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A few years ago, the need for a highly accurate Road Centreline coverage was becoming imperative for a growing GIS user community within the State of Qatar. The availability of 1:1,000 scale topographic Urban base mapping from the Centre for Geographic Information Systems (CGIS) offered the best option of giving the required high quality output. The Civil Engineering Department (CED) Roads Division, GIS unit investigated options that would offer an economical yet cost-effective solution as resource restrictions did not allow a complete site survey of the State's road network. The conventional method of sending survey crews to the field proved to be expensive, time consuming and resource-intensive.
Software was developed and tested on the 1:1,000 scale topographic Urban basemaps in different areas of Doha with encouraging results. The development phase lasted six months resulting in the Automated Road Centreline (Flowline) Generation Process. The road flowline network for the State of Qatar's urban areas was regenerated by processing more than 950 mapsheets. The software is also used as a maintenance tool whenever the Road geometry changes, for example, when new roads are built or when a roundabout junction is converted to a signalised intersection.
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- Identify and select all road-related polygons (including intersections, roads under construction, and accessways) from the URBAN layer of the 1:1000 scale topographic base mapping; then,
- Select and densify all the road edges (i.e., line extents) of the road polygons from the URBAN layer by adding more points to follow the curvature of roundabouts and turns with the exception of the virtual extents; then,
- Convert all the densified lines forming the road edges to roas points; then,
- Convert all road points along the road edges to Thiessen polygons. An ARC/INFO tool called THIESSEN creates a Triangular Irregular Network (TIN) from these points, then forms these polygons from the intersections of the perpendicular bisectors of the TIN; then,
- Select all the lines of the THIESSEN polygons falling within the road polygons to form our newly generated ROAD FLOWLINES; then,
- Post-processing routines are applied to:
(a) DELETE Dangles or Overshoots along the Flowlines and
REMOVE "Forks' at the Road ends
(b) Straighten (ALIGN) Flolines at the intersection of Roads and Accessways. - Carpark polygons whose areas are greater than 1000 sq.m are added to ensure connecivity to isolated roads, thus completing the Flowline generation process.
The program which is now in production was developed using AMLs working in ARC, ARCEDIT, and ARCPLOT with ARC/INFO 7.0.3. The complete technical details are available on the following URL:
http://www.esri.com/library/userconf/proc96/TO400/PAP370/P370.HTM