Projection vs. Transformation

The details are complicated...but Mike Olkin's advice is simple: don't rely on Project-On-The-Fly...transform your data deliberately. Two observations here:

• In some/many cases an offset of a couple of feet won't be noticeable, for example when working with uncorrected GPS data.
• Other times, for example when working with geoprocessing tools or Select by Location, any offset between two data sets can be fatal.