# Use Polygon Map Format

The map background may consist of a set of polygons; these are defined by a table in the Data Pool that must contain at least these fields:

| Field Name | Contents                                                                                                                                                                                                        |
| ---------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| Name       | The name of the layer.  For geography, this is typically the country.  A string.                                                                                                                                |
| Layer      | An integer identifying a group of related **polygons**.  This is 1:1 with Name.                                                                                                                                 |
| Face       | An integer identifying a singled closed **polygon** with the Layer.  A Layer may be composed of multiple Faces.  This supports describing country geometries where a single closed **polygon** is not adequate. |
| X          | X coordinate of a point in the **polygon**.                                                                                                                                                                     |
| Y          | Y coordinate of a point in the **polygon**.                                                                                                                                                                     |

Note that no Z coordinate may be supplied; the **polygons** are only drawn on the 0 Z coordinate plane.

This table is used to draw the background **polygons**. Each set of coordinates for the same Layer Face is drawn as a closed **polygon**. Since this is a Data Pool table, it may be colored by any of these fields or by additional fields in the table. The color for the first coordinate in each Layer Face is used to color all of the **polygon** Faces in that Layer: a layer has a single color. "Name" is otherwise unused currently.

A typical use is to give **polygons** for a **map**, such as a world **map**. In this case, "Name" would be country name, "Layer" would be an unique integer ID for each country, and "Face" would be a closed **polygon** for a portion of the geometry of a country. Countries with simple boundaries that can be described by a single closed **polygon** have a single Face (e.g., Australia); countries with more complex outline use multiples Faces (e.g., Japan). The first two rows of the table must contain the coordinate range of the **map**. For example, for a **map** of the earth the coordinate range is given as:

| Name     | Layer | Face | X    | Y   |
| -------- | ----- | ---- | ---- | --- |
| BOUNDARY | 1     | 0    | -180 | -90 |
| BOUNDARY | 1     | 0    | 180  | 90  |

Note that the **polygon** must be closed: the first and last points must be the same.

To color each country separately, color the table by "Name".

For **polygon** backgrounds, the fourth token may be the name of a numeric field in the **polygon** table that is used to set the heights of the background polygons. A copy (join) may be used to merge numeric data fields from another table into the **polygon** table.