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Unit I: Ocean Color

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Unit I: Ocean Color Ocean Color Ocean Color Lesson II: Data Processing and Imagery Ocean color observations made in the world's oceans. In this from an Earth orbit allow an lesson we will discuss how the oceanographic viewpoint that is data collected by the satellite impossible from ship or shore -- a sensor are processed to obtain global picture of biological activity useful products for researchers. Ocean Color Viewed from Space: What Does the Satellite See? The satellite radiometer “sees” waters. The sensor has the light entering the sensor. In the capability of making case of ocean color measurements at several visible measurements, the satellite is and infra-red (IR) wavelengths, viewing some portion of the corresponding to different bands earth’s surface and measuring of electromagnetic energy. These the amount and color are called the spectral bands of (wavelength) of sunlight the sensor. The earliest ocean reflected from the earth’s color sensor had 6 bands, surface. As ocean color intensity SeaWiFS (Sea-viewing Wide is related to the amount of each Field-of-view Sensor) has 8 of the constituents in seawater, bands, or channels. Future data from the satellite sensor sensors are planned with many may therefore be used to more spectral bands, which will calculate the concentrations of allow scientists to determine more particulate and dissolved seawater constituents and to do materials in surface ocean so more accurately. What data do the satellite collect? Data received from an ocean Sensor (SeaWiFS), a global ocean color satellite like the Sea- color mission launched in August viewing Wide Field-of view 1997, is stored and transmitted in ã Project Oceanography 15 Spring Series 1999 -Ocean Color Ocean Color several forms. Global Area to collect this LAC data and use it Coverage (GAC) data are in their research. stored in the satellite's computers and transmitted to One other difference between receiving stations on earth GAC and LAC data is that they are during the period the satellite is at different resolutions. Each data in the dark. (Remember that point is taken in a particular area passive data can only be of the ocean. This area is called a collected during daylight pixel. LAC data pixels cover an because sunlight is required for area which is 1 km by 1 km in measurements.) Local Area size. GAC data pixels are larger, Coverage (LAC) data are and cover an area of 4 km by 4 transmitted in real-time (as soon km. Actual pixel sizes differ as it is collected) to other slightly, because the earth’s receiving stations on land where surface is curved, and because it is stored in land-based the satellite is sometimes looking computers. Scientists who down at the earth at an angle. We routinely work with the say that LAC data have higher SeaWiFS data have real-time resolution than GAC data. High Resolution Picture However, GAC data have lower Transmission (HRPT) receivers computer storage requirements. ã Project Oceanography 16 Spring Series 1999 -Ocean Color Ocean Color ã Project Oceanography 17 Spring Series 1999 -Ocean Color Ocean Color Data Processing from Level 0 to Level 3 At the time when LAC and GAC the ocean. A typical scene may data are transmitted and stored on be thousands of pixels on a side. computer, they consist of a string of numbers. Each number Using SEADAS, Level 0 data are represents a single reading of one converted to Level 1 data by of the sensors onboard the transforming the string of numbers satellite. For example, one into a data table, or matrix. The number corresponds to the data matrix has columns amount of red light reflected from corresponding to each pixel in a a single pixel. The raw data are scene and rows which correspond classified as Level 0. Fully to a scan line. There are 8 of processed scenes available for these data matrices for each you to view on the NASA websites scene--one for each color band of are Level 3 data. SeaWiFS. Each number, or element, in the matrix represents The raw, or unprocessed, Level 0 a raw radiance value. Other data data include total radiances for such as latitude and longitude for ocean, land, clouds, and ice each pixel, date, time, and all the measured by the satellite. It also other information required to includes actual SeaWiFS spectral calibrate and process the data, are responses, saturation responses, stored separately for each scan telemetry data, global positioning line. data and other data. The table on page 17 is a very Data processing is complicated, small part of an actual Level 1 but it is greatly facilitated by the data file. The raw radiance values use of SEADAS, a software only vary between 817-822 in this package developed by NASA portion of the scene. Level 1 data scientists for processing and can be plotted on a map using viewing ocean color data. All the SEADAS. Each radiance value in data for a Level 0 scene are the matrix can be displayed as a stored as a single file. Each shade of gray corresponding to scene consists of many pixels, the magnitude of the number. High which define a rectangular area of numbers are light, low numbers ã Project Oceanography 18 Spring Series 1999 -Ocean Color Ocean Color dark, and intermediate numbers correction factors are not well are some shade between these known, and can vary due to two. One map can be made for environmental each color of light measured, that factors. is for each radiance channel. The values on the map correspond to Level 2 images often contain black the intensity of radiance measured areas, where all the radiance was for each individual pixel. removed from pixels after atmospheric corrections. These On page 17 is an actual Level 1 are areas where clouds, ice or image taken over the southern tip land were present. The area and of Florida. The brightest areas are shape of individual pixels in Level clouds. Can you find Florida and 1 and Level 2 data may also be the Bahamas in this scene? Level distorted because of the angle at 1 data still largely consist of light which the satellite views the scattered by atmospheric earth's curved surface. Level 3 components, and this must be data processing by SEADAS removed to study ocean color. removes the distortions and also Level 2 processing applies the combines multiple scenes in a atmospheric correction algorithms, given area to reduce the cloud and also applies other algorithms cover. This is called "binning" the to convert the Level 1 data to data, and is similar to taking the concentrations of substances of average of several numbers. interest, for example chlorophyll Scenes can be binned over time concentration. Algorithm intervals, or over intervals of development is still an active area space. Spatial binning is used to of research, since many of the combine incomplete scenes of a particular area of interest. True Color and False Color Images Since white light is a combination 1 data can be added in SEADAS of different colors, we can add to get a true color image. SeaWiFS data collected in single bands to get a true color image of One other way to enhance images the ocean. There are 3 blue for easier interpretation is to make bands, 2 green bands, and a red false color images. This is similar band. One of each color of Level to grayscale images, except that ã Project Oceanography 19 Spring Series 1999 -Ocean Color Ocean Color several colors are used to scale they may look like we imagine the the data. How do we assign scene appears to the eye. False colors to the data? For color results when some scheme temperature, it might seem logical other than natural color is used for to let blue represent cold (low displaying images. temperatures), green intermediate, and red hot (high temperatures), The term "false color" does not but any scheme could be used. mean that the data are wrong or Usually color is chosen to make that the picture is deceiving you. It the image easy to interpret. A only means, don't be deceived; scale or legend by the figure this figure is not a color defines the meaning of the colors. photograph. You should look at Depending on the circumstances, the scale or legend to interpret it. the colors may look real; that is, Ground Truth Measurements and Instruments While the advent of ocean color scientists could not develop the satellites has revolutionized the algorithms they use to calculate way ocean scientists study the such parameters as chlorophyll ocean, it has not totally concentration. eliminated the need to study the ocean from ships. In fact, the A variety of instruments have been measurement of ocean color developed to collect the required parameters in and just above ground truth data. The basic the water is now more measurements needed are important than ever to be sure absorption and scatter of the main that data from the satellite are substances responsible for ocean interpreted correctly. Data color, namely phytoplankton, collected in the field to verify detritus particles, and dissolved and calibrate data from satellite organic substances; water-leaving ocean color sensors is called radiance; and incident solar "ground truth." Without ground irradiance (the amount of truth data, sunlight striking the sea surface). These properties change with ã Project Oceanography 20 Spring Series 1999 -Ocean Color Ocean Color seasons, with depth in the constantly trying to verify that water column, and from one chlorophyll concentrations area of the ocean to another. calculated from satellite sensors So, scientists must make their accurately represent chlorophyll measurements at different concentrations in the ocean.
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