# Then vs Now: day-of-year climatologies

Per-pixel day-of-year statistics of the [rasters](../rasters/README.md) over a baseline period, one
set of three 366-band COGs per (dataset, variable, place, baseline). Published at
`https://storage.oceanmetrics.io/gazetteer/climatology/`.

```
climatology/
  collection.json
  items/<dataset>_<variable>_<place>_<y0>-<y1>.json      # one Item, assets mean / sd / n
  dhw_5km/CRW_SST/NMS:FKNMS/1985-2005_mean.tif           # float32, NaN nodata
  dhw_5km/CRW_SST/NMS:FKNMS/1985-2005_sd.tif             # float32 sample sd (ddof = 1), NaN when n < 2
  dhw_5km/CRW_SST/NMS:FKNMS/1985-2005_n.tif              # uint8 years with a value, no nodata (0 = none)
  dhw_5km/CRW_SST/NMS:FKNMS/2003-2012_{mean,sd,n}.tif
```

Baselines (`erddap-places:baseline` = `[y0, y1]` on each Item):

- **1985–2005**: the climate-dashboard-app's default "first 20 years" (it is 21 calendar years,
  1985 through 2005 inclusive, as the app's slider used it).
- **2003–2012**: the NMS EcoIndicators baseline.

Band i is day-of-year i of a leap-year calendar, exactly as in the year files, so the Then band
for a Now date is the same band number. Band 60 (29 Feb) is computed from the leap years only (n ≈
a quarter of the others); band descriptions are `--MM-DD`.

**No smoothing.** Each band is the raw mean of that calendar day over the baseline years (n ≤ 21 or
10 values per pixel), so the curve is noisy at day scale; smooth in the client if wanted (the app's
moving-average slider). `catalog/build_then_now.py --window N` can instead pool ±N days per band
(`erddap-places:window_days` on the Item and the `WINDOW_DAYS` tag record it; the published files
use 0).

Same grid, encoding and reading pattern as the rasters (EPSG:4326, 0.05°, `INTERLEAVE=BAND`,
256 px tiles, DEFLATE, no overviews): a band is one range request of a few kB.

```js
const img  = await (await fromUrl(`${base}climatology/dhw_5km/CRW_SST/NMS:FKNMS/1985-2005_mean.tif`)).getImage(0)
const then = (await img.readRasters({ samples: [band - 1] }))[0]
// anomaly = now[i] - then[i]; a z-score uses the matching _sd.tif band
```

For a baseline not listed here, average the year files: see "A custom Then" in
[`../rasters/README.md`](../rasters/README.md).

License: CC-BY-4.0 (derived); CRW data NOAA, public domain. Built by
[`catalog/build_then_now.py`](../../build_then_now.py).