ILWIS objects

 

Raster maps    

 

 

A raster map is a data object used to store spatial geographic information and remote sensing data as pixels (picture elements) of a certain size, e.g. 20 x 20 m. These pixels are either codified by IDs, class names, values or colors; this is determined by the domain of the map. The relation between pixels in a raster map and the position on earth is defined by the georeference that the raster map is using.

Raster maps can be displayed in a map window, and can be edited with the pixel editor. You can calculate with raster maps (MapCalc) and you can perform many other raster and image processing operations on them such as: Filter, Cross, Distance calculation, etc. In ILWIS, most spatial operations are performed on raster maps.

A raster map can be obtained:

As mentioned above, the output of spatial operations is usually another raster map.

When working with the multi-spectral bands of a satellite image or with multiple raster map of a time series, you can combine these bands or maps into a map list.

Contents of a raster map and location of pixels:

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

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

The spatial location of the pixels in a raster map is defined by the georeference that the raster 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 raster maps are displayed is specified in a Display Options Raster 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 raster 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 raster map:

Usually, you have to rasterize a point, segment or polygon map with the Points to Raster, Segments to Raster or Polygons to Raster operation. In the Rasterize process, you have to specify the dimensions of the raster map's pixels in the georeference of the raster map.

To display a raster map:

The easiest way to display a raster map is to double-click the map in the Catalog. The map will be displayed in a new map window. For other display methods, see How to display maps and tables.

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

Tip:

To speed up the display of large raster maps e.g. maps consisting of a few thousand lines and columns or more, you can create Pyramid layers. For more information, refer to ILWIS objects : Raster maps - Pyramid layers.

To edit a raster map:

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

Raster maps are edited with the Pixel editor; you can edit the class names, IDs, or values of the pixels in the raster map. Dependent raster maps and read-only raster maps cannot be edited.

Operations on raster maps:

You can calculate with raster maps by typing a Map Calculation formula on the command line of the Main window or in the Map Calculation dialog box. When the definition symbol = is used in a calculation, the output map is dependent.

You can also perform other operations on a raster 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, see Appendices : ILWIS expressions.

When the output map of a calculation or another operation uses a value domain, you can usually define a value range and precision for the output map, see below. Operations on raster maps are for instance Filter, Cross and Distance calculation. For more information on dependent maps, see Basic concepts : dependent data objects. For more information on operations, refer to ILWIS operations.

Technical information:

A raster map consists of an ASCII object definition file (.MPR) and a binary data file (.MP#); when pyramid layers have been created for a raster map, also an .MPY# file is available. The object definition file contains further references to the domain and the georeference that the raster map is using; these are properties of a raster map.

By viewing the properties of a raster map, you can see whether the map is dependent or not, which other objects the map is using, etc. In the properties dialog box of a value raster map, you can select the Interpolation option to obtain interpolated values on sub-pixel level. For dependent maps, you can manage dependencies: break dependencies, make dependent maps up-to-date, delete the dependent map's data file, etc. For more information, see Basic concepts : properties of objects or the Raster Map Properties dialog box.

The IDs, class names or values of pixels are stored line by line, starting from the first pixel (left) on the first line in the map (up), to the last pixel in the last line. Raster maps are stored as bit, byte, 2-byte, 4-byte or 8-byte maps. The store type of a raster map is primarily defined by the domain that the map is using. For value maps this is determined by the value range and precision that you specify for the output map during a calculation or operation. Usually, ILWIS will come up with defaults, but you can adjust these values. Choosing a wide value range and a very small precision results in a large data file on disk as more bytes are needed to store each pixel value in the map, also the speed of display and the speed of calculations may decrease.

For value maps that use 1 or 2 bytes per pixel, the map's histogram is automatically calculated when you open the map and a histogram is not available yet. For value maps that are stored using 4 or 8 bytes per pixel, you need to use the Histogram operation to obtain the map's histogram.

Limitations:

In ILWIS 3.x, limitations on maximum number of lines and columns have been removed. In practice however, general limitations depend on available disk space; for printing purposes limitations depend on available memory. Furthermore, 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.

See also: