Why did NGS change from NGVD 29 to NAVD 88?
NAVD 88 was computed for many of the same reasons as NAD 83. About 625,000 km of leveling had been added to the NGVD since 1929. Thousands of bench marks had been subsequently destroyed and many others had been affected by crustal motion, postglacial rebound, and subsidence due to the withdrawal of underground fluids. Distortions amounting to as much as 9 meters had been seen due to forcing the new leveling to fit the NGVD 29 height values. (“Results of the General Adjustment of the North American Datum of 1988,” Surveying and Land Information Systems Vol. 52, No. 3, 1992 pp. 133-149) • 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 f
