# shom-grd2nc v3.2.3

**Mercurial revision 5803:7164e00e8f3d | 2026-08-18**

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## Description

`shom-grd2nc` is a tool to **convert SHOM grid files** to NetCDF format or **convert gridit-cdf NetCDF atlas files** to SHOM's `.a` files.

Supported input file types:

- SHOM ASCII `.grd` files.
- SHOM binary `.a` and ASCII `.b` file pairs.
- One or multiple **gridit-cdf NetCDF** files.

---

## Usage

```bash
shom-grd2nc [OPTIONS] file
```

or

```bash
shom-grd2nc file1.nc [file2.nc ...]
```

---

## Options


| Option         | Description                                                                          |
| -------------- | ------------------------------------------------------------------------------------ |
| `-h`, `--help` | Display help and exit.                                                               |
| `-3`           | Create a **NetCDF 3** file (uncompressed, potentially large due to dummy variables). |
| `--small3`     | Create a **NetCDF 3** file **without dummy variables** (smaller than `-3`).          |


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## Examples

### **Convert a SHOM `.grd` File to NetCDF**

```bash
shom-grd2nc input.grd
```

### **Convert a SHOM `.a` and `.b` File Pair to NetCDF**

```bash
shom-grd2nc input.a
```

### **Convert a NetCDF File to SHOM `.a` Format**

```bash
shom-grd2nc input.nc
```

### **Convert Multiple NetCDF Files**

```bash
shom-grd2nc file1.nc file2.nc file3.nc
```

### **Force NetCDF 3 Output (Uncompressed)**

```bash
shom-grd2nc -3 input.grd
```

### **Force NetCDF 3 Output Without Dummy Variables**

```bash
shom-grd2nc --small3 input.grd
```

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## Environment Variables

None.

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## Notes

1. **Input File Handling:**
  - Only **one input file** is accepted at a time for SHOM `.grd`, `.a`, or `.b` files.
  - Multiple NetCDF files can be processed in a single command.
  - If the same input file is specified multiple times, a warning is issued, and only one instance is processed.
2. **Output:**
  - By default, the output is a **compressed NetCDF 4** file (smaller and more efficient).
  - Use `-3` or `--small3` to generate **NetCDF 3** files (uncompressed or without dummy variables).
  - The output filename is derived from the input filename (e.g., `input.grd` → `input.nc`).
3. **Coordinate Handling:**
  - Supports `LONGITUDE` and `LATITUDE` axes.
  - Coordinates can be specified as `START,DELTA` or `GIVEN_BELOW`.
4. **Grid Data:**
  - If `--small3` is used, grid data (dummy variables) are **skipped** to reduce file size.

---

## See Also

- [NetCDF Documentation](https://www.unidata.ucar.edu/software/netcdf/)
- [SHOM (Service Hydrographique et Océanographique de la Marine)](https://www.shom.fr/)