Collapsing dual lines (double-lined features) to centerlines is a type of generalization operation (Esri, 1996). When map scale reduces, features represented by dual lines at the larger scale may look "collapsed" at the smaller scale; that is, the spacing between the dual lines may not be visible anymore. Therefore, these features need to be represented by single lines. Since Collapse Dual Lines to Centerline was designed to work with fairly regular, near parallel pairs of lines, such as road casings, you may get unsatisfactory results if you use it to create a skeleton from irregularly shaped features, such as hydrographic data.
Although there are no restrictions on the input data, if it has a proper topology and complete casing pairs, you will get better results. A single, open line or a broken casing may result in confusing conditions.
The tool will derive single lines centered inside casings based on given width parameters. A dual-line feature wider than the Maximum Width or narrower than the Minimum Wwidth will not be centerlined. Use known casing widths when setting the parameters if possible. However, since casings usually become wider at intersections, set the Maximum Width greater than the known maximum width. You may have to experiment to find suitable parameters.
If the width of the input data is relatively constant and the intersections are simple, a complete centerline result can be produced, that is, LTYPE = 1 for all resulting lines. Otherwise, unused lines (such as a single casing or casings with a width beyond the specified range) and outlines around complicated intersections will be flagged with an LTYPE value of 2 for editing them further.
The process will partition input data that exceeds 500 arcs. Centerlines are created for each partition and then merged. The partition lines will be included in the result with LTYPE = 3 so you can check the connections along these lines and delete them.
The items LL#, RL#, L-ID, and R-ID in the out_cover.AAT make it easy to relate the resulting centerlines to their source casings. The items will have the value of zero for noncenterline arcs, that is, the arcs with the LTYPE values of 2 and 3.
If the input data contains casings with distinctive widths, it is better to give the features different codes so you can select them and run this tool with relevant widths separately.
This tool would not be able to recognize the intended casing pairs where a number of lines run parallel with their widths narrower than the specified Maximum Width, and it won't produce all intended centerlines since it skips every other pair of casings. This tool may not always differentiate casings from the space between casings if they both are smaller than the specified Maximum Width. If this is the case, unintended centerlines may be produced.
Technical paper, Esri, "Automation of Map Generalization: The Cutting-Edge Technology", 1996. It can be found in the White Papers section at this Internet address: http://downloads.esri.com/support/whitepapers/ao_/mapgen.pdf