## dem.prepare_dem_for_snap()


Return an external DEM prepared for SNAP's sampling and height conventions.


Usage

``` python
dem.prepare_dem_for_snap(
    dem,
    *,
    source_crs=None,
    download_missing_grids=False,
    cache_dir=None,
    snap_bin=None
)
```


The output geotransform is shifted by half a pixel because SNAP interprets an external DEM's origin as the center of its first pixel, and so samples the file half a pixel south-east of where its georeferencing places it \[1\]. SNAP fixed that in the 14.0.2 update of its `snap-dem` module, so the shift is applied only for an installation older than that, and for one whose version cannot be determined. When `source_crs` is supplied, elevations are also transformed from that compound horizontal and vertical CRS to WGS 84 ellipsoidal heights (EPSG:4979). SNAP cannot do that conversion itself: the only geoid it knows is EGM96, which it applies even to the Copernicus DEM, whose heights are referenced to EGM2008 \[2\].

The result is correct **only** when read by the SNAP installation it was prepared for, and must only be passed as an `externalDEMFile`. Set SNAP's `externalDEMApplyEGM` parameter to `False`. Prepared copies are cached by source content and conversion settings.


## Parameters


`dem: str | Path`  
Local DEM to prepare.

`source_crs: str | None = None`  
Compound CRS describing the DEM's horizontal and orthometric references, for example `"EPSG:4326+3855"` for Copernicus GLO-30. If omitted, elevations pass through unchanged.

`download_missing_grids: bool = ``False`  
Download required open-license PROJ grids when they are not installed.

`cache_dir: Path | None = None`  
Directory for prepared copies. Defaults to `SNAP_DEM_CACHE` (`~/.s1bursts/dem`).

`snap_bin: Path | None = None`  
A SNAP installation's `bin` directory, which decides whether the half-pixel shift is needed. Defaults to `SNAP_BIN`, then `gpt` discovered on `PATH`.


## Returns


`Path`  
Path to the prepared copy.


## Raises


`FileNotFoundError`  
If `dem` does not exist.

`ValueError`  
If the source CRS is invalid or inconsistent with the DEM.

`TransformationGridError`  
If no exact vertical transformation is available, because a PROJ grid it needs is missing or could not be downloaded.

`pyproj.exceptions.ProjError`  
If any DEM pixel falls outside the selected transformation's valid area.

`ConfigurationError`  
If `snap_bin` is given but contains no `gpt` executable.


## References

1.  <span style="font-weight: 600; color: #6c757d;">\[1\]</span> External DEM is sampled half a pixel off. ESA STEP forum. [https://forum.step.esa.int/t/external-dem-is-sampled-half-a-pixel-off/46042](https://forum.step.esa.int/t/external-dem-is-sampled-half-a-pixel-off/46042)
2.  <span style="font-weight: 600; color: #6c757d;">\[2\]</span> Copernicus DEM built-in path applies EGM96 while the data is EGM2008. ESA STEP forum. [https://forum.step.esa.int/t/copernicus-dem-built-in-path-applies-egm96-while-the-data-is-egm2008/46041](https://forum.step.esa.int/t/copernicus-dem-built-in-path-applies-egm96-while-the-data-is-egm2008/46041)