Sewer condition assessment - GIS database without introducing processing errors
Project Scope
The Large Diameter Sewer Condition Assessment and Inspection Program includes a detailed
analysis of the 15-inch through 42-inch pipe including the following task items:
- Provide a CCTV inspection of all the pipes which are within the size requirements;
- Provide manhole observations as the CCTV inspections were completed;
- Place all inspection data onto the COP Water Department GIS including defect locations,
defect types, and photographs of the defects;
- Provide an assessment of all the pipes including defect scoring and impact factor scoring
onto the COP Water Department GIS; and
- Provide a Capital Improvement Program (CIP) based upon the information listed above.
The COP™s expectation of this project was to meet the Condition Assessment and Rehabilitation
requirements of the pending EPA Collection System Regulation CMOM section. These
expectations included:
- CCTV Inspection
- Rating System for Pipe Defects and Establish Priorities for Rehabilitation and Repair
- Identify and Repair the Imminent Failure Locations
- Prepare Long-Term Rehabilitation and Repair Program including priorities
- Prepare Capital Improvement Program including costs and schedule of repairs
City of Phoenix Database Preparation
The COP Water Engineering and GIS Department had to provide a database of the sewer system
prior to the award of the project. The process of extracting the desired data from the COP Water
Department Geographic Information System (GIS) required several steps to achieve the correct
format. The first step was to use ArcInfo and ArcEdit, which is the platform that the data
currently reside in, to query the database for sewer pipe 15-inch to 42-inch of non-concrete pipe
material, such as Vitrified Clay Pipe (VCP) and Ductile Iron Pipe (DIP). A coverage was created
by doing a iogetld on the sewer lines that met this criterion. The data in this coverage had to go
through a clean-up process for the following reasons: (1) the COP GIS information is stored in a
quarter section by quarter section, or what is termed as a tile by tile basis, and (2) to assure for
the highest accuracy possible based on COP GIS current records. Because of the tiled data format, the pipeline segments had to be snapped together across tile (quarter section) boundaries,
and lines needed to be un-split in order to remove unnecessary breaks in lines or pseudo-nodes.
For quality assurance, flow direction was checked, pipe material in question was re-verified, and
missing pipelines were added.
The next steps in the process involved much data manipulation. The information or the
attribution data about the manholes, inverts, stationing, etc., is contained on the nodes (at the
ends of each line segment) and not on the line segment itself. For the purposes of the project, the
consultant needed this information on the line segment. Therefore, the information had to be
transferred from the nodes to the lines (arcs) by relating the two through their common unique
identifier, the arc number. At that point, the COP GIS Department chose to compare the GIS
coverage and its associated data to a Sewer Flow Modeling database that is maintained in the
COP Wastewater Engineering Division.
The Flow Modeling database not only contains the information that the COP GIS Department
maintains in the GIS, but also additional information such as slope and basin designation for
each sewer pipe. In order to compare two databases, the COP GIS Department used the
ArcView TM platform. The GIS coverage that the COP GIS Department created was converted to
an ArcView shapefile and the Flow Modeling database, maintained in Fox Pro and imported to
Microsoft Access, was imported into ArcView as a database file. Before a comparison could be
done, a couple of obstacles had to be overcome. The two databases did not have a common field
that could be compared, and the quarter section numbering system was not the same between the
two tables. It was decided the GIS fields would be manipulated to fit the schema of the Flow
Modeling database. This was done by creating a look-up table in ArcView that would be used to
match the GIS quarter section numbers to that of the Flow Model. Once the quarter section
numbering systems matched, the fields in the GIS database table were manipulated to create the
primary key field that would be used as the common field. Next, the Flow Model table was
joined to the GIS table to create one table containing their combined information. By joining the
tables, the COP GIS Department discovered some records did not match up or, in other words,
discrepancies were found. Some examples of these were missing manholes, pipe segments,
attribution, etc., which was caused because the pipe was in an area that had not been converted to
GIS or updated. Several hours of research on these unmatched sewer pipe segments and updates
to both the GIS data and the Flow Modeling data were done until a 100% match was achieved.
The pipe segments that needed to be added to the GIS database were edited and added in ArcEdit
and then re-converted to an ArcView shapefile. The final shapefile of the combined databases of
sewer lines 15-inch to 42-inch made of non-concrete pipe was then delivered to URS, along with
other shapefiles containing pertinent COP data, such as city limits and street information.
