Mistakes in decoding or plotting are the most easily checked, requiring only a check of the original message. This should always be the first step in questioning inconsistent or suspect data. Plotting of off-time reports on the current chart is another common error. For example, a few 12Z reports may show up within a block of 18Z reports and be inadvertently plotted. Detect-ing this error may or may not be easy.
Reports containing observational and com-putational errors are almost impossible to correct.
However, when stations make consistent, obvious errors in weather element values, maintain a list of these stations, the elements in error, and the probable applicable corrections. Computational errors are found in sea level pressure reductions, dew-point temperatures, and true wind speed and direction reported by moving ships. The first two elements are generally taken directly from tables or graphs. Errors arise principally from misreading the tables or graphs. In the case of true wind, greater reliance should be placed on reports from ships known to carry well-trained observers, such as weather ships, naval vessels, and large commercial liners.