The pink, purple, and gray tints depicted on the USGS maps represent dense residential areas where there were too many houses and other buildings to be individually symbolized. Open, green, or white areas within the tint are collected as Other Urban or Built-Up category 17. Nonlinear residential developments are defined by a systematic road network, a minimum of three houses in 2.5 acres, a main highway, and a community population of at least 500. The evolutionary development of a transportation system requires that implicit and explicit compilation criteria be defined to aid in creating a principal transportation data layer. Implicit compilation criteria are based on various factors that together rate the quality of a transportation route.
Lineage is a measure that describes the documented history of a route.
It refers to the historical presence of a transportation route as depicted in literature or historical events. Mobility is a measure of the inherent road quality or design, which implies the level of service or accessibility for a particular route. Alignment is a measure of the rectilinear characteristics of a transportation route. One of the primary evolutionary features of roads has been based on the ability to straighten roads by overcoming geographic features that once were barriers to travel. The presence of ancillary transportation features, such as bridges, tunnels, and ferries, is one of the factors that support the application of implicit compilation criteria.