Sewer line locating and depth: how the mark on your lawn is made

01 /How does sonde locating work?

Sonde locating works by transmitting a low-frequency radio signal from the camera head and detecting it above ground with a matching receiver. Many residential push cameras use a sonde at 512 Hz, a frequency chosen because it passes well through soil and through metal pipe such as cast iron; some systems offer other frequencies. The operator sets the receiver to the sonde frequency and walks the line.

1. Stop the camera at the point of interest

   Usually a defect, a transition, the property line or the end of the run.

2. Sweep for the peak

   The operator walks across the likely route; the signal rises to a maximum directly above the sonde.

3. Find the nulls

   Because of the sonde's field shape, the receiver shows two null points ahead of and behind the sonde along the pipe. The sonde sits midway between them, under the peak.

4. Read depth

   With the receiver on the ground at the peak and oriented correctly, the unit estimates depth to the sonde.

5. Mark and measure

   The spot is painted or flagged, the depth noted, and distances measured from fixed features such as the foundation wall.

02 /How accurate are sewer locate depth readings?

Sewer locate depth readings are estimates that are usually close enough to plan an excavation but not precise enough to dig blind. Accuracy depends on the equipment, calibration, the operator's technique and interference. Readings tend to be less reliable when the line is deep, when the sonde is inside metal, near other buried utilities, under reinforced concrete, or when the receiver is not directly over the sonde.

03 /Why does locating matter for repairs?

Locating matters because excavation is paid for by volume and difficulty. Knowing the defect sits at 2.4 metres under the lawn rather than 3.5 metres under the driveway changes the method, the equipment and the price. A contractor quoting without a locate has to allow for uncertainty, and that usually shows up in the quote.

• Spot repairs need the defect located to within a reasonable excavation window.
• Trenchless work (lining or bursting) needs access pit locations, which the locate helps plan.
• Ownership questions depend on whether the defect is on private property or past the property line.
• Landscaping, decks and garages are planned around the line's route.

For how repair methods compare, SewerQuotes.com explains typical quote structures. For how inspection fees handle locating, see the sewer inspection cost guide.

04 /How should locate marks be recorded?

Locate marks should be recorded so they survive rain, mowing and snow: measured from permanent features, photographed, and written into the report with depth estimates. Paint on grass can disappear within days. Flags get moved.

If you are in a Canadian winter with snow cover, marks may need stakes rather than paint, and frozen ground can affect how soon an excavation is practical.

Pair the ground record with a record along the pipe. Entering each observation's counter distance and its locate depth in the Lateral Defect Map produces a printable strip from the access point to the main, with depth labels on each marker, that sits alongside your photos of the marks.

05 /What if the line cannot be located?

If the line cannot be located, the usual reasons are that the camera could not reach the point, the signal is blocked or distorted, or the line is deeper than the equipment reads reliably. The report should say which points could not be located and why.

• Try from a different access point to get the camera closer to the defect.
• Use a different frequency if the equipment allows.
• For long runs, locate every few metres to trace the route rather than just the defect.
• Ask whether the municipality has service connection records showing approximate tie-in location; in Calgary, 311 is the starting point for such requests.

06 /Can homeowners locate a sewer line themselves?

Homeowners generally cannot locate a sewer line accurately without a camera and sonde. Visible clues help estimate the route: the direction the main stack or cleanout faces, the location of the municipal main (often under the street or lane), and older lawn settlement lines. But an estimate is not a locate. Renting a camera does not usually close the gap, because rental kits rarely include the matching locator; see renting a sewer camera.

Ready to have the line located and inspected? Request a quote and mention that you need locate marks and depth for each defect.

07 /How are the property line and the tie-in located?

The property line crossing and the tie-in to the main are located the same way as a defect: the camera is stopped at the point and the sonde is found from above. These two points matter because they frame ownership and access. The tie-in is often under a road, lane or boulevard, where the reading may be affected by pavement reinforcement or other utilities.

• Stop the camera at the curb line, lot line or a known distance, and locate it to show where the line crosses.
• At the tie-in, record the distance, the estimated depth and the surface feature above it (lane, street, boulevard).
• Note if the camera entered the main or stopped at the connection fitting.
• Compare the tie-in location with any municipal records you can obtain.

Property boundaries themselves come from a survey or real property report, not from the sewer locate. If a defect sits close to the line, the survey and the locate together tell you which side it is on. If the municipality has inspected your connection from the main with a lateral launch camera, its record can help confirm the tie-in; see CCTV sewer inspection.

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