# Data - Preparation

This module allows you to create topological hedge layers:

- A **line layer** representing the median axis of each polygon.  
- A **point layer** representing line extremities.  
- A **polygon layer** split at each node with a degree greater than or equal to 3.

These three layers together represent your hedge objects. The hedge geometries are linked via unique index fields created by this module:

- `pid` – Polygon ID.  
- `eid` – Edge ID.  
- `vid` – Vertex ID.

```{admonition} Remark
:class: remarque
These fields act as Primary Keys (PK) and Foreign Keys (FK):

- A **Primary Key** (PK) is a unique identifier for each record in a table (e.g. each polygon, line, or point has its own unique ID).
- A **Foreign Key** (FK) is a field that references a **Primary Key** in another table, creating a link between related data.

In HedgeTools, these IDs allow us to:

- uniquely identify each geometry element (polygon, line, point)
- maintain relationships between them (e.g. which line belong to a polygon, which points define a line) and build the hedge object.
```

An optional algorithm can be applied to simplify the median axis, reducing geometric irregularities and smoothing the line.

```{toctree}
:maxdepth: 1
:caption: Hedge creation steps

topological_arc
topological_node
topological_polygon
simplify_arc
```