Pseudocolor : Sea Surface Temperature

Sea Surface Temperature from MODIS AQUA

The highest-quality sea surface temperature data is derived from the Moderate-resolution Imaging Spectroradiometer (MODIS) instruments aboard the NASA Terra and Aqua satellites. Data are currently available globally at 4 km, 36 km and 1 degree resolutions with daily, weekly and monthly time intervals and all are available via OPeNDAP. This plot shows how easy it is to instantly generate a pseudocolor with no configuration using the ODC.

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Step-By-Step

Go to the dataset list Expand NASA/JPL Physical Oceanography...(PODAAC) Expand Sea Surface Temperature Select 36 km Gridded Daily (Aqua) to the retrieve panel discussion: This is one of the datasets derived from the AQUA satellite. With a lot of the remotely sensed data there may little or no semantic information in the data description itself and the file names do not help much either. To get handle on the data, after you have done the basic directory retrieval try this technique: copy the initial part of the URL from the location bar at the top of the retrieval screen (in this case the URL starts with http://dods.jpl.nasa.gov and visit the web site. You will often find detailed information on the normal website and that is true of the PODAAC datasets such as MODIS AQUA. Drill down to 2004/001 and select the first file (MY36MDN1.sst...) Retrieve the structure for MY36MDN1.sst by double-clicking on the file name Select the main array by selecting Structure, Structure, SST Mean [all three radio buttons should be on] Click the red Output to (plotter) button [red indicates a network access will occur] [you will now automatically be moved to the View / Plot panel] [the data, about 2M, will download; when it completes the variable selector tab will populate] Click the blue Plot to (preview) button [the default settings should be fine for this plot] Without changing or specifying an values we have created a pseudocolor plot. In the published plot there are a couple of minor changes: the axes are indexed, the ajusted temperature values are shown and there is different text on the plot. Below is how to make these changes so your plot looks just like the one above: Modifying the Text: Go to the Plot Options tab Turn off "Generate Axis Captions" Add the new title, date and colorbar caption Note that a blocky sans-serif font has been selected and that the color of the text has been changed to charcoal (set saturation to 0 and brightness to around 60-80). The title has been layed out left-aligned to the plot area with a 10-pixel vertical offset and the date is right-aligned to the plot area with a 4-pixel vertical offset. If you need more information on managing text see special instructions on working with text. Indexing the Axes: Make sure pseudocolor is the plot type selected Go to the variable selector tab of the plot panel [to left (y-axis) and to the bottom (x-axis) of the axes you will see the axis mapping selectors] In each of the selector drop-down boxes select "[indexed]" Setting the Colorbar Bias: [The actual temperature values from the satellite are on a different scale than the standard of measurement (degrees celsius). We need to find out what this bias is and adjust for it. Usually you can see the linear bias values (slope and intercept) in the structure description on the retrieval pane if you click the "show descriptions" check box, but here a syntax error is preventing the DAS (description) information from being read. You can see the warning that this happened if you go to View / Text and click the "Show Errors" button. This warning also briefly appears in the status bar when it occurs. So we need to manually get the DAS.] Accessing a DAS Manually:Copy the file URL from the location bar up to but not including the question mark and append ".DAS" and paste it in a browser. The DAS text will be shown. At the bottom of this long file you will see the slope is 0.01 and the intercept is -300. Now we can proceed... Go to the plot options tab In the legend section there is a sub-panel entitled "Linear Bias" Turn off the check "No adjustment" Enter the slope and intercept determined as in the paragraph above Tip: Finding More Data The dataset list only contains a collection of sites and resources we have identified, but there are a lot of other directories not listed. When you are interested in big resource site like PODAAC go to their web site to find out what data they have and then look for the OPeNDAP URLs. You will often be able to find URLs that are not in our dataset list.

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OPeNDAP Data Connector Plot Gallery · Revised 25 January 2004 ·