In the next figure, we show an ensemble PA fit for a very filamentary dataset in Perseus. Both the derived D and K are consistent with previous determinations. The method used to do this is very simple and will be detailed in the reference given above. The individual fits to these two contours are superposed on the results of the global analysis. Open squares are the ensemble.
Closed squares are the two selected large contours.
Open triangles are the IPA results. Dotted line is global fit. Solid lines are IPA fits. From inspection of the two IPA results it is possible to see why global PA is missing the true result. The derived ellipticities are in good qualitative agreement with the appearance of the contoured data.