ILWIS objects

Polygon maps

A polygon map is a data object used to store spatial geographic information that consists of polygons, i.e. closed areas including the boundaries making up the areas. The areas are either codified by IDs, class names or values; this is determined by the domain of the map. The relation between polygons in a polygon map and the position on earth is defined by the coordinate system that the map is using.

Maps containing uniquely codified areas such as cadastral plots, or mapping units such as geological formations, land use classes, or soil units, can all be stored as polygon maps. Polygon maps are usually used as a stepping stone to raster maps. Polygon maps can be displayed in map windows, and edited with the polygon editor.

A polygon map can be obtained:

by importing an existing polygon map from ILWIS version 1.41,
by creating a segment map and editing it with the segment editor (with or without digitizer); then polygonize the segment map either in the segment editor or with the Segments to Polygons operation,
by performing ILWIS operations that return a polygon map as output, for instance the Attribute map of polygon map operation.

Like in ILWIS 1.4, you first have to digitize segments and then polygonize these segments to obtain a polygon map.

Contents of a polygon map and location of polygons:

The contents of a polygon map are defined by the domain that the polygon map is using. A polygon map may store for example:

individual area features with or without attributes; each polygon has its own mostly unique ID, for instance cadastral plot IDs; the map uses an ID domain,
classes of area features with or without attributes; polygons may occur in several places in the map, for instance land use classes or soil classes; the map uses a class domain,
area features representing measurable values; polygon values representing for instance population densities or concentration levels of air pollution; the map uses a value domain,
true, false or undefined polygons; the map uses a Bool domain,
output values (any domain) after performing an operation, e.g. IDs after the Unique ID operation,
attribute values (any domain) when an attribute map was created from an attribute table with the Attribute map of polygon map operation.

For more information on domains, refer to Basic concepts : working with domains.

The spatial location of polygons in a polygon map is defined by the coordinate system that the polygon map is using.

For maps using a class or ID domain, you can create an attribute table which stores additional information on the classes or IDs in the map. Use the same class or ID domain for the attribute table as you used for the map. Then, add the attribute columns to the table. When you have for example a map with building blocks (coded by IDs), you can add an attribute table with the predominant landuse per building block and the number of residents per building block. Or, when you have a soil map (coded by classes), you can add an attribute table with pH, soil texture, etc. for each soil unit.

To create an attribute table, refer to How to create an attribute table.

The manner in which polygon maps are displayed is specified in the Display Options Polygon Map dialog box.

When a polygon map uses a class domain, you can display polygons in a single color, or by using or creating a representation class. In a representation class, every class can have its own color, and optionally you can use hatchings or patterns. For more information, see Representation Class editor and the Edit Repr Class item (polygon) dialog box.
When a polygon map uses a value domain, you can display polygons in a single color or you can use or create a representation value or a representation gradual. For more information, see Representation Value/Gradual editor.
When a polygon map uses an ID domain, you can display polygons in a single color or in multiple system colors. However, an ID polygon map will usually have an attribute table and you can then choose to display the map by one of its attribute columns. For class columns, you can select or create a domain class; for value columns, you can select or create a representation value or a representation gradual, etc. For more information, see How to display a map by one of its attributes.
When a polygon map uses a Bool domain, you have to specify colors each time you display the map.

For more information, see ILWIS objects : representations.

Tip:

To store the display setting of one or more maps displayed in a map window, save the map window as a map view; open the File menu in a map window and choose the Save View or the Save View As command.

Names of polygon maps:

In ILWIS 3, object names comply with Windows long file names. Also Universal Naming Convention (UNC) paths are supported. For more information, see How to use long object names.

To create a polygon map:

To create a polygon map, you first have to create a segment map in which you can digitize polygon boundaries. Furthermore, you can create a point map in which you can digitize the polygon names or IDs as label points. When you display both the segment and point map in one map window, you can easily see whether the areas enclosed by segments have a label point. Then, polygonize the segments from within the segment editor. For more information, refer to How to create polygon maps or to Segment editor. To check after polygonization whether all polygons obtained a class name, ID or value, you can use the Polygons to Points operation.

