Here is what the ArcView help files say about datums: A datum is a set of parameters defining a coordinate system, and a set of control points whose geometric relationships are known, either through measurement or calculation. One part of defining the coordinate system is the spheroid used to approximate the shape of the earth. A spheroid is defined by a radius and an eccentricity. These two constants are used as inputs to the equations which calculate a projected coordinate from a coordinate in decimal degrees. When a projection is created, it is associated with a default spheroid so that these constants will be available. This default spheroid varies from projection to projection, but is usually the SPHERE for small-scale projections and CLARKE 1866 for large-scale projections. Certain predefined projections (for example, those stored in default.prj in ArcView's etc directory which are displayed as "standard" projections in Projection Properties), are associated with specific spheroids based on their use. For instance, the "State Plane - 1927" projections are associated with the CLARKE 1866 spheroid (with the exception of Michigan), the "State Plane - 1983" projections are associated with the GRS 80 spheroid, the "New Zealand National Grid" projection is associated with the INTERNATIONAL 1909 (also known as the INTERNATIONAL 1924) spheroid, the "Great Britain National Grid" projection is associated with the AIRY spheroid, and the National Grids of Malaysia, Singapore, and Brunei are associated with the EVEREST spheroid. When a projection associated with a certain spheroid is used, ArcView assumes that the decimal degrees data being projected was collected in a datum based on that spheroid. So while ArcView has no knowledge of datums per se, it does know about spheroids, which are part of the definition of a datum. Therefore you do need to know what datum your data is in, and set the spheroid of the projection accordingly. This can be done either through the Projection Properties dialog or by using Avenue.   Projections