[aspect-devel] A bug in gplates plugin

Sun Nov 2 09:41:25 PST 2014

Hi Siqi,
I have looked a bit into the interpolation issue and it seems indeed
that the previous implementation did a better job at avoiding the
gplates grid effects. Since GPlates now also has the interpolation
possibility, I would not oppose switching back to the old formulation,
however this is a bit involved, because I did some changes in the
meantime that are unrelated to this. So simply reverting gplates.cc and
gplates.h will also revert these improvements and bugfixes. Do you want
to sort out the stuff and make a new pull request, or should I take a
look into it in a while when I have some more time?

Best,
Rene

On 10/27/2014 05:25 AM, Siqi Zhang wrote:
> Hi Rene,
> 
> About the bug, here is a simplified parameter file to reproduced this bug
> (as shown in Pole_velocity_bug). Using velocity data from gplate 1.3 with 4
> degrees resolution.
> I have already send the pull request.
> 
> As for the plate deformation problem, I don't think changing interpolation
> width will solve it.
> Here is an example of the strain rate (Strain_rate1.png), using
> interpolation width=0.
> The resolution of gplate data is 4 degrees, and computation grid is refined
> 5 times which the resolution is much better than the gplate data.
> In comparison, if using the old lat-lon interpolation (using cartesian
> velocity instead of spherical velocity), the strain rate looks much better
> (Strain_rate2.png). It seem working well for my problem.
> And it doesn't seem to have problem with convergence for my test, even with
> strain rate dependent rheology.
> 
> And it seems that the gplate 1.4 has an option to do some smoothing while
> writing the gpml file. May be we don't need to do it here.
> Or another alternative will be only do smoothing at the plate boundary
> which can be determined by checking the velocity gradient on lat-lon grid.
> 
> Regards,
> 
> Siqi
> 
> 
> On Mon, Oct 27, 2014 at 11:55 AM, Siqi Zhang <siqi.zhang at mq.edu.au> wrote:
> 
>> Hi Rene,
>>
>> I am using the development version, and data from gplate 1.3. (gplate 1.4
>> has the similar problem)
>> And the problem only appears while using coarse gplate data grid and more
>> refined surface grid. (only points in the first and last latitude grid will
>> be affected).
>> I have put pull request for this, please take a look.
>>
>> Thanks for the clarification of the interpolation, I will try
>> interpolation width to 0 see if it solve the deformation problem.
>>
>> Regards,
>>
>> Siqi
>>
>> On Mon, Oct 27, 2014 at 9:44 AM, Rene Gassmöller <rengas at gfz-potsdam.de>
>> wrote:
>>
>>> 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
>>> ------------------------------
>>> 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
>>>
>>> _______________________________________________
>>> Aspect-devel mailing list
>>> Aspect-devel at geodynamics.org
>>> http://lists.geodynamics.org/cgi-bin/mailman/listinfo/aspect-devel
>>>
>>
>>
>>
>> --
>> 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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