How to Use a Magnetic Locator for Boundary Survey Work

Monument recovery — finding buried iron pins, rebar, and monuments set at property corners — is one of the most time-consuming and consequential tasks in boundary surveying. Miss a monument and you may set a line in the wrong place. Spend an hour probing in the wrong spot and you've blown your field schedule. A magnetic locator, used with good technique, turns a potentially long search into a few-minute exercise. Here's how to do it right.

Equipment You'll Need

• Magnetic locator: Schonstedt GA-72Cd (professional choice) or Schonstedt GA-52Cx (entry-level)
• Fresh 9V alkaline battery: Check before each field day — a weak battery reduces sensitivity without warning
• Tile probe or rebar probe: A steel rod used to confirm the buried monument's exact location before digging
• Small shovel or trowel: For carefully exposing the monument once located
• Flagging tape or marking paint: To mark your search area and found location
• Field notes: Record description and condition of found monuments

Step 1: Start with the Record Research

Before you pick up the magnetic locator, do your homework. Review the deed, prior survey plats, subdivision plans, and any recorded notes about existing monuments. Where was the monument set? By whom, and when? What was the monument type — iron pin, rebar, iron pipe, aluminum cap, concrete monument with iron center?

Knowing what you're looking for matters. A large iron pipe monument buried 18 inches deep signals differently than a half-inch iron pin flush with the surface. If prior surveys show an iron pipe, expect a stronger signal from farther away. If the monument is a small rebar with an aluminum cap, the cap may not contribute much signal — the rebar is what you're finding.

Step 2: Set Up the Magnetic Locator

Turn the locator on in an open area away from vehicles, buildings, and other buried iron. Let the instrument stabilize for 10–15 seconds. Set sensitivity using the control knob:

• In open rural areas: use medium-high sensitivity to maximize detection range
• Near buildings, chain link fences, or vehicles: reduce sensitivity to filter out competing signals
• In areas with known iron infrastructure (old neighborhoods, industrial sites): use lower sensitivity to focus on close, strong signals

You should hear a steady, low background tone. If the tone is erratic or constantly peaking without a clear target, move away from interfering sources and re-adjust.

Step 3: Grid Search the Target Area

Based on your record research, you have an expected location for the monument. This might be "at the end of the deed call" based on distances and directions from the prior corner, or it might be a location shown on a subdivision plat. Your GPS or total station has probably given you a calculated position — but monuments can be off by feet due to old survey error, ground movement, or re-monumentation. Search a reasonable area around your expected position.

Sweep technique:

1. Hold the wand about 6–12 inches above the ground, parallel to the surface
2. Walk parallel passes across the search area, about 18–24 inches apart
3. Move at a slow, deliberate walking pace — too fast and you'll walk over a weak signal
4. Listen carefully for any change in pitch — even a subtle rise indicates something ferrous below
5. Cover the entire search area before concluding nothing is there

Common mistake: Holding the wand too high. Every inch of additional height reduces detection sensitivity. Keep the wand as close to the ground as you comfortably can without dragging it through vegetation or debris.

Step 4: Pinpoint the Signal Peak

When you detect a signal change, stop and work the peak. The peak signal — the highest pitch, loudest tone — is directly above the target. To find the exact center:

1. Walk through the signal in your current direction. Note where the pitch is highest.
2. Now walk perpendicular to your first pass. Note where the pitch is highest on this axis.
3. The intersection of the two peak positions is the center of the target.
4. Mark the center with a small flag or a scratch in the dirt.

With a GA-72Cd and a small iron monument, you can typically pinpoint to within 2–3 inches with consistent technique. Practice makes this faster and more accurate.

Step 5: Probe Before You Dig

Before breaking ground, probe the location with a tile probe or rebar probe. Push the probe straight down at the pinpointed center. If you hit iron at a reasonable depth (6 inches to 3 feet for most monuments), you've confirmed the target. The probe also tells you the depth — valuable for deciding whether to dig or core drill.

Probing also distinguishes between buried monuments and other buried ferrous debris — old nails, wire, hardware, scrap metal. If the probe hits a small, point-like object at the expected depth for a monument, it's likely a pin. If you hit a large flat object, it could be scrap or a buried lid.

Step 6: Expose and Document

Once you've probed and confirmed the monument, carefully expose it with a small shovel or trowel. Avoid striking the monument with the shovel — scratches or movement can affect its admissibility as a legal monument. Expose enough to read the cap or identify the monument type.

Document the found monument in your field notes:

• Monument type (iron pin, rebar, pipe, concrete, etc.)
• Cap or stamp information (surveyor name, license number, date)
• Condition (set, leaning, disturbed, in concrete, etc.)
• Depth found below surface
• Photograph the exposed monument

Tips for Difficult Monument Recovery

• Monuments under pavement: Asphalt and concrete don't block magnetic signals. Sweep the paved surface normally; the signal passes through.
• Monuments near fences: Chain link and iron fencing creates broad, noisy magnetic signals. Work from the side away from the fence. Reduce sensitivity. Look for the peak signal that stands out from the fence noise.
• Very old monuments: Corroded iron has less magnetic signal than new iron. Use maximum sensitivity. Search a larger area.
• Aluminum or concrete monuments: These are NOT magnetic — don't expect a signal from an aluminum cap unless it has an iron center or an iron pin beneath it. For aluminum or concrete monuments with no iron, use a GPS receiver and reference measurement for recovery.

Related products and guides:

• Schonstedt GA-72Cd Magnetic Locator
• Schonstedt GA-52Cx Magnetic Locator
• Browse Utility Locating Equipment
• Browse Field Supplies
• Tools for Utility Locating and Pre-Excavation Marking