Implement xESMF based regridding operation #127
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philippjfr
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@jbednar Whenever you have a minute it would be good if you could review this. |
jbednar
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Jan 29, 2018
geoviews/operation/regrid.py
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| grids using the xESMF library, supporting all the ESMF regridding | ||
| algorithms including bilinear, conservative and nearest neighbour | ||
| regridding. In addition to these different interpolation options | ||
| it supports |
geoviews/operation/regrid.py
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| if issubclass(self.p.target.interface, XArrayInterface): | ||
| ds_out = self.p.target.data | ||
| ds_out.rename({tx.name: 'lon', ty.name: 'lat'}, | ||
| inplace=True) |
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I didn't look very far into it, but is renaming in place safe (here and below)? Seems like that would leave people with altered data.
| "source": [ | ||
| "The xESMF library is specifically designed to provide an easy way to correctly resample grids defined in geographic coordinate systems and differs significantly from the simpler approach used by datashader, which applies simple upsampling and downsampling. xESMF is a wrapper around the [ESMF regridding algorithms](https://www.earthsystemcog.org/projects/esmf/regridding), which compute an interpolation weight matrix which is applied to remap the values of the source grid onto the destination grid. \n", | ||
| "\n", | ||
| "In GeoViews these algorithms are made available via the simple ``weighted_regrid`` operation, which supports the different interpolation modes including: 'bilinear', 'nearest_s2d', 'nearest_d2s' and 'conservative'. Since generating the sparse weight matrix takes much longer than applying it the operation will cache the weight matrix on disk for later use, this optimization can be disabled via the ``reuse_weights`` parameter or customized by defining a custom ``file_pattern``." |
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Why "simple"? I'd take that out, unless you are comparing it against some more complicated option we also provide (in which case that should be described here). Also, as usual, "use, this" needs to be "use; this" or "use. This".
| "cell_type": "markdown", | ||
| "metadata": {}, | ||
| "source": [ | ||
| "If we want to explore a very large grid it therefore often makes sense to resample the data onto a rectilinear grid, which can be rendered much more efficiently. Once again we have the option of using the datashader based approach or the more correct xESMF based approach." |
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I'd say "more accurate" rather than "more correct"; there are varying levels of accuracy but correctness is Boolean. Neither approach is unequivocally "correct"; one can always use more and more complex models (given that the earth isn't actually precisely spherical, for instance).
| "cell_type": "markdown", | ||
| "metadata": {}, | ||
| "source": [ | ||
| "To regrid a ``QuadMesh`` using GeoViews we can import the ``rasterize`` operation. In the background the operation will convert the ``QuadMesh`` into a ``TriMesh``, which datashader understands. To optimize this conversion so it occurs only when aggregating the ``QuadMesh`` for the first time we can activate the ``precompute`` option. Additionally we have to define an aggregator, in this case to compute the mean ``Tair`` value in a pixel:" |
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I feel stupid now, I've been puzzling over this a while.
| "cell_type": "markdown", | ||
| "metadata": {}, | ||
| "source": [ | ||
| "Another commonly used mesh/grid type are trimeshes, here we will load a 3dm file and demonstrate how to visualize it using HoloViews:" |
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Another comma splice; needs to be "trimeshes; here".
| "cell_type": "markdown", | ||
| "metadata": {}, | ||
| "source": [ | ||
| "This is a reasonably large mesh with >1 million triangles and cannot easily be displayed directly with matplotlib or especially bokeh. Therefore we will once again regrid this dataset onto a rectilinear grid. Additionally, to speed up plotting of meshes, we can do two things, first of all since all data displayed with bokeh is first projected to Web Mercator coordinates we can project the mesh immediately, secondly we can use the ``precompute`` during rasterization, which will cache an optimized mesh datastructure:" |
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"two things: first, since ....; second, we can"
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Tests passing. Merging. |
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Adds a xESMF based regridding operation along with a user guide on how to use it. Depends on holoviz/holoviews#2265. The draft user guide can be seen here: https://anaconda.org/philippjfr/resampling_grids/notebook