Best practices to determine what coordinate system to use when data is in 2 state plane Coordinate System
It's a good question and there probably isn't one "correct" answer. I lived in Seattle, WA when I got my graduate degree in GIS, so a lot of the data we worked with was from Seattle and King County which uses Washington State Plane North. However, I worked in Tacoma at the time, and when I used Tacoma/Pierce County data, the standard was to use Washington State Plane South. Perhaps this is a "classic" example as it is included in this wikipedia topic: https://en.wikipedia.org/wiki/State_Plane_Coordinate_System
Often an administrative agency (city, county, etc.) will have established a standard coordinate system for their data. So depending on whether your GIS work is being done as a deliverable to an agency or organization that has an established standard, you would want to ensure you're working with their standard (e.g. if you were delivering work to the City of Tacoma, you would use WA State Plane South, but for Seattle you would deliver it in State Plane North).
If that isn't the case, and you have determined that you need to use a State Plane coordinate system, then it will be more accurate to use the coordinate system that covers the largest extent of your data.
Hope this helps!
