What is QL-C?
QL-C is obtained by surveying visible utility features (e.g., manholes, valve boxes, hydrants, pedestals) and correlating those observations with record/QL-D information to infer the underground line between them. It does not use surface geophysics (that would be QL-B) and is unreliable for design conflicts.
Method: Survey Visible Features • Purpose: Inferred Alignments • Standard: ASCE 38-22 • Also Known As: "Feature Correlation"
Acceptable Methods to Achieve QL-C
Systematic Inventory of Surface Appurtenances
Walkdown and catalogue all visible features (e.g., sewer/storm MHs, water valves, hydrants, utility pedestals, junction boxes, risers, transformer pads, utility poles/risers). Photograph and tag each feature.
Water Utilities
- • Fire hydrants
- • Water valve boxes
- • Water meter boxes
- • Shutoff valves
Sewer/Storm
- • Manholes
- • Catch basins
- • Cleanouts
- • Junction structures
Power/Telecom
- • Utility pedestals
- • Junction boxes
- • Transformer pads
- • Utility poles/risers
Survey to Project Control
Collect precise X/Y (and ground Z) for each visible feature using total station or GNSS tied to project control; capture feature type, IDs, rim elevations (for mapping context), and notes.
Survey Requirements
- • Total station or GNSS positioning
- • Tied to project control network
- • Precise X/Y coordinates
- • Ground elevation (Z)
- • Rim/invert elevations where accessible
Documentation Required
- • Feature type and utility owner
- • Feature ID/asset numbers
- • Physical condition notes
- • Digital photographs
- • Measurement metadata
Records Correlation (QL-D → QL-C)
Correlate surveyed features with owner records/as-builts and other second-party information to infer which features are connected and the likely centerline between structures. Draw the inferred alignments with appropriate symbology (e.g., dashed lines) and metadata that the feature is QL-C.
Correlation Process
Data Sources
- • Owner as-built drawings
- • Construction records
- • Previous SUE investigations
- • Municipal GIS databases
Analysis Requirements
- • Match surveyed features to records
- • Infer logical connections
- • Use appropriate symbology
- • Document assumptions
Use Visual Clues at the Surface
Where present, consider pavement cuts/utility patches, trench scars, utility corridor norms, and service entries as corroborating clues—always reconciled to records and surveyed points.
Surface Indicators
- • Pavement cuts and patches
- • Trench scars in landscaping
- • Utility corridor patterns
- • Service entry alignments
Important Note
Visual clues are corroborating evidence only. Always reconcile with records and surveyed points. Never rely on surface indicators alone.
Targeted Applicability by Utility Type
Favorable for QL-C
Favor QL-C for networks where surface structures reliably indicate alignment.
Gravity Sewers
Manholes typically indicate direct pipe connections
Storm Drains
Surface structures show clear flow patterns
Large Water Mains
Valve boxes and hydrants indicate main alignment
Use with Caution
Be cautious with utilities where pedestals or cabinets may not indicate the buried alignment between structures.
Telecommunications
Pedestals may not reflect underground routing
Electrical Distribution
Cabinets may connect to multiple buried routes
Gas Distribution
Service connections may not indicate main alignment
What Does Not Qualify as QL-C (Alone)
Records Only (No Field Survey)
Records only (no field survey of features) → remains QL-D.
Surface Geophysics
Surface geophysics (EM, GPR, sondes) → that work product is QL-B, not QL-C. If you used geophysics, classify accordingly.
Assumptions Without Correlation
Assuming alignments between features without correlation (no record support or conflicting evidence) → insufficient; document the basis or leave as unknown.
Minimum Documentation to Claim QL-C
Surveyed Features List
Surveyed features list with coordinates, feature type/ID, ground elevation, photos.
Required Information
- • Precise X/Y coordinates
- • Ground elevation
- • Feature type and owner
- • Asset ID numbers
- • Digital photographs
Correlation Log
Correlation log referencing the record sources used (drawing numbers/dates, owner replies).
Documentation Requirements
- • Record drawing numbers and dates
- • Owner contact information
- • Data source reliability assessment
- • Correlation methodology notes
Inferred Alignments Tagged
Inferred alignments explicitly tagged "ASCE 38-22 QL-C" with notes on assumptions/limitations.
Mapping Requirements
- • Appropriate symbology (dashed lines)
- • Clear QL-C attribution
- • Assumptions documented
- • Limitation notes included
Practical Cautions (Especially for Rail/LRT Corridors)
Congestion & Misleading Clues
Expect congestion and misleading surface clues; QL-C is not design-grade where conflicts are possible—upgrade to QL-B (geophysics) and QL-A (test holes) at crossings and tight clearances.
When to Upgrade
- • Potential conflicts identified
- • Tight clearances required
- • Critical crossings
- • Design-grade accuracy needed
FHWA Advisory
FHWA notes QL-C/records correlation can omit or misplot buried plant; use it primarily for planning or for networks with strong surface expression.
When QL-C Is Most Effective
Planning Phase
Ideal for preliminary planning when general utility locations are needed.
Network Mapping
Effective for utilities with strong surface expression and logical connections.
Cost-Effective
Lower cost than QL-B geophysics when appropriate for the application.
Best Practice
Use QL-C for initial network understanding, then upgrade to QL-B/QL-A where conflicts are identified or design-grade accuracy is required.
Ready to upgrade from QL-C Standards?
Contact us to learn how our Visual Ground Disturbance System can help you document and manage QL-C feature surveys with proper correlation and workflow management.