I need to measure the distance (with the ruler) of points from two different layers. However, one layer is NAD83 and the other is NAD27. I also have a standard ESRI basemap downloaded. The issue I have is that while ooints on the map all look fine, I don't know if they are messed up by the differing datum. I also don't know if I accurately converted from NAD27 to NAD83. When I uploaded my csv files an automatic message showed up, indicating that they would be converted to the NAD83. However, I admit to having no idea what the difference is between 'defining a projection' and 'projecting data.'
For Layer Properties -> Source , the following appears:
Geographic Coordinate System: GCS_North_American_1983 Datum: D_North_American_1983