Thanks Martin! I am not 100% with the inner workings of most coordinate transformations so I am learning a lot watching you work through these features
Cheers, Adam On Fri, Nov 8, 2013 at 6:07 PM, Martin Desruisseaux < [email protected]> wrote: > Hello all > > The main work this week has been an effort to increase the accuracy of the > matrix computed by the BursaWolfParameters.getPositionVectorTransformation() > method. This was a known problem in Geotk. The intend is not to have an > accurate "position vector transformation" (that would be pointless since > this transformation method, like all datum shifts, is only approximative > anyway). The intend is to get back an identity matrix when concatenating a > chain of operations having "A -> B" followed by "B -> A". More specifically > the problem was: > > * "Position vector transformation", when expressed in matrix form, is > close to an identity matrix (values are quite small). > * When performing matrix inversions and multiplications, rounding > errors accumulate relatively fast. > * As a consequence of combination of the two above points, some > concatenation of transformations resulted in matrices difficult to > distinguish from noise. > > > The usual strategy for floating point values: > > if (abs(value) < epsilon) > > didn't worked in this case, because the overlapping between "signal" and > "noise" were too high in some situations: sometime a real datum shift was > considered as noise, and conversely. This problem does not happen for a > BursaWolfParameters object alone, but appears after a few concatenations. > The work of the last few weeks is an attempt to improve the discrimination > between signal and noise in coordinate transformation chains. > > My plan for next week is to add a new property in BursaWolfParameters: its > domain of validity. This information was missing in Geotk, and experience > has shown that this was a problem. Then, I plan to complete the > GeodeticObjects class. When GeodeticObjects will provide at least the WGS84 > GeographicCRS, I think that the class would be okay for a SIS 0.4 release > (remaining work would be to provide a basic Feature class for the Shapefile > reader). > > Martin > >
