Monday, April 30, 2012

Am I projecting or transforming that coordinate system? Or both?

We received a shipment of data from Esri for Philadelphia business locations.  We needed it transformed from geographic coordinate system (lat/long) to Penna State Plane South (3702) coordinate system (in feet).  Esri told us we have to do that ourselves.

Projections, coordinate systems, and transformations are always a little confusing for those of us who don't mess with them a lot.  Esri's tool labeling doesn't help any.

We had a team huddle and decided we were not projecting a coordinate system -- we were transforming from one coordinate system to another.  I looked in ArcToolbox under  Data Management > Projections & Transformations.  There is no Transform tool.  So I tried the only tool in there for Features:  Project.

I opened the Project tool and ignored the options first time around.  Got an error for not choosing the option:


Read the Help.  The option is required if you are changing coordinate systems.  Which is what we came here for in the first place.

How to choose?


Not sure.  I saw no hints in the input or output coordinate system descriptions.  So I used the first option on the list shown .  Looks like to worked fine.  This tells me where in State Plane:

Questions:

1.  Were we transforming or projecting?  Or both?  

2.  Is "re-projecting" a necessary step in transforming a data set's coordinate system from one to another?

3.  How do we decide which Geographic Transformation to apply?

I appreciate your comments.