Update sample README

zarr_sample/README.md

@@ -1,37 +1,76 @@
 # calvdb — Dataset Sample
 
-This folder contains a small sample of the full **calvdb** dataset, provided so that reviewers can inspect the data format without downloading the complete archive (6.02 GB).
+This folder holds one glacier from **calvdb**, left **unzipped** so you can inspect
+the store layout in the file browser above without downloading the full dataset
+(5.4 GB). Click into `RGI60-07.00025.zarr/` and open any `zarr.json` to see the
+shape, dtype, chunking and dimension names of an array.
 
-## Contents
+The sample is Braasvellbreen (RGI60-07.00025), 55 observations, from the **train**
+split. Its arrays are identical to `zarr_zipped/RGI60-07.00025.zarr.zip`.
 
-The sample includes Braasvellbreen glacier (RGI60-07.00025) taken from the **train split** of the benchmark.
+> **For actual use, download from [`zarr_zipped/`](../zarr_zipped) instead** — one
+> file of 275 MB rather than the 617 loose files here, and readable without
+> unzipping. This copy exists for browsing.
 
-## How the sample was created
+## Reading it
 
-Each Zarr store in this folder is taken directly from the full dataset without modification. The stores were copied from the full `zarr_zipped/` collection and are structurally identical to every other glacier store in calvdb.
+Requires **zarr >= 3.0** and **Python >= 3.11**. These are Zarr **format 3** stores;
+zarr-python 2.x cannot read them and fails with
+`GroupNotFoundError: group not found at path ''`. See the
+[main README](../README.md#requirements) for the full requirements and
+troubleshooting.
+
+```python
+import zarr
+from huggingface_hub import snapshot_download
+
+path = snapshot_download(
+    repo_id="enscg/calvdb",
+    repo_type="dataset",
+    allow_patterns="zarr_sample/RGI60-07.00025.zarr/*",
+)
+group = zarr.open_group(f"{path}/zarr_sample/RGI60-07.00025.zarr", mode="r")
+print(group["sdt"].shape)          # (55, 541, 1052)
+```
+
+The zipped archives are read the same way but without unzipping:
+
+```python
+store = zarr.storage.ZipStore("RGI60-07.00025.zarr.zip", mode="r")
+group = zarr.open_group(store, mode="r")
+```
+
+A complete runnable example is in
+[`example_loading.ipynb`](../example_loading.ipynb).
 
 ## Zarr store structure
 
-Each glacier is stored as a Zarr v3 group (zstd compression). Opening any store reveals the following arrays:
+Each glacier is a Zarr v3 group (zstd compression) with `T` observations on an
+`H × W` grid:
 
 | Array | Shape | Description |
 |---|---|---|
-| `sdt` | `(T, H, W)` | Signed distance transform of the calving front; values clipped to ±2000 m |
+| `sdt` | `(T, H, W)` | Signed distance transform of the calving front, metres, clipped ±2000; the front is the zero level set |
 | `imagery` | `(T, 5, H, W)` | Multispectral imagery: blue, green, red, NIR, SWIR-1 |
-| `geophys` | `(T, 6, H, W)` | Geophysical channels: bed elevation, surface elevation, ice thickness, surface velocity, strain rate, ice–fjord mask |
-| `climate` | `(T, 2)` | Climate forcing: surface runoff, ocean temperature |
-| `trace` | `(T, H, W)` | Rasterised calving-front line (used for evaluation only, not a model input) |
-| `dates` | `(T,)` | Observation dates in YYYY-MM-DD format |
-| `valid_sdt` | `(T,)` | Boolean mask — True if the SDT observation is valid |
-| `valid_img` | `(T,)` | Boolean mask — True if the imagery observation is valid |
-| `valid_clm` | `(T,)` | Boolean mask — True if the climate observation is valid |
-| `valid_trace` | `(T,)` | Boolean mask — True if the trace observation is valid |
-| `sensors` | `(T,)` | Sensor identifier for each imagery observation |
+| `geophys` | `(T, 6, H, W)` | Bed elevation, surface elevation, ice thickness, surface velocity, strain rate, ice–fjord mask |
+| `climate` | `(T, 2)` | Surface runoff, ocean temperature |
+| `trace` | `(T, H, W)` | Rasterised calving-front line (evaluation only, not a model input) |
+| `dates` | `(T,)` | Observation dates, YYYY-MM-DD |
+| `sensors` | `(T,)` | Sensor code: `0=S2, 1=LE07, 2=LC08, 3=LC09, 255=unknown` |
+| `valid_sdt` | `(T,)` | True if the SDT observation is valid |
+| `valid_img` | `(T,)` | True if the imagery observation is valid |
+| `valid_clm` | `(T,)` | True if the climate observation is valid |
+| `valid_trace` | `(T,)` | True if the trace observation is valid |
 
-All arrays share the same time axis `T`. Spatial dimensions `H × W` vary per glacier. Spatial resolution is 30 m; CRS is EPSG:3995 (Arctic Polar Stereographic). 
+All arrays share the same time axis `T`. Spatial dimensions `H × W` vary per
+glacier. Resolution is 30 m; CRS is EPSG:3995 (Arctic Polar Stereographic).
 
-> **Note on `ice_fjord_mask` (geophys channel 5):** Although `ice_fjord_mask` is stored in the zarr archive as geophysical channel 5 for completeness, it is excluded from the model input tensor in the baseline experiments due to inaccuracies in the mask boundary for several glaciers. It is retained in the dataset to support future work that may use or improve upon this variable.
+> **Note on `ice_fjord_mask` (geophys channel 5):** it is stored for completeness
+> but excluded from the model input tensor in the baseline experiments, because
+> the mask boundary is inaccurate for several glaciers. It is retained to support
+> future work that may use or improve upon this variable.
 
 ## Further details
 
-- **Full dataset, splits and normalization:** see the main [README](../README.md)
+- **Full dataset, requirements, splits and normalisation:** see the main
+  [README](../README.md)