One item per (MGRS square, year). Twelve single-band COGs per item,
10 m, in each square's own UTM zone.
Query the mirror, do not walk the tree
items.parquet holds every item of every year. Walking {year}/{tile}/item.json
is one HTTP request per tile and there are hundreds per year.
Asset vocabulary
Twelve keys, and the key is the filename stem. Polarization comes before the
QA role, so a sorted listing keeps each QA layer directly beneath the
measurement it describes.
asset key
title
stored as
nodata
to display
coh12_vv
Coherence VV
float32
NaN
0.1 to 0.75, cmc.lipari
coh12_vv_qa_num
Coherence VV – Contributing Observations
uint16
0
0 to 90, cmc.glasgow
coh12_vv_qa_cqm
Coherence VV – Composite Quality Map
float32
NaN
10*log10(x), then -3 to 10 dB, cmc.glasgow
coh12_vh
Coherence VH
float32
NaN
0.1 to 0.55, cmc.lipari
coh12_vh_qa_num
Coherence VH – Contributing Observations
uint16
0
0 to 90, cmc.glasgow
coh12_vh_qa_cqm
Averaging rtc_* or *_qa_cqm must happen in the linear domain. A mean of
decibels is not the decibel of a mean. The renders block on the collection
carries the same stretches with portable matplotlib colormap names.
Provenance is in the file
Each GeoTIFF carries the dataset tags recording the processing chain
(PRODUCT_TYPE, PIPELINE_SOFTWARE*, COMPOSITING_*, BACKSCATTER_*,
COHERENCE_*). The item properties mirror them under glace:* and
processing:*, so you can filter on them without opening a raster, but the
file is the source of truth and stays self-describing if separated from this
catalog.
Every tile item of a year carries the same start_datetime and end_datetime:
the compositing window, chosen once per year from the lowest-snow period. Which
acquisitions actually contributed varies per tile and product and is on the item
as glace:coh_first_acquisition / glace:coh_last_acquisition and the rtc
pair; coherence ends about twelve days before backscatter because a pair needs
both dates inside the window.
Pairing with the mosaics
The mosaics collection holds the same measurements resampled onto one fixed
equal-area grid, one item per year. Join on the year. Do not join on geometry:
the tiles are per-UTM-zone and the mosaics are ETRS89-LAEA.
Gotchas
QA-NUM's nodata value is 0, which coincides with zero contributing
observations.
NaN is nodata on every float32 band. Some browser GeoTIFF decoders drop
LERC's validity mask and hand a NaN through as 0; that is a client defect,
not a property of these files.
Tiles are LERC_ZSTD compressed with a per-quantity error bound anchored to
that quantity's own measurement noise.
Coverage is the Alpine Convention perimeter; a square outside it has no item.