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.


2 comments:

  1. At least you're watching for those sleepers... I personally would've checked the last option, but it is counter-intuitive as you say. And while you stumbled on it, 'projection' is seen here as 'transformation'... as it means 're-projection' in fact!

    Melita Kennedy (mkennedy@esri.com) was a great resource, and is largely responsible for adding the exotic systems us oilers found. She did not, however, manage to excise those damn spherical projections, to render projection parameters more explicit, or uneditable in the properties.

    "Too much of one, too little of the other, WTF"

    ReplyDelete
    Replies
    1. Hello,

      From what I understand:

      1) You were both projecting from one coordinate system to another as well as needing to perform a transformation on the data. At one time, the WGS84 and NAD83 spheroidmodels were pretty close. Through advancements in GPS and modeling they have moved away from each other and now may be more thana meter away, thus requiring that tranformation. If you do a Google search on "Evolution of wgs nad" a pdf should come up on it

      2) Re-projecting is usually necessary going from one coordinate system to another in order to change the spheroids being used. The transforming depends on a lot of factors, but I usually try to do so.

      3) Deciding on the geographic transformation relies on knowing how the data was captured, your need for it to be within (.5/1/3/5) feet off etc. I think there is a file in the bin directory or somewhere that has an explanation of all of the coordinate transformations. But as I understand, at leas the NAD 83 coordinate transforms from WGS improve as they go down the list. As data collection methods have improved, ESRI has tweaked their tranformations.

      I would echo the other poster though, Melita at ESRI is a great resource.

      Paul
      Riley County KS GIS

      Delete