Datum conversions are called transformations. They are not exact mathematical formulas but require localized estimates and fitting. Several different methods may be used, and some methods are unique to particular datum and cannot be used for others. Thus, it is not desirable to convert map layers back and forth repeatedly from datum to datum. Usually one picks a datum to be used for a particular project and sticks with it, converting layers as needed. The choice of datum is sometimes a matter of convenience. If most layers to be placed in a database already use NAD27, it is easier to stick with the older datum. Alternatively, an organization may decide that all layers must use the NAD83 datum as a standard policy, in which case nonmatching layers would require conversion. Most GPS and other data collection system can be set to work with a variety of datums.