Sea surface temperature Landsat

Sea Surface Temperature with Landsat

By Koukoula Valsamina

1st year student of the Master’s degree ¨Electronics and processing of

Information¨

The first Landsat satellite, formerly known as the Earth Resources Technology Satellite (ERTS), was launched in 1972 to take images of the Earth's surface. The images have been used to analyze changes on the earth, such as deforestation, sea level rise, droughts, famines, destruction of natural habitats, and urbanization. Landsat-2, -3, -4, and -5 were developed and launched afterwards (Landsat-6 failed to reach orbit).

Landsat class data with the Landsat 7 acquisition strategy (LTAP) has been highly successful in satisfying multiple user needs

– GLCF has regular downloads of 25,000 + scenes per month.

How can Landsat images be useful for us?

The earth's environment is always changing. Famines, floods, deforestation, and urbanization are just a few of the negative examples. Since Landsat-7 takes images every 16 days, scientists can use the images to monitor, compare and study these changes in the environment. Although Landsat-7 will cover the entire globe every 16 days, it actually acquires only about 1/3 of the area at a time because of limited processing and storage capacity on the ground and thermal/power constraints on the spacecraft. Some example scenes that would be in high demand include science, agriculture or commerce applications or where change is likely occuring, such as the spring green-up. Landsat-7 also attempts to avoid cloudy areas. With these types of constraints, Landsat-7 will actually take about 3 months to acquire pictures of the entire earth.

• How does Landsat-7 work?

Landsat images are obtained through a process called remote sensing. Remote sensing is the process of gathering data from a distance. The images of the earth's surface are taken using a satellite sensor called the Enhanced Thematic Mapper (ETM+). Different objects emit different types of electromagnetic radiation. The TM sensor detects the electromagnetic energy from the objects and records this information in 7 different wavelengths in the form of an image.

No other system will match Landsat's combination of synoptic coverage, high spatial resolution, spectral range and radiometric calibration. In addition, the Landsat Program is committed to provide Landsat digital data to the user community in greater quantities, more quickly and at lower cost than at any previous time in the history of the program.

Why are Landsat-7 images interesting

Landsat-7 takes images of the earth every 16 days. The ETM+ will take pictures of the Earth 705 km (440 miles) out in space. Landsat-7 can operate in extremely cold temperature. Landsat-7 follows instruction from computers on Earth. Landsat-7 can operate for many years. Landsat-7 travels around the Earth in 99 minutes (5933.05 seconds to be more precise). Landsat-7 will provide affordable images (around $500 per image). Each ETM+ image covers 115 miles on a side.The pixel size from the ETM+ is 30 m (98.4 ft) for seven bands. However, there is a panchromatic band at a resolution of 15 m (49.2 ft).

Sea surface temperature

• Information concerning Sea Surface Temperature (SST) is needed in the assessment of remote sensing for fisheries application, for example: potential fishing zone, and site selection for marine culture (grouper, snapper, seaweed, and pearls). The information of SST is used to identify some marine phenomenon (e.g.: upwelling, front, and eddies) (Nontji, 1987), which are normally these areas are rich of nutrient. SST is also used as indicator of the environment required for the living of some marine biota.

• In the assessment of potential area for marine culture, some physical parameters with relatively high accuracy of coastal waters are needed (e.g.: SST, Total Suspended Matter (TSM), and Chlorophyll). This information, for example, can be derived from Landsat data which has 30 meters spatial resolution for visible, NIR (Near Infrared), SWIR (Short Wave Infrared) bands, and 60 meters spatial resolution for thermal bands. Therefore, it is need to do the assessment of SST from high spatial resolution, such as Landsat data.

Difficulties met when detecting SST

In addition to the effects of atmospheric transmission and upwelling, cloud contamination and the sea surface effect (SSE) may degrade or obscure a water surface thermal signal. Due to the infrequent temporal sampling of high-resolution sensors such as Landsat (16 days), completely cloud-free scenes are rare. Dense clouds are typically cooler than surface water temperatures, lowering the perceived temperature. However, the differentiation of clouds may become difficult when the temperature difference is less pronounced . Clouds can contaminate classification studies as these clustering techniques are more likely to group cloud coverage as a unique feature instead of differing water bodies.In addition, high cirrus clouds may significantly lower the perceived water temperature and create false values. Uncertain boundaries of thin cirrus clouds and cloud margins are often compensated by manual cloud-tagging, or thresholding and cloud-edge erosion, methods both time-consuming and subject to observational error.

Methodology TM landsat images have been used for the years 1990 and 2000 as shown in below figures

. Landsat sensor measures radiation in seven bands of the electromagnetic spectrum with spectral resolution of 30m except for band 6 which measures emitted thermal infra-red radiation and has a resolution of 120 m. The processing of these color images (two dates) has been carried out to to map the thermal distribution. ERDAS Imagine 8.6 software package is used to process and analyze the acquired images.

As surface water temperature is the basic parameter for the deviation of the thermal behavior of the environment, a thermal classification of both images has been conducted using the thermal infrared (10.4 to 12.5 um) band 6 that measures the amount of infrared radiant flux emitted from surfaces. The apparent temperature is a function of the emissivities and true or kinetic temperature of the surface. It is useful for locating geothermal activity, thermal inertia mapping for geologic investigations, vegetation classification, vegetation stress analysis, and soil moisture studies. The resulted temperature represents an “effective at-satellite temperature of the viewed Earth-atmosphere system under the assumption of unity emissivity”. The consideration of emissivity of the surface cover types would include the additional problem of mixed pixels in a 60 x 60 m 2 area. Otherwise the mixed signatures help to accept the assumption of unity emissivity for this kind of application. The thermal bands of the satellite images were transformed into surface temperature values.

The digital numbers were transformed into absolute radiance in the two landsat sensors (TM5 and TM7), using the following equation :

L = (Lmax - Lmin)/255 * DN + Lmin (1),

where L is the spectral radiance, Lmin and Lmax [mW cm-2 sr- 1 μm-1] are spectral radiances for each band at digital pixel numbers 0 and 255 respectively.Using this equation with the TM landsat 5 Lmin and Lmax the values 0.124 and 1.560 [mW cm-2 sr-1 μm-1] respectively.

On the other hand, the using of this equation with the TM landsat 7 the following reference values are given: ETM + Spectral Radiance Range:Low Gain: Lmin - 0.0 Lmax - 17.04 [mW cm-2 sr-1 μm-1]

High Gain: Lmin - 3.2 Lmax - 12.65 [mW cm-2 sr-1 μm-1]

The spectral radiances L lambda were converted into effective satellite temperatures T by

T= K2 / ln ( K1 / L +1) (2)

where K1, K2 are calibration constants. By applying this equation on the two types of TM sensors, we calculated the following;

For Landsat TM 5 the constants are K1= 60,776 and K2 = 1260,56 [mW cm-2 sr-1 μm-1]

For Landsat TM 7 the constants are K1= 666,09 and K2 = 1282.71 [mW cm-2 sr-1 μm-1]

ConclusionSeawater thermal mapping of the study area was generated by using the thermal band of the two landsat images. No atmospheric correction was carried out for this comparative study.In this way the term surface temperature is not correct but should describe the representation of

the thermal behaviour of surface cover types under these discussed conditions. The temperatures are also not directly comparable with air temperatures normally measured 2

meters above ground.