Addressing the Needby Alnoor Ladak, GIS Coordinator for Roads, Civil Engineering Department |
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Eighteen months ago, Roads GIS in conjunction with the Centre for GIS embarked on an ambitious project that was to culminate in the replacement of the existing Roads Centreline coverage. The goal of the project was to deliver a high quality current and accurate roads centreline network for the State of Qatar complete with attributes such as zone and street numbers for each road, street names, road hierarchy information, and perhaps most important, address ranges. It was envisioned that the availability of such a high quality coverage with the appropriate data would have a tremendous impact on GIS users as the data has many potential uses.
Implementing the goal in a cost-effective, time-efficient manner proved to be a significant task, especially if the whole exercise, such as digitizing the country's entire road network and assigning the required attributes; were to be implemented manually. To overcome this problem, the first phase, which was launched in May 1995, focused on generating road centrelines automatically from existing data sources. The development phase of this project lasted six months resulting in the Automated Flowlines Generation Process. This process generates road centrelines automatically. A brief description of this process was given in Al-Khabar's Summer 1996 issue. The complete process is explained in a paper available on the 1996 ESRI User Conference Proceedings CD. The roads centreline network for the State of Qatar's urban areas was regenerated over a span of six months processing approximately 950 mapsheets. Since then, the road network is automatically updated once a week based on changes to the Urban layer.
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A road centreline with the resulting attributes transferred automatically by the Address Range Process |
Completing Phase I was a major milestone in the Roads Spatial Data Infrastructure project. With the geometry part accomplished, the second phase of the project focused on determining how to automatically attach specific attributes to the road network arcs. The premier task was to determine how street numbers, street names and address ranges could be captured. This data was already available on the Planning Department's Land Plan polygon layer.
The Land Plan layer contains what is known as right-of-way or road corridor polygons. These polygons demarcate the area within which roads and associated services are constructed. Each polygon is assigned certain attributes that are stored in an Oracle table and accessed via the polygon's unique key. Among these attributes are the zone and street numbers. Once these numbers are accessed, the street name can be determined from the master street names Oracle table.
The technique developed to automatically attach the zone and street numbers to the road arcs, therefore works on one road corridor polygon at a time. Each time the road network coverage is overlaid on the polygon, all road arcs that pass through the polygon are selected. The attributes are then transferred to the selected arcs after some processing. This calculated coincidence is actually a good test of how the urban planning process and the actual road construction process - that are carried out by two separate government agencies - are congruous. Basically there should not be a road centreline where there is no road corridor.
Capturing the address ranges proved to be a more challenging task. The Land Plan layer also contains polygons that demarcate individual plots of land. Each plot is assigned an address by the Planning Department that comprises a plot number and a street number that links the plot to a given street. Theoretically it was thus possible to focus on a road corridor polygon and determine its associated plots using the street number that is common to both features. Once the plots are identified, the address ranges - the maximum and minimum odd and even plots numbers - for the street can be determined. These values can then be transferred to the appropriate arcs.
Although this approach worked, tests revealed that the placement of individual addresses on a given street could be improved to mimic reality more closely. In short, the placement of an individual address needed to be as close to the actual plot as possible. This was accomplished by focusing on the individual arcs that made up a street. Thus rather than only determining ONE address range for the entire street, sub-address ranges were determined for each individual arc in the street. This was accomplished by locating the closest arc to each plot and transferring the address to that particular arc. Thus each arc that is part of the street, has its own mini address range that enables addresses to be located more precisely.
Once the address ranges have been determined for each arc, the arc's direction is checked to ensure it is pointing in the direction of increasing addresses. This is important as all addresses that fall within the address range are extrapolated. This extrapolation is calculated based on the lowest value in the address range being arbitrarily located at the from-node of the arc and the highest value at the to-node. Thus if the arc is not pointing in the direction of increasing addresses, the results could be misleading.
To complete the address range transfer process, all arcs that are located at road intersections are processed. Intersections are special cases due to the numerous special situations that occur. Arcs that meet at an intersection could each belong to a different street; or pairs of arcs could belong to the same street. Also depending on the type of intersection, the number of arcs varies from 3 to as many as 16 in the case of roundabouts. These arcs are thus individually processed to determine which street they belong to, and assigned the proper attributes. Once the above process is complete, an address coverage is generated and quality controlled. Street name routes are also generated. At this point the coverage is ready for production.
The above process has been in development for a few months and is now in production. Attribute transfers are done on a zone basis with one to two zones processed daily. As anticipated, the attribute transfer has been completed in time for the GIS/GPS Conference 1997, and is available for public use on GISnet.