# Pystac client query to get cloud masked data

**URL:** <https://discourse.pangeo.io/t/pystac-client-query-to-get-cloud-masked-data/2871>\
**Category:** Data\
**Created:** [October 27, 2022, 5:15pm UTC](https://discourse.pangeo.io/t/pystac-client-query-to-get-cloud-masked-data/2871 "2022-10-27T17:15:40Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![Guy\_Maskall](https://yyz2.discourse-cdn.com/flex030/user_avatar/discourse.pangeo.io/guy_maskall/32/765_2.png) [@Guy\_Maskall](https://discourse.pangeo.io/u/Guy_Maskall)\
**Post date:** [October 27, 2022, 5:15pm UTC](https://discourse.pangeo.io/t/pystac-client-query-to-get-cloud-masked-data/2871/1 "2022-10-27T17:15:40Z")

</div>

I hope someone can give me a quick pointer here. I’m using the pystac\_client to query the Element 84 AWS stac API (and still finding my way around the API). Okay, fine. I see catalog “sentinel-s2-l2a-cogs” from which I’ve pulled an example tile, and I see catalog “sentinel-s2-l1c”, where I understand the cloud mask data lives. Do I need to pull data from both, in two operations, and merge to then filter the L2 product pixels on the cloud mask? Or is there a convenient way to do a query (one data “pull”) on the L2 product as I have been doing but specify the cloud mask product as a filter to only get the non-cloudy pixels? If that makes sense.

Apologies for the haste of this post, I’m happy with an “RTFM” answer with a link to the FM, but I have a 6 month child howling and wanting a feed and could do with a pointer to get me starting first thing in the morning back at work!

---

<div class="post-metadata">

**Author:** ![Guy\_Maskall](https://yyz2.discourse-cdn.com/flex030/user_avatar/discourse.pangeo.io/guy_maskall/32/765_2.png) [@Guy\_Maskall](https://discourse.pangeo.io/u/Guy_Maskall)\
**Post date:** [October 27, 2022, 11:41pm UTC](https://discourse.pangeo.io/t/pystac-client-query-to-get-cloud-masked-data/2871/2 "2022-10-27T23:41:48Z")

</div>

## stac\_cfg

Okay, so without a fractious child on my lap, I believe I’ve found the cloud mask I’d want is actually in the SCL asset. From an example I was playing with, I’ve been using `stac_load` with a `stac_cfg` of

```auto
cfg = """---
sentinel-s2-l2a-cogs:
  assets:
    '*':
      data_type: uint16
      nodata: 0
      unit: '1'
    SCL:
      data_type: uint8
      nodata: 0
      unit: '1'
    visual:
      data_type: uint8
      nodata: 0
      unit: '1'
  aliases: # Alias -> Canonical Name
    red: B04
    green: B03
    blue: B02
"*":
  warnings: ignore # Disable warnings about duplicate common names
"""
cfg = yaml.load(cfg, Loader=yaml.SafeLoader)

```

This gives me a `red`, `green`, and `blue` variable in my xarray Dataset.

The docs say, for stac\_cfg

> Mostly used to specify “missing” metadata like pixel data types.

and I’ve not found a very comprehensive documentation for creating this config.

## Bands

Saving the best til last, I just added the `SCL` asset to the list of bands to extract, thus

```auto
xx = stac_load(
    items,
    bands=("red", "green", "blue", "SCL"),
    crs=crs,
    resolution=10 * zoom,
    chunks={}, # <-- use Dask
    groupby="solar_day",
    stac_cfg=cfg,
)

```

and now have an extra variable, `SCL`, in my Dataset. Yay. And first glance suggests it contains features that look rather like clouds. So this is making progress.

Perhaps my question now boils down to ('cos I’m going to bed now):  
_Is the numeric encoding of `SCL` contained in a convenient attribute in the data somewhere, or do I need to refer back to the Sentinel docs to know what value corresponds to cloud free, cirrus etc?_