The basic problem appears to be that SkyWcs persistence is not exact. Note that assertWcsAlmostEqualOverBBox does not allow 0 as the max error, so it cannot test equality. Fix that and fix the persistence test in afw test_skyWcs.py to use it.

I have attached a script that uses astshim and shows small differences between a celestial WCS FrameSet and its un-persisted copy. I also tried editing AST to persist floats with 19 digits instead of 17 and it made no difference. I have sent the script to David Berry in hopes that he can fix AST. When I run the script I see the following:

delta= [[  0.00000000e+00   7.95807864e-13   0.00000000e+00]

 [  0.00000000e+00  -6.82121026e-13   0.00000000e+00]]

Traceback (most recent call last):

  File "temp.py", line 51, in <module>

    assert np.array_equal(pixRoundTrip, pixRoundTrip2), "inverse transform is different"

AssertionError: inverse transform is different

The basic problem was that AST was not saving the inverse coefficients for MatrixMap and there were subtle numerical differences when they were computed when un-persisting. David Berry fixed that in starlink_ast.

I didn't see this earlier because I had mis-written assertWcsAlmostEqual... to require positive maximum error instead of allowing 0. I fixed that and changed two tests in afw and test_coaddBoundedField in meas_algorithms to use an error of 0.

In addition I added accessors to CoaddBoundedField in order to make debugging problems such as this simpler. I also modified operator== for CoaddBoundedField and CoaddBoundedFieldElement to test value equality, not pointer equality (provided neither pointer is null) and wrapped CoaddBoundedField::operator== with pybind11.

Please make sure you are comfortable with my changes to CoaddBoundedField in meas_algorithms. I hope you will find the changes in afw innocuous. There is no need to review the starlink_ast changes, though I made a pull request in case you want to look at them.

Looks good.  One small comment on the PR.