Appendix A — Glossary and crosswalk
A.1 Terms as this book uses them
Frame. The coordinate system positions are written in: geographic longitude and latitude, a projected CRS, rotated longitude and latitude, a scan-angle system, or none. Grid type is always relative to a frame.
Regular (in a frame). Positions follow from the index by origin plus step in each direction. Six numbers for a 2D grid: dimension and extent, or equivalently an affine transform.
Rectilinear (in a frame). Each axis has its own 1D sequence of positions, with varying spacing; cells are at their outer product.
Curvilinear (in a frame). Cells are indexed by row and column but their positions are not an outer product of two sequences. Genuinely curvilinear means curvilinear in every frame tried.
Unstructured. No row and column index; positions and connectivity are explicit.
Binned grid (L3 bins). NASA’s integerized sinusoidal scheme: rows of equal latitude height, each with a whole number of near-equal-area bins. Unstructured (no (i, j) index) but implicit (positions from the bin number).
Logically rectangular. Indexable as (i, j); covers regular, rectilinear and curvilinear (Balaji).
Degenerate rectilinear. A regular grid stored as 1D coordinate vectors.
Cryptic curvilinear. A grid stored as 2D coordinate arrays that is regular in some other frame.
Geolocation array. GDAL’s term for per-pixel coordinate arrays. Covers cryptic curvilinear grids, model grids and sensor swaths alike.
Dimension (arrays). An axis of an array. Not a coordinate count and not a shape’s topological dimension.
A.2 The same idea in different vocabularies
| This book | VTK, visualisation | ESMF, Balaji | CF, netCDF | GDAL classic | xarray | stars (R) |
|---|---|---|---|---|---|---|
| regular | image data, uniform | uniform | 1D coordinate variables, evenly spaced | geotransform | 1D coords; RangeIndex or rasterix RasterIndex |
regular dimension |
| rectilinear | rectilinear grid | rectilinear | 1D coordinate variables | geolocation, or geotransform if regular enough | 1D coords | rectilinear dimension |
| curvilinear | structured grid | curvilinear | 2D auxiliary coordinates (coordinates attribute) |
geolocation arrays | 2D coords | curvilinear |
| unstructured | unstructured grid | (outside logically rectangular) | UGRID | not modelled | (via uxarray and similar) | not modelled |
| frame | (none) | coordinate system | grid_mapping, crs_wkt |
spatial reference | not modelled (xproj adds a CRS index) | crs |
| centre vs edge | point vs cell data | centre vs corner stagger | bounds, corner arrays |
PixelIsPoint vs PixelIsArea | (by convention) | offset, cell vs point |