Several datum are commonly used in North America. The north American datum of 1927, often abbreviated NAD 1927 or even NAD27 , is based on the Clarke 1886 spheroid. The north American datum of 1983, or NAD 1983, is based on the GRS_1980 spheroid. These two datums include a network of surveyed points to help define the fit and provide accurately located positions. Another datum commonly used for satellite data is the world geodetic survey of 1984, or WGS 1984. Because NAD 1983 is relatively recent. One finds many maps that still use NAD27. However, the coordinates of a particular location can differ by up to several hundred meters between NAD27 and NAD83. Thus, one usually converts all the layers within a particular database into a common datum to avoid alignment problems.