectance is because water is able to effectively absorb the Near IR band. Since a lot of it is absorbed, there is less to be reflected out into the atmosphere.

Q3:

Sig 2

Highest reflectance = Band 1 (0.45 – 0.52 µm); Lowest reflectance = Band 6 (10.40 – 12.50 µm)

Sig 3

Highest reflectance = Band 4 (0.76 – 0.90 µm); Lowest reflectance = Band 6 (10.40 – 12.50 µm)

Sig 4

Highest reflectance = Band 5 (1.55 – 1.75 µm); Lowest reflectance = Band 3 (0.63 – 0.69 µm)

Sig 5

Highest reflectance = Band 5 (1.55 – 1.75 µm); Lowest reflectance = Band 2 (0.52 – 0.60 µm)

Sig 6