# Lidar preprocessing
<!-- <div class="algorithm-id">
<strong>Algorithm ID</strong>: <code>hedgetools:lidarpreprocessing</code>
</div> -->
```{container} algorithm-id
**Algorithm ID**: `hedgetools:lidarpreprocessing`
```

Preprocess lidar point cloud COPC files. COPC files are point clouds files optimized for spatial request and can be created from common LAS/LAZ files using QGIS [pdal:createcopc](https://docs.qgis.org/3.44/fr/docs/user_manual/processing_algs/qgis/pointclouddatamanagement.html#create-copc) algorithm. Preprocessing operations available are display below panels for parameters and outputs and are calls to [PDAL](https://pdal.org/en/2.10.0/) library algorithms.

Preprocessed point clouds files are saved in a destination folder.


## Parameters

| Label                                        | Name           | Type                                                   | Description                                                                                                                                 |
| -------------------------------------------- | -------------- | ------------------------------------------------------ | ------------------------------------------------------------------------------------------------------------------------------------------- |
| COPC lidar files                             | `LIDAR_FOLDER` | [folder]                                               | Folder with COPC lidar files.                                                                                                               |
| Lidar preprocessing operations <br> Optional | `OPERATIONS`   | [string]  <br> Default: None                           | JSON string containing preprocessing operations with parameters. Choice of operations is made using `Lidar preprocessing operations` panel. |
| Mask    <br> Optional                        | `MASK`         | [vector: polygon]                                      | Polygon layer to clip lidar point cloud.                                                                                                    |
| Parrallel threads                            | `WORKERS`      | [numeric: integer]                                     | Number of parrallel threads.                                                                                                                |
| Destination folder                           | `OUTPUT`       | [string]    <br> Default: `[Save in temporary folder]` | Folder to save preprocessed point cloud COPC files.                                                                                         |

## Operations

### Height Above Ground normalization
Normalize height with nearest neighbor or delaunay filer.

PDAL documentation:

- https://pdal.io/en/2.8.4/stages/filters.hag_delaunay.html
- https://pdal.io/en/2.8.4/stages/filters.hag_nn.html


| Name       | Type                        | Description                                  |
| ---------- | --------------------------- | -------------------------------------------- |
| `Strategy` | [string] <br> Default: `nn` | Normalization strategy (`delaunay` or `nn`). |

### Assignment

Assign values for a dimension range to a specified value.

PDAL documentation:

-  https://pdal.io/en/2.8.4/stages/filters.assign.html


| Name         | Type     | Description                                                  |
| ------------ | -------- | ------------------------------------------------------------ |
| `Condition`  | [string] | Condition to assign value. Must be writted with PDAl syntax. |
| `Assignment` | [string] | Assignation. Must be writted with PDAL syntax.               |


### Range filter

Filters dimension within a given range.

PDAL documentation:

-  https://pdal.io/en/2.8.4/stages/filters.range.html


| Name             | Type                                    | Description                                                               |
| ---------------- | --------------------------------------- | ------------------------------------------------------------------------- |
| `Field`          | [string] <br> Default: `Classification` | Point cloud dimension/field to filter (`Classification`, `X`,`Y` or `Z`). |
| `Inferior bound` | [numeric: integer]   <br> Default : 1   | Inferior bound for range filter. Keep values above.                       |
| `Superior bound` | [numeric: integer]   <br> Default : 99  | Superior bound for range filter. Keep values below.                       |

### Statistical Outliers Removal

Remove point cloud outliers.

PDAL documentation:

-  https://pdal.io/en/2.8.4/stages/filters.outlier.html


| Name                           | Type                                            | Description                                             |
| ------------------------------ | ----------------------------------------------- | ------------------------------------------------------- |
| `Mean number of neighbors`     | [numeric: integer] <br> Default: 8              | Mean number of neighbors (statistical method only).     |
| `Standard deviation threshold` | [numeric: double]  <br> Default : 2.0           | Standard deviation threshold (statistical method only). |
| `Method`                       | [string]           <br> Default : `statistical` | The outlier removal method (`statistical` or `radius`). |
| `Radius`                       | [numeric: double]   <br> Default : 1.0          | Radius (radius method only).                            |