Datum

A datum is a mathematical model that defines the size, shape, and orientation of the Earth (or a reference surface) used as the basis for calculating geographic coordinates. The datum provides the reference framework against which all positions are measured.

Types of datums

• NAD83 (North American Datum of 1983): Current standard for North America; used by BLM, USGS, and most US agencies
• WGS84 (World Geodetic System of 1984): Global datum used by GPS; practically identical to NAD83 for most purposes
• NAD27 (North American Datum of 1927): Historical datum based on the Clarke 1866 ellipsoid; superseded by NAD83

• NAVD88 (North American Vertical Datum of 1988): Current standard for elevations in North America
• NGVD29 (National Geodetic Vertical Datum of 1929): Historical vertical datum

Why datums matter: - Coordinates in different datums refer to different physical locations - The difference between NAD27 and NAD83 can be up to 100 meters (300+ feet) in some areas - Using the wrong datum can place features in the wrong location, causing boundary errors - GPS data (WGS84) and BLM data (NAD83) are close but not identical

Datum transformations: When combining data from different datums, transformation parameters are applied to convert coordinates from one datum to another. GIS software handles these transformations, but the user must specify the correct input and output datums.

Best practices: - Always document the datum for spatial data - Use NAD83 as the standard for US mining and land management data - Transform legacy NAD27 data to NAD83 before analysis - Verify datum when integrating data from multiple sources