Remote Sensing Lab #8: Spectral Signature Analysis

Goals and Background

The main objective of this lab is to understand how to obtain and interpret spectral reflectance signatures from various Earth surfaces captured by satellite images.  To perform the objective I will be using Erdas Imagine 2015 to collect, graph, and analyze spectral signatures to determine if they pass the spectral separability test.

Methods

I was provided a satellite image from the year 2000 of the Eau Claire and Chippewa Falls area in Wisconsin.  I was instructed to collect spectral signatures from the following list of Earth and near surface materials.

1.  Standing Water
2.  Moving Water
3.  Vegetation
4.  Riparian Vegetation
5.  Crops
6.  Urban Grass
7.  Dry Soil (uncultivated)
8.  Moist Soil (uncultivated)
9.  Rock
10.  Asphalt Highway
11.  Airport Runway
12.  Concrete Surface (Parking lot)

To obtain the spectral signatures in Erdas I will be utilizing the Polygon tool under the Drawing tab (Fig.1).  After drawing an outline of the area of interest (AOI) to collect the spectral signature from, I opened the Signature Editor from the Supervised menu in the Raster tab of Erdas.  With the Signature Editor open I used the Create New Signature from AOI and Display Mean Plot Window to add the signature from the polygon to the window and display the graph of the spectral signature (Fig. 2).

(Fig. 1) Polygon tool (highlighted in yellow below Manage Data) found in the Drawing tab.

(Fig. 2) AOI outline (center,with blue nodes surrounding), Signature Editor (left) and the Signature Mean Plot graph (right)

I proceeded to repeat this process for the entire list I was given.  In the Signature Editor I was able to change the Signature Name (label) for each of the spectral signatures, which transferred to the graph as well.  The majority of the surface materials were easy to identify from the given image.  Utilizing the Google Earth Link/Sync feature with in Erdas helped me to identify surface features which were not easily identifiable.

Analyzing the spectral signatures is the next step.  Selecting two of the signature at the same time in the editor window and then selecting Multipule Signature Mode on the plot window allows you to view more than one signature (Fig. 3).

(Fig. 3) Two signatures selected and displayed in the plot graph.

Results


Analyzing the graph of reflectance from (Fig. 3) we are able to see the variation between standing water and moving water.  The variation displayed between the two water surfaces is explained through Specular and Diffuse reflection.  The movement and ripples in the moving water give it diffuse reflection which send the reflections in all directions and reduces the intensity.  Where as the standing water is smooth has a more specular reflection, which sends the reflectance back with a higher intensity.  Multiple variations of this type of analysis can be done from the spectral information gathered in this manner.  

(Fig. 4) Final graph will all 12 surface spectral signatures plotted.

Sources

Satellite image is from Earth Resources Observation and Science Center, United States
Geological Survey.