Answers to Questions:

1.Q1. The DFW metropolitan area is most distinct in the channel 1 image. Other features include bodies of water and clouds.

1.Q2. The water bodies are better defined in the channel 2 image. The channel 2 image alone is not sufficient to distinguish bare soil from vegetation. As shown in Figure 1.3, the reflectance values for vegetation and bare soil are nearly the same over the range of channel 2. While contrast in reflectance between bare soil and vegetation decreases for these surfaces from channel 1 to 2, the contrast between water surfaces and either vegetation or bare soil increases from channel 1 to 2 of the AVHRR.

1.Q3. The dominant colors in the false color image are deep red and gray. The gray area appears to include the urban section of the DFW region. However, it also appears to include areas due north of Ft. Worth, and so urban area s are not uniquely defined by gray. By inference from Figure 1.3, a lack of red (the presence of which corresponds to high reflectance in channel 2) could be described not only by urban surfaces, but also by surfaces that are neither dominated by bare soil or vegetation. Such surfaces are typical in agricultural areas, which often contain a mix of bare soil and sparce or short vegetation.

1.Q4. From the generalized reflectance values in Figure 1.3 the following NDVI calculations can be made:

NDVI =(chan2-chan1)/(chan2+chan1), 

where chan2 and chan1 are channel 2 and channel1
generalized reflectance values respectively. 

i.) Lakes (water) NDVI=(0-5)/(0+5)= -1.

ii.) Bare soil NDVI=(30-20)/(30+20)= 0.20

iii.) Dense vegetation NDVI=(35-10)/ (35+10)= 0.56

The comparison between calculated NDVI values (above) and the intensity of brightness in the NDVI image yields a closely-correlated result. Lakes are very dark, vegetated areas near the lakes are very bright, and other areas vary (including the urban area, which is a dark shade of gray).

1.Q5. The DFW urban area is better defined in the NDVI image. The ambiguity between the urban area and non-urban area to the north of Ft. Worth in the false color image is not present in the NDVI image. This confirms that the integrated values of channel 2 over this ambiguous area were lower probably due to a mix in vegetation and bare soil. Such a 'mixed' pixel has lower reflectance values for both vegetation and bare soil than would be observed if either surface dominated.

Table 1.2. NDVI, T4sfc, and the ratio of T4sfc/NDVI computed for the pixels associated with the weather observation stations from the 10 July 1991 and 5 July - 1 August 1991 images.

1.Q6. The NDVI composite image shows more homogeneous groupings of values throughout the image in comparison to the single-scene image. Additionally, the scattered clouds that appear in the southeast portion of the single date NDVI image have been removed in the composite. The T4sfc composite is slightly more improved than its single-scene counterpart, particularly in the discernment of the DFW urban area. NDVI and T4sfc are both influenced by day-to-day variations in solar illumination, however, the influence on T4sfc is greater.

1.Q7. The single-scene AVHRR data shows a slightly curvilinear relationship between T4sfc/NDVI and NDVI, while the composite image shows a nearly linear relationship. Since the composited data selects the maximum values of NDVI, and often T4sfc, over a period of several weeks, it is more likely that a physically limiting value of both variables will be attained. Additionally, the composite process minimizes the influence of atmospheric attenuation on the NDVI. Given these conditions, a linear relationship between T4sfc/NDVI and NDVI appears to prevail.

1.Q8. Low T4sfc and high NDVI values likely correspond to vegetated surroundings, while high T4sfc and low NDVI values likely correspond to urban or hot, dry arid surfaces.

1.Q9. Station #285 appears to be urban, while station #282 appears to be rural. The other five stations exhibit a response in NDVI and T4sfc that confirm them as neither urban or rural.

2.Q1. Calculation of average percentage urbanization: Station #282 = 0% and station #285=44.5%. Higher T4sfc/NDVI and lower NDVI values are well-correlated to higher percentages of urban land cover.

2.Q2. Station #282 is rural, station #285 is urban, and the other stations appear to be related to the agricultural land class.

2.Q3. Yes, it would be more difficult to discern urban land cover in an arid region. Hot, dry bare soil conditions result in low values of NDVI and high values of T4sfc. Given low values of NDVI and elevated values of radiant surface temperature, the contrast with urban surfaces would be minimized and thus, the distinction between urban and non-urban locations would be difficult to make.

3.Q1. Yes, several bright areas that could be associated with urban areas exist outside of the massive bright area associated with the DFW region.

Table 3.2. OLS values computed for the pixels associated with the weather observation stations.

3.Q2. Stations 285, 281, and 279 appear to be at more urban locales than rural. Previously only station 285 was considered as located in an urban locale. These three stations were also identified as urban through a manual procedure described in Gallo et al. (1993a).