# Attaching IQPC Files

## What is IQPC?

IQPC (IQsoft Point Cloud) is TunnelCAD's native cached point cloud format. It stores point coordinates, colors, and a spatial octree index in a compact binary layout optimized for fast loading and spatial queries.

When you import a raw point cloud (LAS, E57, etc.), TunnelCAD automatically creates an IQPC cache file alongside the project. On subsequent sessions, the IQPC file is loaded directly instead of re-parsing the original format -- typically an order of magnitude faster.

## Attaching an existing IQPC file

To re-link a previously created IQPC file without re-importing raw data:

- **Menu**: Point Cloud > Attach (.iqpc)...

A file dialog opens. Select the `.iqpc` file and it is loaded into the project immediately.

## When to use Attach

- **Reopening a project** -- if the IQPC cache was moved or the automatic cache path no longer resolves, use Attach to point TunnelCAD at the correct file.
- **Sharing between projects** -- multiple tunnel projects can reference the same IQPC file. For example, if two tunnel alignments pass through the same scan area, both projects can attach the same cached cloud.
- **Working on a different machine** -- copy the IQPC file to the new machine and attach it rather than re-importing the original raw data (which may be very large or unavailable).

## File management

```{note}
IQPC files can be large (comparable in size to the original point cloud). Keep them on a local or fast-access drive for best performance. Loading IQPC files over a network share may be noticeably slower.
```

- IQPC files are independent of the raw source data. Once created, the original LAS/E57/PLY file is not needed for viewing or section extraction.
- Deleting an IQPC file does not affect the project -- you can always recreate it by re-importing the raw data.
- The IQPC format includes the octree spatial index, so no additional indexing step is required after attaching.