[aspect-devel] A bug in gplates plugin

[Rene Gassmöller]

Hi Siqi,
thanks for posting these problems. Could you please post the aspect version and gplates version you use? I fixed some problems with the poles and 2d models since the 1.1 release, but maybe there is still a bug (as always ;) ). It might also be related to the change in the gplates format between version 1.3 and 1.4. Could you sent a simple input file that reproduces the problems? If you already have the fixed code, then sure - go ahead and sent a pull request, I will take a look at it.

In fact the interpolation we currently use is a moving window average with no distance weighting. The weighting you see in the code is to account for the different area each point represents in a latitude-longitude grid (sine of latitude). 
I think the reason for switching from latlon interpolation to cartesian moving window was that I introduced this interpolation mostly for smoothing purposes, not so much for the most accurate velocity at a point, but I see your point that this might be important for many models. Would it help your purpose to set the interpolation width to 0? Then there should be no interpolation at plate boundaries.   Or do you mean that with the current interpolation there is actually deformation occuring within plates? That would need to be fixed of course. I would just like to keep the possibility to smoothen the surface velocities as it helps convergence in my models. We can certainly discuss better interpolation methods or provide several options.

Best 
Rene

----- Ursprüngliche Nachricht -----
Von: "Siqi Zhang" <siqi.zhang at mq.edu.au>
Gesendet: ‎24.‎10.‎2014 02:55
An: "aspect-devel" <aspect-devel at geodynamics.org>
Betreff: [aspect-devel] A bug in gplates plugin

Hi All,


I found a bug in the gplates plugin.
While the gplate gpml file set longitude to 0 for all points at the pole, the plugin assume the longitude vary from 0~360 for the grid points at two poles. This gives the wrong cartesian velocity while reading the data and result weird surface velocity field around the pole. I can create a bug fix for this if Rene agrees.


And I have another question regarding the gplate plugin for Rene. 
I have noticed that the interpolation method of this plugin has changed from latitude-longitude grid interpolation using spherical velocity to distant weight interpolation using cartesian velocity. I totally agree that we should do interpolation using the cartesian velocity, but is there any particular reason to use the distant weight instead of using four points bilinear interpolation on the latitude-longitude grid? 
Because while using the new interpolation in 3D model, I found some artificial strain rate within plate, it gives me some trouble while I am also using yielding stress in the rheology. After I reverse back to the old grid approach and interpolate using the cartesian velocity, it seems working better. Although it is not completely removed, the artificial strain rate is much less than the new interpolation approach. I wonder if anyone using this plugin experience the similar problem and if we need the change the interpolation method again.




Regards,




-- 

Siqi Zhang


Research Associate

ARC Centre of Excellence for Core to Crust Fluid Systems (CCFS)

Department of Earth and Planetary Sciences

Macquarie University

NSW 2109



Telephone: +61 2 9850 4727

http://www.CCFS.mq.edu.au

http://www.GEMOC.mq.edu.au
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