A sonde is a small battery powered transmitter built into or clipped near the camera head, and a locator is the handheld receiver that picks up its signal from the surface. Together they let you find the exact spot and depth of a pipe defect without guessing where to dig.
What a sonde actually does
A sonde is not a camera. It records no video and sends no images. Its only job is to broadcast a steady electromagnetic signal from wherever it sits inside the pipe. Most sewer camera sondes transmit at 512Hz, chosen because it carries well through soil, concrete, and standing water without much interference from house wiring. As the camera head or pushrod moves through a line, the sonde moves with it. An operator walking the surface above with a locator follows that moving signal step by step. That is how a technician traces a pipe route that was never mapped, or marks the exact point of a crack, collapse, or root intrusion for excavation.
Sondes come in different sizes to match different head diameters. A compact sonde built for a 17mm or 23mm mini camera head must stay small enough not to hang up the head in tight one and a half to two inch lines. Larger sondes paired with 25mm or 30mm heads can carry more transmitting power, often extending usable locating depth. Some systems attach the sonde behind the head on the pushrod instead of inside it, keeping the head slimmer for narrow pipe work.
How a locator reads the signal
A locator is a handheld wand or box style receiver tuned to the sonde’s frequency. Hold it over the ground and it reads signal strength, growing louder as you pass directly over the sonde. Most locators also estimate depth, usually by comparing signal strength at two points. That reading is an estimate, not a survey measurement, but close enough to guide a marked dig point.
Basic locators give a simple audio tone or needle gauge. Better ones show a numeric depth figure and an arrow guiding you back over the peak signal. Some also display the sonde’s transmitting frequency, useful when a job site has more than one active transmitter nearby, such as a utility locator working the same street.
Built-in sonde versus add-on sonde
Camera systems handle sondes two different ways, and it changes how you shop for one.
- Built-in sonde: the transmitter is sealed inside the camera head itself. There is nothing extra to attach, but if the sonde fails the whole head usually needs replacing.
- Add-on sonde: a separate small transmitter clips onto the pushrod a short distance behind the head. It can be swapped or replaced on its own, and some kits let you remove it entirely for jobs where locating is not needed.
- No sonde at all: entry level cameras sometimes skip the sonde completely to keep cost down. These work fine for visual inspection but cannot be traced from the surface, which limits their use for locating buried defects.
If your work is diagnostic only, inspecting a line whose layout you already know, a sonde is optional. If you regularly mark dig spots for a repair crew, a working sonde and matching locator earn their keep fast.
Depth accuracy and what affects it
No sonde and locator pair gives a perfectly exact depth every time. Several things push the reading off.
- Soil composition. Clay and wet soil conduct differently than dry sand or gravel fill.
- Nearby metal. Rebar, chain link fencing, parked vehicles, and other buried utility lines can distort the field.
- Depth itself. Signal strength drops fast the deeper a sonde sits, so readings below roughly ten to twelve feet get less reliable on most consumer level locators.
- Battery condition. A weak sonde battery transmits a fainter signal and can read shallower than the pipe actually sits.
Most experienced operators treat the depth number as a starting estimate, then confirm the exact spot by pinpointing peak signal strength before marking paint on the ground. A second sweep from a slightly different angle helps confirm the reading before anyone breaks ground.
Choosing a locator for your camera system
Match the locator’s receiving frequency to the sonde’s transmitting frequency first. A 512Hz sonde needs a 512Hz capable locator; mismatched frequencies simply will not detect each other, no matter how good either device is on its own. Beyond that, decide based on how the equipment gets used day to day.
A homeowner tracing one line a few times a year does fine with a simple audio tone locator and whatever sonde came with the camera kit. A contractor marking dig points weekly benefits from a numeric depth display, adjustable sensitivity, and a comfortable wand grip. Cable length matters too. If the pushrod runs 100, 165, or 200 feet, confirm the sonde still transmits a usable signal at that full distance, since some compact sondes lose range before the cable runs out. The camera head reference and the pipe size checker on this site can help match a head, sonde, and pipe diameter before buying.
Self levelling heads and sonde placement also interact. A sonde mounted directly behind a self levelling head sits close to the pipe’s centerline, giving steadier readings than one riding loosely through a bend. Minimum bend radius still applies. Pushing a rigid sonde housing through an elbow it is not rated for risks getting it stuck, so check the bend rating before working sharp turns.
Frequently asked questions
How do you locate a sewer camera underground?
Turn on the sonde and matching locator, then walk the surface above the suspected pipe route while watching the locator’s signal strength. Move slowly and sweep side to side until the reading peaks, which marks the point directly above the sonde. Note the depth estimate, mark the spot, then double check with a second pass before digging.
What is 512Hz and why does it matter?
512Hz is the frequency most sewer inspection sondes use to transmit their locating signal. It was adopted widely because it travels reasonably well through soil and concrete while staying distinct from common electrical interference. A locator must support 512Hz to detect a 512Hz sonde; other frequencies will not register.
Can you use a locator without a sonde?
A locator on its own cannot find a plain camera head, since there is no signal to detect without a transmitting sonde. Some locators can trace metallic pipe or existing tracer wire directly by induction, which is a separate function from sonde tracing. For finding the camera’s exact position in a non metallic line, a sonde is required.
Does pipe material affect sonde accuracy?
Pipe material itself has little effect, since the sonde signal travels through the surrounding soil rather than along the pipe wall. What matters more is what is near the pipe. Metal casing, rebar in a nearby slab, or another buried utility line can distort readings regardless of whether the pipe is plastic, clay, or cast iron.