Lab 8: Spectral Signature Analysis

Goal and Background:
The purpose of this lab is to gain experience collecting and analyzing spectral signatures. In order to do this, 12 spectral signatures of different surface features were collected by creating AOIs with the polygon drawing tool in Erdas and then graphing their curves.

Methods:
There was one part of this lab where I was given 12 different features and I needed to obtain the spectral signatures of those features through the use of a Landsat EMT+ image. The 12 features include:

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)

In order to find the spectral signatures, I had to figure out how to interpret each feature and then zoom into those features throughout the image. I used Google Earth to help distinguish some of the features such as soil and rock. After I used the polygon tool to make AOIs, I used the supervise tool to select the signature editor. I then evaluated the values in a table and then graphed them based on their band signatures individually and then all of them together to observe trends and differences.

Results:

Fig. 1: I first observed the graph for standing water of Lake Wissota to see the signature reflectance.

Fig. 2: I then observed the differences between wet and dry soil based on reflectance of different bands/wavelengths.

Fig. 3: Next I looked at all of the spectral bands together to observe similarities and differences between the various features.

The following table denotes both the bands of highest and lowest reflectance for each feature I observed:

                                                        Highest                 Lowest

Moving Water:                                  Band 1                  Band 4 or 6
Vegetation:                                        Band 4                  Band 3 or 6
Riparian Vegetation:                        Band 4                  Band 6
Crops:                                                 Band 4                  Band 6
Urban Grass:                                     Band 4                  Band 3 or 6
Dry Soil (Uncultivated):                   Band 5                  Band 3
Wet Soil (Uncultivated):                  Band 5                  Band 6
Rock:                                                   Band 5                  Band 4
Asphalt Highway:                              Band 5                  Band 4 or 6
Airport Runway:                                Band 5                  Band 4
Concrete Surface (Parking Lot):      Band 1                  Band 4

Band wavelengths (micrometers)

Band 1 (Blue): 0.45-0.52
Band 2 (Green): 0.52-0.60
Band 3 (Red): 0.63-0.69
Band 4 (NIR): 0.77-0.90
Band 5 (Short-wave Infrared): 1.55-1.75
Band 6 (Thermal Infrared): 10.40-12.50

The spectral signatures for both standing and moving water are significantly different because many of the other features reflect infrared in one way or another but water tends to absorb infrared radiation. The spectral difference for the runway is also quite different, I expected it to kind of be similar to the concrete parking lot signature though which is definitely not the case. The runway reflects a large amount of radiation in all bands because it is a white flat surface, which is true of concrete so I am curious why the concrete surface did not have a similar signature. All of the plant features more or less follow the same trend in that they reflect a large amount of NIR and absorb quite a bit of red light. I did find it interesting that the runway reflected such large amounts of radiation in all of the bands except for NIR where it had a large dip.


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