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Presenting Interactive Satellite Image Loops Using Java, Idl, Perl, C, and Fortran


During more than a decade of developing meteorological workstations, the Forecast Systems Laboratory (FSL) has gained considerable knowledge about the effectiveness of various display techniques for operational forecasters. Regarding the display of satellite image loops, FSL has determined that forecasters desire the ability to control all of the following: * the kind of loop: forward, backward, or swing (forward + backward) * looping speed o dwell on the first and/or last frame * manual stepping through images They also desire to see the most recent image as soon as possible, to have loops in which the image doesn't jump around due to navigation problems, and to see satellite data on the same projection as they see other meteorological data. To achieve this kind of flexibility we use a Java applet that allows users to control manipulation of images on their own computers. The applet (accessible at communicates with a cgi script (written in perl) on the server that tells what images are available to download. The applet interrogates the server at one of several pre-determined time intervals, and automatically downloads new images as they become available. The applet was written so that it can be adapted to display any set of images in either jpeg or gif format. It has been used successfully by a weather hobbyist to display GOES data downloaded through his own WEFAX/APT groundstation (see Though Java has provided good flexibility and speed adequate to allow looping of images at 10 frames/sec on a 166 MHz pc, we have encountered several problems in implementation that will be discussed in our talk. These include difficulty with memory-management, multi-threading, and a few still-obscure bugs that seem to be browser-dependent. We will also discuss how the FSL satellite images displayed by the applet are generated: GOES satellite data ingested by FSL's ground station are remapped to an invariant Lambert Conformal Conic projection. The navigation is particularly important because it eliminates substantial image jerking for loops of high-resolution images on the national scale or smaller. (The ingest software is written in C, and the navigation software is a combination of FORTRAN and C.) The data are then subsampled to reduce the size of the resulting image, color-coded, and a map of political boundaries is superimposed in a way that does not obscure the data. The result is put out as a jpeg image. We find that the jpeg format produces considerably smaller files than the gif images we previously used, with no noticeable degradation in quality. (This software is written in the IDL graphics language.)

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December 05, 1997

This publication was presented at the following:

Symp. on Water Resources and the World Wide Web


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