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On Fri, 22 Apr 2005 19:55:22 GMT, Nathan Young
wrote: It is not as trivial as it should be. METAR decoder software is difficult to write because of the special weather statements that can be included in a METAR entry (things like RVR, multiple precip types, etc). Simple parsers can grab winds, date, time, and cloud conditions. But to be all-encompassing requires a bit more. I found a package (via NOAA?) that would do METAR decodes, and it included approximately 30KB of source code, which seemed like a lot for the extra bit of functionality it provided. 30K just for the parser, or did that include station name lookup tables? Seems like the code should just do one pass through a relatively straightforward nest of if/thens/elseif. Load the METAR into an array since each element is space-delineated, the first two elements are going to be the station name and time of issue (assuming no SPECI). Then the rest of the code would be if/then for the multiple elements. So, 19015G32KT 1SM -RA BR BKN010 BKN035 OVC050 would be IF (windtype) THEN (printwind) ELSIF (visibility) THEN (printvis) ELSEIF (preciptype) THEN (printprecip) ELSIF (cloudtype) THEN (printcloudtype) ELSIF (temp) THEN (printtemp) (blah blah blah) - rinse/recycle/repeat the analysis for all non-null elements in the array.. I think the problem with most of the decoders I've seen is that they expect METAR elements in fixed positions and only code for the base case and don't do any processing. Old weather injectors for MS Flight Sim are good examples of this. Get an RVR in there and they'd have cloud base numbers for the temp and all sorts of nasty errors. |
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![]() "Peter Clark" wrote in message ... On Fri, 22 Apr 2005 19:55:22 GMT, Nathan Young wrote: It is not as trivial as it should be. METAR decoder software is difficult to write because of the special weather statements that can be included in a METAR entry (things like RVR, multiple precip types, etc). Simple parsers can grab winds, date, time, and cloud conditions. But to be all-encompassing requires a bit more. I found a package (via NOAA?) that would do METAR decodes, and it included approximately 30KB of source code, which seemed like a lot for the extra bit of functionality it provided. 30K just for the parser, or did that include station name lookup tables? Like most computer code, 20 percent of it probably does the real work.... and the 80 percent is there to make sure that it exits elegantly and does not run into an infinite loop, crash-to-operating-system, or report ridiculous value... whenever it is fed garbage input or exceptional conditions. |
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