Andrew Coleman Dr. Lein Remote Sensing 15 November 2012 Thermal Mapping and Change Analysis Problem Definition The goal of this project is to map regions of thermal change in Santa Clara County, California over 20 years using two Landsat 5 satellite images; One from May 6, 1991 and another from May 13, 2011. Southern Santa Clara County is home to portions of the city of San Jose. Much of the warming depicted in this analysis can be traced back to urban sprawl derived from San Jose. Image Information Similar time of year is essential for mapping thermal change. Certain objects will emit different thermal characteristics during different seasons. Having a consistent sun angle is also very helpful since objects emit thermal energy they conduct from the sunâ€™s rays. Source: Landsat 5, Dates- 5/6/1991, 5/13/2011, Resolution: 30 Meters. Location: Santa Clara County, CA.
Thermal Imagery (band 6) of Southern Santa Clara County, California May 6, 1991
May 13, 2011
The warmest areas on this map are represented by warm colors (deep red, red, orange, and yellow) while cooler areas are mapped in cool colors (light & dark blues).
Coleman 2 Basic Statistics Logical statistics on thermal characteristics can be acquired by applying a series of mathematical expressions to the images. First converting the images’ brightness values to radiance, then to degrees Kelvin, and lastly to degrees Celsius. This can be obtained utilizing ENVI’s Band Math tool.
May 13, 2011
May 6, 1999
Minimum Change = + 11.706 ˚C, Mean Change = + 8.066 ˚C, Maximum Change = + 8.199 ˚C The rise in mean temperature was expected due to more urban land in 2011 vs. 1991. This can clearly be seen in the 2011 histogram. On the 2011 graph the ‘hottest’ peak is much more significant. More of the study area has been filled with hot urban spots. However, the drastic rise in minimum and maximum temperatures are questionable. There is more urban area but is there somewhere in this region that is actually 8 ˚C warmer on average than anywhere on the map in 1991? The minimum temperature is even more suspicious. 20 years alone should not account for a rise in minimum temperature from 3.430 ˚C to 15.136 ˚C (assuming some areas remained strictly rural for 20 years as the image suggests). Even when global warming is discussed it is
Coleman 3 usually in terms of a few, or couple degrees Celsius in over 50 years. I would have expected the change between minimum and maximum to be much less significant. What accounts for these changes? Significantly Contrasting Atmospheric Conditions? San Mateo Bridge 1991
San Mateo Bridge 2011
The imagery above are derived from the full Landsat 5 image north of the study area in the San Francisco Bay. The 2011 image on the right displays the San Mateo Bridge in a very crisp, defined nature while the San Mateo Bridge in 1991 seems to be hidden by some kind of haze or fog. This may suggest that even though the two images had 0% cloud cover other atmospheric and climatological conditions could have significantly affected results. In addition, the image in 2011 was taken a week later. The sun angle was slightly stronger. Maybe a clearer atmosphere and a slightly stronger sun accounted for the huge spike in maximum temperature? Other thoughts on the subject; Maybe a very large blacktop parking lot was very recently completed in a newly developed section of the city and it spiked the maximum temperature for a certain pixel in the raster. A fresh blacktop parking lot is going to conduct much more energy than an ageing grey one. Climatological aspects could also be different. Maybe 1991 was a much wetter winter and the
Coleman 4 ground heated up slower yielding a very low minimum temperature. Either way something is different which cannot be fully explained by a temporal change of 20 years. The minimum and maximum temperature changes along with the images of the San Mateo Bridge definitely raise some flags pertaining to the consistency of data when used for change analysis. Conclusions Utilizing band ratioing two final maps were composed. One showing regions of the study area which became warmer over the 20 year span and one which highlighted areas which became cooler. Some urbanized areas of San Jose seemingly cooled. Especially on the left central portion of the map. However the new areas of urban sprawl are very warm. Probably due to lack of fully mature vegetation or abundance of a certain type of construction material which conducts thermal energy very efficiently. Overall, the mean temperature contrast between 1991 and 2011 showed that the study area has certainly became warmer over the past 20 years. However the huge contrast between the maximum and minimum temperatures raised some questions about varying characteristics of the images such as sun angles, soil temperatures, haze, and construction materials. The final change analysis maps can be found on the next two pages.