xarray-grass 0.4.0
An Xarray backend for GRASS raster data.
Requires Python: >=3.11

# xarray-grass

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A [GRASS](https://grass.osgeo.org/) backend for [Xarray](https://xarray.dev/).
Explore all your GRASS rasters with Xarray.
Import zarr or NetCDF into your GRASS database.

## Installation

Install the package using `uv` or `pip`:

`uv add xarray-grass`

You need to install GRASS independently.

## Loading GRASS data as an Xarray Dataset

```python
import xarray as xr
import grass_session

import grass.script as gscript


with grass_session.Session(
    gisdb="/home/laurent/data/grassdata/",
    location="nc_spm_08_grass7_test",
    mapset="PERMANENT",
):
    # Make modis_lst mapset accessible
    gscript.run_command("g.mapsets", mapset="modis_lst", operation="add")
    test_ds = xr.open_dataset(
        "",  # No need to pass a path, the information is taken from the active grass session
        engine="xarray_grass",  # If no path is given, then the engine must be specified
        raster=["boundary_county_500m", "elevation"],
        strds="LST_Day_monthly@modis_lst",
    )

    print(test_ds)
```

```
Search path not modified
<xarray.Dataset> Size: 6MB
Dimensions:                     (y: 165, x: 179, start_time_LST_Day_monthly: 24)
Coordinates:
  * y                           (y) float32 660B 2.196e+05 ... 2.212e+05
  * x                           (x) float32 716B 6.377e+05 ... 6.395e+05
  * start_time_LST_Day_monthly  (start_time_LST_Day_monthly) datetime64[ns] 192B ...
    end_time_LST_Day_monthly    (start_time_LST_Day_monthly) datetime64[ns] 192B ...
Data variables:
    boundary_county_500m        (y, x) float64 236kB ...
    elevation                   (y, x) float32 118kB ...
    LST_Day_monthly             (start_time_LST_Day_monthly, y, x) int64 6MB ...
Attributes:
    crs_wkt:      PROJCRS["NAD83(HARN) / North Carolina",BASEGEOGCRS["NAD83(H...
    Conventions:  CF-1.13-draft
    history:      2025-11-02 23:51:57.141257+00:00: Created with xarray-grass...
    source:       GRASS database. project: , mapset:
```

xarray-grass requires an active GRASS session to work.
Here we're using the [grass-session](https://github.com/zarch/grass-session) package to set it.

You can choose which maps you want to load with the `raster`, `raster_3d`, `strds` and `str3ds` parameters to `open_dataset`.
Those accept either a single string or an iterable.
If none of those are specified, the whole mapset will be loaded, ignoring single maps that are already registered in either a `strds` or `str3ds`;
those maps will be loaded into the Xarray Dataset as part of the GRASS Space Time Dataset.
Any time-stamp associated to a single map not registered in a stds is ignored.

The extent and resolution of the resulting `Dataset` is defined by the region setting of GRASS, set with the `g.region` GRASS tool.
Note that in GRASS the 3D resolution is independent from the 2D resolution.
Therefore, 2D and 3D maps loaded in Xarray will not share the same dimensions and coordinates.
The coordinates in the Xarray `Dataset` correspond to the center of the GRASS cell.

In GRASS, the time dimension of various STDSs is not homogeneous, as it is for the spatial coordinates.
To account for this, xarray-grass will create one time dimension for each STDS loaded.

## CF conventions attributes mapping

### DataArray attributes

|CF name  |Origin in GRASS|
|---------|---------------|
|long_name|The "title" field from "r.info", "r3.info", or "t.info"|
|source   |Concatenation of "source1" and "source2" from "r.info" or "r3.info". In case of STDS, taken from the first map.|
|units    |The "unit" field from "r.info" or "r3.info". In case of STDS, taken from the first map.|
|comment  |The "comments" field from "r.info" or "r3.info". In case of STDS, taken from the first map.|

The attributes of the coordinates are in line with CF Conventions.

### Dataset attributes

The attributes set at the dataset level are:
  - `crs_wkt` from the g.proj command
  - `Conventions`, the CF Convention version
  - `history`, the time of creation and version of xarray-grass
  - `source`, the name of the current grass project and mapset

## Writing an Xarray Dataset or DataArray to GRASS

Continuing the script fro above, we can now write back the STRDS to GRASS.

There are two requirements of note to write to GRASS using xarray-grass:
  - The Dataset or SataArray needs a `crs_wkt` attribute with the CRS information in the WKT format.
  - The `dims` parameter needs to map every dimensions which is non standard to its standard name.
  The standard names are [x, y, z and start_time]

```python
    from xarray_grass import to_grass

    # Let's write the modis time series back into the current (PERMANENT) mapset

    da_modis = test_ds["LST_Day_monthly"]
    # xarray-grass needs the CRS information to write to GRASS
    da_modis.attrs["crs_wkt"] = test_ds.attrs["crs_wkt"]

    to_grass(
        dataset=da_modis,
        dims={
            "LST_Day_monthly": {"start_time": "start_time_LST_Day_monthly"},
        },
        overwrite=False,
    )
```

The above `print` statement should return this:

```
<xarray.Dataset> Size: 3MB
Dimensions:                     (y: 165, x: 179, start_time_LST_Day_monthly: 24)
Coordinates:
  * y                           (y) float32 660B 2.196e+05 ... 2.212e+05
  * x                           (x) float32 716B 6.377e+05 ... 6.395e+05
  * start_time_LST_Day_monthly  (start_time_LST_Day_monthly) datetime64[ns] 192B ...
    end_time_LST_Day_monthly    (start_time_LST_Day_monthly) datetime64[ns] 192B ...
Data variables:
    boundary_county_500m        (y, x) float64 236kB ...
    elevation                   (y, x) float32 118kB ...
    LST_Day_monthly             (start_time_LST_Day_monthly, y, x) int32 3MB ...
Attributes:
    crs_wkt:      PROJCRS["NAD83(HARN) / North Carolina",BASEGEOGCRS["NAD83(H...
    Conventions:  CF-1.13-draft
    history:      2025-11-01 02:10:24.652838+00:00: Created with xarray-grass...
    source:       GRASS database. project: <nc_spm_08_grass7_test>, mapset:<P...
```

## Roadmap

### Goals for version 1.0

- [x] Load a single raster map
- [x] Load a single Space-time Raster Dataset (strds)
- [x] Load a single raster_3d map
- [x] Load a single str3ds
- [x] Load a combination of all the above
- [x] Load a full mapset
- [x] Support for the `drop_variables` parameter
- [ ] Write from xarray to GRASS
  - [x] Write to a 2D raster
  - [x] Write to STRDS
  - [x] Write to 3D raster
  - [x] Write to STR3DS
  - [x] Transpose if dimensions are not in the expected order
  - [x] Support time units for relative time
  - [ ] Support `end_time`
  - [ ] Accept non homogeneous 3D resolution in NS and EW dimensions (GRASS 8.5)
- [x] Lazy loading of STDS on the time dimension
- [ ] Properly test with lat-lon location

### Stretch goals

- [ ] Read CRS definitions from CF compatible fields
- [ ] Lazy load on the spatial dimension