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Delineation of Road Sections:
The lateral extent of each roadway section boundaries of each roadway section (2 lane sections and 2 shoulder sections for each segment) were necessary to determine which of the LIDAR data points corresponded to a particular section. In order to define lane and shoulder regions, the location of the edge of pavement, centerline, and edge of shoulder was necessary. The four individual sections for each segment included:

• northbound shoulder (NS)
• northbound pavement (NP)
• southbound pavement (SP)
• southbound shoulder (SS)

Roadway boundaries were defined using each of three different datasets including the 6-inch resolution orthophotos, 12-inch resolution orthophotos and a surface model. The surface terrain model was created from the LIDAR data. The point cloud from randomly spaced LIDAR for Iowa Highway 1 corridor, which had an average point density of 1 point per 27 square feet, was used to develop a triangular irregular network (TIN) using the Spatial Analyst module in ArcView 3.2â. The surface model was tested since it is a direct product that would be available with any LIDAR data collection effort. Aerial images are frequently taken in conjunction with LIDAR data collection and can be planned to meet desired resolution requirements for final orthophotos, but add to the cost and require additional processing. Therefore, the ability to use the surface model alone to determine roadway boundaries would be the ideal situation.

Each of the three datasets was used individually to create polygons that defined each region of the roadway segment. The polygons were then used to select the LIDAR points that corresponded to each section by polygon overlay to be used in the regression analysis. Figure 3 illustrates the use of each dataset to create boundary polygons.


Figure 3: Roadway delineation from: a) 6-inch Orthophoto b) 12-inch Orthophoto c) TIN from LIDAR
With the surface terrain model, the outer edge of the shoulder was the only feature that could be visually determined. As shown in Figure 3(c), only a rough outline representing the entire roadway was available using the TIN. The outer edges of the shoulders were established as the outer edges of the outline. The centerline of the road was determined by finding the midpoint of the area enclosed by the outer edges of the shoulder. Information relating to the lane width and the shoulder width were queried from GIMS database for each of the 10 test segments. Once a centerline was established, pavement edges were determined by adding the lane width for each section from the GIMS. The centerline and pavement edges defined the polygon for the northbound pavement and southbound pavement sections.

In the 12-inch resolution orthophotos, the shoulder edges could be determined for all 10 segments, but the centerline could not be consistently identified in the images. When the centerline was not readily identifiable, it was estimated by finding the midpoint from the delineated outer edge of the shoulders. Each of the four roadway sections was determined for each segment using the 12-inch resolution orthophotos. The edge of pavement, centerline, and edge of shoulder were clearly visible for each of the 10 segments in the 6-inch resolution orthophotos. Consequently for the 6-inch orthophotos, the boundaries of each of the four sections (NS, NP, SP and SS) were defined using the images alone.


Figure 4: Comparison of road segments derived by using the three baselayers

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