Add reorder_to_cellsize function for reordering CellGrids.

CHANGES.rst

@@ -6,6 +6,7 @@ v0.2.3
 ======
 
 - Fix bug in calc_lut in case of differently ordered subset of a grid.
+- Add function to reorder grid based on different cell size. (See grids.reorder_to_cellsize)
 
 v0.2.2
 ======

pygeogrids/__init__.py

@@ -1,4 +1,4 @@
-from .grids import BasicGrid, CellGrid, genreg_grid, lonlat2cell
+from .grids import BasicGrid, CellGrid, genreg_grid, lonlat2cell, reorder_to_cellsize
 import pkg_resources
 
 try:

pygeogrids/grids.py

@@ -1070,3 +1070,44 @@ def _element_iterable(el):
         iterable = False
 
     return iterable
+
+
+def reorder_to_cellsize(grid, cellsize_lat, cellsize_lon):
+    """
+    Reorder grid points in one grid to follow the
+    ordering of differently sized cells. This is useful if
+    e.g. a 10x10 degree CellGrid should be traversed
+    in an order compatible with a 5x5 degree CellGrid.
+
+    Parameters
+    ----------
+    grid: :py:class:`pygeogrids.grids.CellGrid`
+        input grid
+    cellsize_lat: float
+        cellsize in latitude direction
+    cellsize_lon: float
+        cellsize in longitude direction
+
+    Returns
+    -------
+    new_grid: :py:class:`pygeogrids.grids.CellGrid`
+        output grid with original cell sizes but
+        different ordering.
+    """
+
+    cell_grid = grid.to_cell_grid(cellsize_lat=cellsize_lat,
+                                  cellsize_lon=cellsize_lon)
+    cell_sort = np.argsort(cell_grid.arrcell)
+    new_arrlon = grid.arrlon[cell_sort]
+    new_arrlat = grid.arrlat[cell_sort]
+    new_arrcell = grid.arrcell[cell_sort]
+    new_gpis = grid.gpis[cell_sort]
+    new_subset = None
+    if grid.subset is not None:
+        full_subset = np.zeros(new_arrlon.size)
+        full_subset[grid.subset] = 1
+        new_full_subset = full_subset[cell_sort]
+        new_subset = np.where(new_full_subset == 1)[0]
+    return CellGrid(new_arrlon, new_arrlat, new_arrcell,
+                    gpis=new_gpis,
+                    subset=new_subset)

tests/test_grid.py

@@ -533,5 +533,25 @@ def test_genreggrid():
     assert lat == 88.5
 
 
+def test_reorder_to_cellsize():
+    """
+    Test reordering to different cellsize
+    """
+    lons = np.array([-177, -177, -176, -176])
+    lats = np.array([51, 57, 51, 57])
+    gpis = np.array([1, 2, 3, 4])
+    cells = np.array([14, 14, 14, 14])
+    orig_grid = grids.CellGrid(lons, lats, cells, gpis=gpis)
+    reordered_grid = grids.reorder_to_cellsize(orig_grid, 5.0, 5.0)
+    nptest.assert_almost_equal(reordered_grid.gpis,
+                               np.array([1, 3, 2, 4]))
+    nptest.assert_almost_equal(reordered_grid.arrlon,
+                               np.array([-177, -176, -177, -176]))
+    nptest.assert_almost_equal(reordered_grid.arrlat,
+                               np.array([51, 51, 57, 57]))
+    nptest.assert_almost_equal(reordered_grid.arrcell,
+                               np.array([14, 14, 14, 14]))
+
+
 if __name__ == "__main__":
     unittest.main()