# Legacy TCM Conversion

TunnelCAD can convert project files created with earlier versions of the application. The old format uses the `.tcm` extension; the conversion produces a native `.tun` project.

## Converting a TCM file

1. Open **File > Convert TCM...**.
2. Browse to the `.tcm` file and confirm.
3. TunnelCAD reads the legacy data, converts it, and opens the result as a new project.

```{note}
This is a **one-way conversion**. TunnelCAD does not export back to the TCM format. Once converted, save the project as a `.tun` file to preserve it in the current format.
```

## What gets converted

The converter reads all major data from the TCM file:

- **Alignment geometry** -- the alignment section of a TCM file uses the same format as ALM (alignment) files. TunnelCAD reuses its ALM parser to read this data, so all element types (lines, arcs, spirals) are fully supported.
- **Templates** -- design profile geometry including arc segments. Template arcs are stored in TCM files as bulge values rather than tessellated polylines, which preserves curve accuracy during conversion.
- **Cross-sections** -- measured section data at each station.
- **Line definitions and styles** -- section line properties and display settings.

## Character encoding

Older TCM files may contain text encoded in Windows-1253 (Greek character set) rather than UTF-8. The converter automatically detects and cleans up non-UTF8 characters so that station names, descriptions, and other text display correctly after conversion.

## Recommended workflow

1. **Convert** the TCM file using **File > Convert TCM...**.
2. **Review** the imported data -- check alignment geometry in the Alignment tab, inspect sections in the Sections tab, and verify templates in the project tree.
3. **Save As** a `.tun` file to preserve the converted project in the current format.

```{tip}
Keep the original `.tcm` file as a backup. The conversion does not modify the source file.
```