8.3 Blueprints, GIS, Asset Management & System Mapping

8.3 Blueprints, GIS, Asset Management & System Mapping

Exam Focus: Two criteria tasks: interpret blueprints, GIS, and sketches of the system showing location and configuration of collection system components, and update GIS and/or asset management (e.g., manhole location, GPS, pictures). Plan-and-profile literacy is a genuine Class I skill.

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Reading a Sewer Plan and Profile

Sewer construction drawings are drawn as paired views on a single sheet.

Plan view (upper half) shows the horizontal alignment looking down: the pipe centerline, manhole locations and numbers, street right-of-way, easements, existing utilities, and service laterals. It carries a north arrow, a graphic scale, and a legend.

Profile view (lower half) shows the same run in vertical section, aligned column-for-column with the plan above it. It carries:

• Existing ground surface as one line and finished grade as another.
• The pipe drawn between manholes with its invert elevations called out at each structure.
• Rim elevations at each manhole.
• Slope, length, size, and material annotated on each run, such as 210 LF – 8" PVC SDR 35 @ 0.40%.
• Utility crossings shown at their measured elevations.

Stationing

Horizontal position along the alignment is expressed in stations of 100 feet. Station 0+00 is the baseline origin; 3+50 is 350 feet from that origin; 12+75 is 1,275 feet. Stationing lets the plan view and the profile view refer to the same point, and it is how field locations are called out.

Elevations, Datum, and Benchmarks

All elevations reference a vertical datum (commonly NAVD 88, though older drawings may use NGVD 29 or a purely local datum). A benchmark is a physical monument of known elevation used to transfer that datum to the work. Two rules follow:

1. Never mix datums. A drawing on NGVD 29 and a GPS reading on NAVD 88 differ by a foot or more in most of the country — enough to lay a sewer at the wrong grade.
2. Verify the benchmark before setting grade. Benchmarks get disturbed by construction.

Invert is the inside bottom of the pipe; crown is the inside top; rim is the top of the manhole casting. Cut depth to invert is rim or ground elevation minus invert elevation (Section 6.3).

Design Drawings Versus Record Drawings

Design drawings show what was intended. Record drawings — commonly called as-builts — show what was actually built, including field changes, revised alignments, and the true depths encountered. For any field work, record drawings govern. Where they conflict with the design set, the record set is the better information, and where neither exists the crew must locate and pothole.

Sketches — the hand-drawn field record of a manhole, a tap location, or a repair — are the raw material from which record drawings and GIS updates are made. A good sketch carries at least two measured swing ties from permanent objects (a hydrant, a building corner, a property pin), not just an approximate note.

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GIS Fundamentals

A collection system geographic information system stores each asset as a spatial feature with an attribute record.

• Point features: manholes, cleanouts, lift stations, valves, service connections.
• Line features: gravity mains, force mains, laterals.
• Attributes: unique asset ID, diameter, material, install year, upstream and downstream invert, length, slope, last inspection date, PACP condition score, basin, ownership.
• Connectivity: which pipe drains to which — this is what allows the GIS to trace upstream from a complaint to find every contributing property, or downstream to find what a spill will reach.

GPS Accuracy

Not all GPS is equal, and the distinction matters when you are recording an asset that someone will later dig for.

Vertical accuracy from GPS is typically two to three times worse than horizontal, which is why invert elevations are established by survey level, not by GPS.

Updating the Record

The criteria name updating GIS as an operator task, and the specific items listed — manhole location, GPS, pictures — are exactly what field crews contribute:

• Correct a manhole plotted in the wrong place or missing entirely.
• Record newly discovered assets: buried manholes found during cleaning, unmapped laterals found during CCTV.
• Attach photographs of the frame, the interior, and the surroundings to the asset record.
• Record what was actually found: material and diameter that differ from the map, a pipe flowing the opposite direction from what the map shows.
• Log the inspection and its PACP scores against the asset.

A GIS is only as good as the field feedback loop. A crew that finds an error and does not report it guarantees the next crew repeats the wasted trip.

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Asset Management

Asset management is the discipline of getting the required level of service from the system's physical assets at the lowest life-cycle cost and an acceptable level of risk. It answers five questions: What do we own? Where is it? What condition is it in? What is its remaining life? What does it cost to keep it in service?

Risk-Based Prioritization

Work is ranked by risk:

$\\text{Risk} = \\text{Likelihood of Failure} \\times \\text{Consequence of Failure}$

• Likelihood of failure (LoF) comes from condition — PACP structural grades, age, material, corrosion history, break history.
• Consequence of failure (CoF) comes from criticality — diameter and flow carried, depth, whether it crosses a stream or a highway, whether it serves a hospital, and what a failure would discharge to.

This is why a Grade 5 defect in a 6-inch line on a quiet cul-de-sac may rank below a Grade 3 defect in a 36-inch interceptor under a river crossing. Condition alone does not set priority; consequence does as much work.

Outputs of the program are a capital improvement plan, a rehabilitation-versus-replacement decision for each asset, and a defensible answer when a regulator asks how the utility decided what to fix.