Digitizing chart recorder data: Coordinate system conversion for rain gauges and similar recording instruments

Though chart recorder data are widely available for many environmental parameters over long periods of record, the nonorthogonal coordinate systems of many types of recorder charts complicates data extraction such that it is almost always manual; a tedious chore involving hand-counting tiny red boxes. Perhaps the most common data available on these types of charts are precipitation from weighing-type recording rain gauges. Unfortunately, because most digitizer software assumes the data being digitized have an orthogonal coordinate system, the nonorthogonality inherent in these charts prevents simple digitization of the data. If methodologies exist for electronically extracting rain gauge data, they are not widely known; recent texts still outline manual break-point tabulation in detail with no reference to any software or published methodologies for electronic data extraction. A simple procedure is presented here to transform digitized chart recorder data into meaningful, accurate data using readily available software. A comparison of manually read precipitation data and precipitation data extracted with a digitizer showed average differences of less than half the smallest marked increment on the charts for both precipitation amount and time. Differences were almost exclusively due to human bias in locating break points rather than true error.