Calculating length with Python and the PYTHON_9.3

Question

(Please critic wholly if possible) If I have polyline data in NAD83 and I want to calculate geometry using the PYTHON_9.3 parameter from the arcgisscripting.create(9.3), I am getting mixed results. If I calculate geometry in ArcMap, I set the dataframe to the state plane that my data resides, then calculate geometry in the attribute table view but I don't know that I am doing it right. I don't project the data to the state plane that the polyline record resides that I want to calculate geometry on but instead just set the dataframe and let the project on the fly by arcmap work or not work.

Now trying to automate the process, I do a gp.selectbylocation against state plane data then clip the polyline data for each selected state plane and then project the data to that state plane and do the "PYTHON_9.3" in gp.calculatefield_management piece. What environment settings should I consider or research more. Thanks guys!

Answer 1

I would adopt a workflow that uses only projected data with polylines that represent that their "true" shape (densified sufficiently to represent their curvature), or calculate geodesic distances between points. I would not rely on the magic of dataframe settings and mixed coordinate systems between the underlying data and those of the dataframe. Projection on-the-fly, regardless of the GIS being used, should only be for viewing convenience and nothing else.