[mdsumner]

Requested by Michael · project thread

What

New page store.html (nav: after Rectilinear chunks). Point it at an Icechunk repository or a Zarr store and it shows where a variable's cells and chunks sit on the earth:

• reads Icechunk with icechunk-js 0.6.0 (EarthyScience) and Zarr with zarrita 0.7.5, the same pair gridlook uses; both are pinned devDependencies and loaded from jsDelivr +esm in the page
• finds the CRS from a CF grid_mapping (14 appendix F mappings, plus rotated pole), the EPSG code in crs_wkt, GeoZarr attributes, or 1D lat/lon coordinates (WGS 84 assumed, and the page says so)
• finds the cells from a GeoTransform, spatial:transform or 1D coordinates (km units are scaled to m)
• draws a mesh over index space, moving from the globe to a gridlook projection (plus LAEA and the store's own CRS)
• colours by data (the whole slice when it is small, otherwise a spread of chunks or the chunk you click), by cell area on the earth, or by the angular distortion of the store's CRS
• shows a readout for the clicked cell: chunk key, x/y, lon/lat, area, h/k, omega, value
• uses gridlook's URL hash form (#<store>::varname=..::projection=..) and has an "Open in gridlook" link

Checks

test/check-storecrs.mjs (added to npm test):

• every CF mapping goes through proj4js and is compared with pyproj CRS.from_cf, all within 1 mm. The exception is ellipsoidal aeqd: proj4js 2.12.1 is about 1 m off and inverts it into the wrong hemisphere. 2.22 fixes this. The test reports it rather than failing.
• cell areas are compared with pyproj Geod, using densified parallel edges
• six fixture stores (LCC, polar stereographic in km, a global grid inside a group, rotated pole, a plain Zarr store with GeoTransform, and a GLAD-shaped metadata-only store) are served over HTTP and read through js/store-read.js. The first-cell lon/lat is checked against PROJ.

The browser check used Playwright + swiftshader with the CDN routed to local bundles: all fixtures open and draw.

Not yet

• curvilinear (2D lat/lon) grids are only noted (Regular in what? could fit a CRS to them later)
• the public icechunk-public-data bucket sends no CORS header, so the browser cannot open it from another site
• the +esm jsDelivr builds of icechunk-js and zarrita have not been loaded from the real CDN yet

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Generated by Claude Code