The Digital Line Graph (DLG) reader enables FME to import Level 3 DLG data and export it to any of the FME output formats. DLG is a published ASCII format developed by the United States Geological Survey (USGS) Federal Agency and is intended to assist in data exchange with the National Digital Cartographic Data Base (NDCDB).
The DLG reader supports all three distinct types of DLG data:
- large-scale DLG data (1:24,000 scale)
- intermediate-scale DLG (1:100,000 scale)
- small-scale DLG data (1:2,000,000 scale)
The three scales of DLG data are physically formatted into files in one of these ways: standard, optional, and graphics formats. FME supports both the standard and the optional DLG distribution formats. However, the graphics format is not supported. Most DLG data is distributed in the optional format.
Overview
DLG data files consist of ASCII fixed field records. The records may or may not be stored with embedded carriage returns or end of line markers. The DLG reader intelligently determines the end of each record, and interprets files with or without explicit end of record markers.
The DLG file structure was designed to accommodate all categories of spatial data represented on a conventional line map. Node, line, and area data types are present within the DLG format, along with linkages and attribute codes.
Linkages are references to other features within the same DLG data set, used in a variety of contexts.
DLG files do not explicitly store attribute values but use a feature coding approach in which unique feature codes are assigned to the different types of features stored within the data set. Each geometric entity present in a DLG file may be assigned major and minor attribute codes which always appear as a pair. Together these codes often form complex relationships to assign specific attributes for each feature. The attribute coding scheme is designed to accommodate basic cartographic data categories such as hypsography, hydrography, or political and cultural features, as well as additional thematic data categories. The FME supports a maximum of 12 attribute code pairs per feature.
The FME looks for an extension of either .dlg or .opt for the input DLG files, but accepts any DLG file as input regardless of file name or extension.
Although mapping files may be created from scratch to work with the features as presented directly by the DLG reader, starting with an FME generated mapping file provides an easy way to harness the enhanced semantic interpretation of all attribute codes and linkages built into the FME distribution. This section will first outline the features and attributes produced directly by the DLG reader. These features and attributes produced by using an FME generated mapping file are presented at the end of this section.