Lab 1: Surface Temperature Extraction

Goal and Background:

This lab is designed to introduced the workflow of extracting land surface temperature information from thermal bands of satellite images and account for variations in land surface temperature over space. This lab will take satellite imagery and run it through two models in order to get surface temperature in radiance not in true temperature.  A later lab will introduce the workflow to produce true temperature of surface features.


Methods:

When thermal bands come straight from the source, they do not give surface temperature right away.  Some manipulation needs to be done to the imagery.  This is done by doing two different calculations to the images.  The first calculation is to get the spectral radiance readings at the sensors aperture. The equation is visible below.  It consists of multiplying the rescaled gain with the image band then adding on the rescaled offset.  The gain and offset are obtained by looking at the sensors metadata.  In order to get the Grescale or gain, a simple equation is used.  All this information can be gained from the image metadata.

Figure 1

This at aperture conversion equation can be seen in the image below (figure 2) for band 62 of a landsat ETM+ image of the Eau Claire Wisconsin area. The equation is from figure 1 with the proper values for grescale and brescale plugged in as well as the image bands. Once this is ran, another conversion needs to be done in order to get the blackbody surface temperature.

Figure 2

The following equation (figure 3) is the equation to convert the at satellite radiance to blackbody surface temperature.  Tb is at satellite temperature in kelvin, K1 and K2 are pre launch satellite constants that can be obtained online or from the metadata and the L value is the spectral radiance which is gained from the first conversion that was done. 

Figure 3

The following image (figure 4) is the above equation with the right numbers plugged in.

Figure 4

Results:

The resulting image is after running both conversions on the Landsat ETM+ Eau Claire image from the year 200.  This image was brought into ArcMap and symbolized.  From this image, surface temperature can be found by using the identify tool in ArcMap.  Note that these surface temperatures are in Kelvin.  A simple conversion needs to be done in order to get the temperature in fahrenheit or  celcius.  Now these temperatures are not the true kinetic surface temperatures.  To get true surface temperature, the material each object the earth is made of needs to be taken into consideration.  This method also assumes the unity of emissivity and uses pre-launch constants.

Figure 5

The final figure (figure 6) is a image of the surface temperature in Eau Claire and Chippewa county taken from the Landsat 8 Satellite.  This image was put through the same process as the other image but using the correct constants.

Figure 6

This method is a quick and easy way to extract surface temperature from thermal remote sensed data.  there is more steps needed to get true surface temperatures that will be highlighted in future labs.

Sources:

Cyril Wilson
Landsat