To display a polygon map:

The easiest way to display a polygon map is to double-click the map in the Catalog. The polygon map will be displayed in a new map window. You can also drag a polygon map from the Catalog to an existing map window, in order to show that polygon map on top of other information shown in the map window. For other methods, see How to display maps and tables.

In the Display Options Polygon Map dialog box, you can specify how the map should be displayed.

To edit a polygon map:

Class names, IDs, or values of selected polygons can be edited in the polygon editor. To change the shape of polygons, or to delete or undelete polygon boundaries you need to use the segment editor; then re-polygonize the segment map.

To open the polygon editor, you can click a polygon map with the right mouse button in the Catalog and subsequently choose Edit from the context-sensitive menu. When a polygon map is already displayed in a map window, you can also choose the Edit Layer command from the Edit menu in the map window. For other methods, see How to edit point, segment, polygon and raster maps. In the polygon editor, you can use a digitizer but you can also use the mouse pointer. Dependent polygon maps and read-only polygon maps cannot be edited.

Operations on polygon maps:

The most frequently used operation on polygon maps is probably the Polygons to Raster operation which rasterizes a polygon map. During this operation, you have to select or create a georeference for the raster map to define the size of the pixels in the raster map. Another operation on polygon maps is for instance Attribute map of polygon map.

You can perform operations on a polygon map by selecting an operation from the Operation-list or from the Operations menu (for other methods, see How to start operations). Subsequently, fill out the appearing dialog box of that operation and click the OK button; this generates an ILWIS expression on the command line. Output objects that are obtained through an operation's dialog box are always dependent. Advanced users can type the complete ILWIS expressions on the command line of the Main window or create a script to execute a series of expressions.

For more information on dependent maps, see Basic concepts : dependent data objects. For more information on operations, refer to ILWIS operations.

Technical information:

An ILWIS 3 polygon map consists of an ASCII object definition file (.MPA) and two binary data files (.MPAP#, .MPAT#). The object definition file contains further references to the domain and the coordinate system that the polygon map is using; these are properties of a polygon map.

When opening or displaying an ILWIS 2 polygon map, the ILWIS 2 data files of the polygon map (.PC#, .PD#, .PL#, .PS#, .TP#) will be used. Only when an ILWIS 2 polygon map is edited, the map will be converted to and saved in the new ILWIS 3 data file format for polygon maps (.MPAP#, .MPAT#).

By viewing the properties of a polygon map, you can see whether the map is dependent or not, which other objects the map is using, etc. For dependent maps, you can also manage dependencies: break dependencies, make dependent maps up-to-date, delete the data files of a dependent maps, etc. For more information, see Basic concepts : properties of objects.

A polygon map stores one or more polygons. A polygon is nothing more than one or more segments (see also ILWIS objects : segment maps), which together define the complete boundary of the polygon, and an ID, a class name or a value to identify the polygon.

For topological polygons, each segment is used both to define the boundary for the polygon on the left as well as for the polygon on the right of that boundary.
The boundary of non-topological polygons is defined by a single segment for the outer boundary and optional segments for possible inner boundaries (holes in the polygon). Segments are not re-used.

If you wish, you can open a polygon map as a table: click the polygon map with the right mouse button in the Catalog, and subsequently choose Open as Table from the context-sensitive menu. In the table window, you can view the class names, IDs, or values of all individual polygons in the map, the area of each polygon, and the coordinates of the bounding box around each polygon.

Limitations:

In ILWIS 3.x, limitations for number of polygons per polygon map, number of polygon boundaries per polygon, or number of points per polygon boundary, only depend on vailable memory. However, ILWIS is not designed to work with very large data sets: to speed up processing you may need to split maps into smaller logical parts.