Lab 8: Analysis of Spectral Reflectance Signatures for Earth Surfaces and Near Surface Materials

Goals and Background:

The purpose of this lab was to develop skills in measuring and interpreting spectral reflectance signatures of Earth surfaces and near surface materials. Spectral signatures were collected from satellite images, graphed, and analyzed. This lab will prepare me to collect and analyze spectral signatures for any Earth surface and near surface feature from multispectral remotely sensed images.

Methods:

The Landsat ETM+ image eau_claire2000.img (United States Geological Survey, 2015) was used to collect and analyze spectral signatures of near surface features and various Earth surfaces.The image covered Eau Claire and parts of Wisconsin and Minnesota. Spectral signatures of twelve materials and surfaces were collected in the lab as follows:

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)

Spectral signatures were collected in Erdas Imagine 2013 using the spectral tools. The collection process first started by using the Polygon tool under the Drawing heading to create a polygon around the desired material/surface. The spectral signature of the polygon was then plotted using the Signature Editor tool. The plot was displayed using the Display Mean Plot Window tool.

Spectral signatures for each individual material/surface can be seen in Figure 1-12. Descriptions of the results are discussed in Table 1. Differences between dry and moist soil spectral signatures are seen in Figure 13. All spectral signatures are plotted on one plot in Figure 14.

Results:

Signature	Band with highest reflectance	Band with lowest spectral reflectance
1	Blue (0.45-0.5um)	Near and mid infrared bands (0.7-3.0um).
2	Blue (0.45-0.5um)	Mid infrared (1.3-3.0um)
3	Red and near infrared (0.62-1.3um)	Green (0.5-0.58um)
4	Red and near infrared (0.62-1.3um)	Green (0.5-0.58um)
5	Red and near infrared (0.62-1.3um)	Green (0.5-0.58um)
6	Blue (0.45-0.5um)	Red and near infrared (0.62-1.3um)
7	Near and mid infrared (0.72-3.0um)	Red and near infrared (0.62-1.3um)
8	Near and mid infrared (0.72-3.0um)	Red and near infrared (0.62-1.3um)
9	Near and mid infrared (0.72-3.0um)	Green (0.5-0.58um)
10	Near and mid infrared (0.72-3.0um)	Red and near infrared (0.62-1.3um)
11	Near and mid infrared (0.72-3.0um)	Red and near infrared (0.62-1.3um)
12	Blue (0.45-0.5um)	Red and near infrared (0.62-1.3um)

Table 1

Figure 1: Mean plot for standing water.

Standing water had the highest reflectance in the blue band because of Rayleigh scattering. Low reflectance in the near and mid infrared bands was due to water absorbing radiation. 

Figure 2: Mean plot for moving water. 

Figure 3: Mean plot for vegetation.

Vegetation had the highest reflectance in the red and near infrared bands (0.62-1.3um) because the plants block out harmful radiation present within those bands. The lowest reflectance was in the green band (0.5-0.58um) because plants absorb the radiation for photosynthesis.

Figure 4: Mean plot for riparian vegetation. 

Figure 5: Mean plot for crops.

Figure 6: Mean plot for urban grass.

Figure 7: Mean plot for dry soil (uncultivated).

Figure 8: Mean plot for moist soil (uncultivated).

Figure 9: Mean plot for rock.

Figure 10: Mean plot for asphalt highway.

Figure 11: Mean plot for airport runway.

Figure 12: Mean plot for concrete surface (parking lot).

Figure 13: Mean plot shows differences between dry and moist soil.

Dry and moist soil varies the most in the mid infrared band (1.3-3.0um) because water in the moist soil absorbs more radiation.

Figure 14: Mean plot showing spectral signatures for all surfaces.

Spectral signatures for vegetation, riparian vegetation, and crops were very similar. This is because they are all types of vegetation that reflect radiation in very similar ways. Additionally, asphalt and airport runway spectral signatures were very similar. This is because airport runways are commonly made out of asphalt. The spectral signature for the rock was very different from other hard surfaces like asphalt, airport runway, and concrete surface. This is because asphalt, airport runways, and concrete surfaces have been made by humans, which alters their natural reflective properties that rock displays. Urban grass had a very different spectral signature compared to vegetation, riparian vegetation, and crops. This is because urban grass has less moisture content, which causes it to have higher reflectance in the blue and green bands.

Sources:

United States Geological Survey. (2015). [Satellite image is in img. format]. Earth Resources Observation and Science Center. Retrieved from http://eros.usgs.gov/