Ilwis

CHAPTER 4

Spatial data management
In the previous chapter you have seen how spatial data is entered in ILWIS via digitizing. Another method for obtaining spatial data is to convert digital data into other file formats. Sometimes, you obtain data that might not be directly usable in an analysis. The data may still need to be edited. The data may be in the wrong data type for a specific type ofanalysis. You may have to convert data from one vector type to another (e.g. from segments to polygons), or from vector to raster (rasterization). The data may also contain the wrong coordinates, georeference, domain or representation. In this chapter the various spatial data management operations are shown, which precede the actual data analysis. The first part of this chapter (section 4.1)deals with the import of raster and vector maps from different file formats. In section 4.2, the topic of map projections is treated, and how you can change maps from one projection to another. In section 4.3, tools for vector data management are shown, and in section 4.4, the vector to raster conversion is demonstrated. Then in section 4.5, the tools for the management of raster maps are explained.The final section 4.6 focuses on the conversion of domains. Before you can start with the exercises, you should start ILWIS and change to the subdirectory C:\ILWIS 3.0 Data\Users Guide\Chapter04, where the data files for this chapter are stored.



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Double-click the ILWIS icon on the desktop. Use the Navigator to go to directory the C:\ILWIS 3.0 Data\Users Guide\ Chapter04.

ILWIS 3.0User’s Guide

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Spatial data management

4.1 Importing digital data
In the previous chapter you have seen how to digitize maps. However, sometimes maps may already be available in digital format from another source. They may have been generated using different software, they may have resulted from scanning maps, or they may be available as digital satellite images or scanned images. Inorder to use these digital data in ILWIS, the files need to be imported to the ILWIS file formats that are required. Importing can be done with the Import module, in which you can select the import method and format. While importing files into ILWIS, you must know what type of data you are importing: whether it is a point, segment, polygon or raster map, and what type of information is storedinside the map. The content of a map determines the domain type (class, ID, value, image, picture, color, etc.) that will be assigned to the imported map. Within ILWIS, you can use the normal ILWIS import, but you can also import external files via GeoGateway. GeoGateway is a product of PCI Geomatics (http://www.pcigeomatics.com) which allows programs to access data in many geomatics file formats, ina uniform manner, without having to translate them before use in applications. GeoGateway includes access to imagery, vectors, attributes, projections and other auxiliary information of interest to users of geomatics applications. When using GeoGateway, you can choose to: - perform a genuine import and translate the external file to an ILWIS format: - the external data will be translated intoILWIS file formats: ILWIS object definition files (e.g. .MPR, .TBT) as well as ILWIS data files will be created. - you can view the imported data, calculate with it, use it in ILWIS operations, etc. - the imported data can be edited in ILWIS. - keep the external file in its original format (Use As): - the external data will not be translated into ILWIS file formats: only ILWIS object definition files(e.g. .MPR, .TBT) will be created; as the same data file will not appear twice on your hard disk in different formats, you will save some hard disk space in this way. - you can view the imported data, calculate with it, use it in ILWIS operations, etc. - the imported data cannot be edited in ILWIS however. After you used Import via GeoGateway, you will obtain an ILWIS object collection . The...