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I'm using data from the Transportation section of the National Map (US). They provide three layers for roads: Trans_RoadSegment, Trans_RoadSegment2, and Trans_RoadSegment3. I seem to recall this scheme from other GIS data--the same road is split into multiple layers.

Example road segments

In the example above, the road is split into three layers. Some of the segments overlap others, some take a slightly different route, and some do not overlap at all. In any case, if you were to display only one of the layers, there would be gaps in the road. Why is this done? Why isn't the road data all on a single layer? The National Map provides a data key poster, but I don't see any information about this split there.

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I imagine this is due to the file size limitation of Shapefiles. From "Geoprocessing considerations for shapefile output" (link):

There is a 2 GB size limit for any shapefile component file, which translates to a maximum of roughly 70 million point features. The actual number of line or polygon features you can store in a shapefile depends on the number of vertices in each line or polygon (a vertex is equivalent to a point).

The roads are broken into separate layers to keep the files under the legal limit. I grabbed the Texas SHP from the USGS National Map downloader and the roads are in eleven shapefiles that are about 1.44GB each, plus the spare change in a twelfth .

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