All my data are in the NAD 27 or NGVD 29 datum. Do I have to purchase new data or is software available to convert old data to new data?
Horizontal coordinates for most points in NAD 27 exist in NAD 83 and may be obtained relatively inexpensively from NGS via the World Wide Web or NGS’ Information Services Branch. If the coordinate data are not held by NGS, two possibilities exist. First, the original field observations can be used to compute new coordinates via least squares adjustment software using NAD 83 control coordinates. If this is not possible, NAD 27 coordinates can be converted to NAD 83 coordinates using available software from NGS called NADCON (v.2.1). There is usually a loss of accuracy using this process, but it often is sufficiently accurate for mapping purposes. The accuracy for the conversion is estimated to be 10-15 cm RMS (one sigma) at the data points used to derive the model, with occasional outliers approaching 50 cm. Original field observations are unaffected by the change in datums. See the directory of PC Software on the NGS Web Site. Elevations for many points in the NGVD 29 exist in the NAVD
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