# Regridding

In order to intercompare different models, they must be interpolated from the native grid on which they were calculated to a common output grid. This procedure is known as regridding or remapping.

Fundamentally, regridding consists of two steps:

- Creation of an interpolation matrix (weights file), which gives the amount that each point on the source grid contributes to each point on the target grid. These weights will depend on the type of interpolation desired. ISMIP6 has standardized on First-order Conservative Remapping (Jones, P.W. 1999,
*Monthly Weather Review*,**127**, 2204-2210), which does a better job preserving fluxes (or other integrals) than, eg., bilinear interpolation. - For each point on the target grid, sum the data values on the source grid multiplied by the corresponding weights. The same weights file may be used for more than one input variable, provided all are defined on the same source grid.

Different software may do one or the other or both of these steps.

To create the weights file, you must have *grid description files* for your source and target grids. These files give the latitudes and longitudes of every grid point, along with the corners of the boundary of its surrounding cell. There are several ways to write these files, depending on the software you're using.

We have tested a number of software packages, and so far, the easiest way we have found to do regridding for structured (Heiko Goelzer) and unstructured triangular (NASA Goddard) grids is using the Climate Data Operators (CDO):

Will Roberts (U. Bristol) has had success regridding unstructured BISICLES output using a combination of ESMF to generate weights and NCO to calculate the regridded data values. ESMF is also the tool used by CESM and CISM.

Regridding BISICLES output with ESMF and NCO

Notes on successful regridding using other packages, or further notes on using these, are welcome!