ILWIS objects

 

Segment maps    

 

 

A segment map is a data object used to store spatial geographic information that consists of lines, for example roads, rivers or contour lines. Segments are either codified by IDs, class names or values (height map); this is determined by the domain of the map. The relation between segments in a segment map and the position on earth is defined by the coordinate system that the map is using. Segment maps can be displayed in map windows, and edited with the segment editor.

A segment map can be obtained:

Contents of a segment map and location of segments:

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

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

The spatial location of segments in a segment map is defined by the coordinate system that the segment 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. To create an attribute table, refer to How to create an attribute table.

The manner in which segments are displayed is specified in the Display Options Segment Map dialog box.

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 segment 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 segment map:

Open the File menu of the Main window and select the Create Segment Map command, or double-click the New SegMap item in the Operation-list. In the appearing Create Segment Map dialog box, enter a name and description for the segment map, select or create a coordinate system, enter boundary values for the map and select or create a domain. For other create methods, see How to create a map.

To display a segment map:

The easiest way to display a segment map is to double-click the map in the Catalog. The map will be displayed in a new map window. You can also drag a segment map from the Catalog to an existing map window, in order to show that segment 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 Segment Map dialog box, you can specify how the map should be displayed.

To edit a segment map:

You can edit a segment map by clicking it with the right mouse button in the Catalog and subsequently choose Edit from the context-sensitive menu. When a segment 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.

Segment maps are edited with the Segment editor: you can insert new segments and delete and undelete existing ones, edit the class names, IDs, or values of segments, change the shape of segments, split and merge segments, and pack, check and polygonize segments. You can use a digitizer but you can also use the mouse pointer. Dependent segment maps and read-only segment maps cannot be edited.

Operations on segment maps:

Frequently used operations on segment maps are for instance the Segments to Polygons operation to polygonize a segment map and the Segments to Raster operation which rasterizes a segment map. During the rasterize 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 a segment map is for instance Contour interpolation.

You can perform operations on a segment 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 segment map consists of an ASCII object definition file (.MPS) and a binary data file (.MPS#). The object definition file contains further references to the domain and the coordinate system that the segment map is using; these are properties of a segment map.

When opening or displaying an ILWIS 2 segment map, the ILWIS 2 data files of the segment map (.CD#, .SC#, .SG#) will be used. Only when an ILWIS 2 segment map is edited, the map will be converted to and saved in the new ILWIS 3 data file format for segment maps (.MPS#).

By viewing the properties of a segment 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 segment map stores one or more segments. A segment is nothing more than a sequence of XY-coordinates and an ID, class name or value to identify the segment. Each XY-coordinate in a segment is stored as a pair of 8-byte floating point reals (doubles). The first and the last XY-coordinate in a segment are called nodes. The XY-coordinate of a node can be used by multiple segments. Thus, nodes are required to begin and end individual segments and they can be used to connect two or more segments. Inserting a new node into an existing segment is called splitting; connecting a new segment to an existing node is called snapping.

If you wish, you can open a segment map as a table: click the segment 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 segments in the map, the length of each segment, and the coordinates of the bounding box around each segment.

Limitations:

In ILWIS 3.x, limitations for number of segments per segment map, or number of points per segment, only depend on available 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. You can also try to reduce the number of coordinates in segment maps by using the Tunnel Segments operation.

See also: