Not every corner you survey is fresh concrete. A lot of the hardest ones are already standing — the corner of an existing foundation you're tying an addition into, a masonry wall a new structure has to meet, an old footing that has to be located before anything new gets designed around it. The concrete cured years ago. Your job is to find out where reality actually is, so the new work can be built to it instead of to an assumption.
Existing corners don't cooperate
A fresh-formed corner is at least clean. An existing one has been through weather, traffic, and time. Edges are chipped, spalled, or rounded. Faces aren't perfectly plumb or perfectly square anymore. And you frequently can't get a rod anywhere near where you'd want it. So the "corner" you're trying to locate is both physically imperfect and impossible to touch at the exact point — which pushes crews right back into holding the rod off a rough edge and eyeballing an offset.
Why the stakes are higher on retrofit work
On existing-conditions work, your survey usually is the design basis. A detailer or engineer is going to take your as-built and lay new steel, a new pour, or a new connection against it. There's no "the plan says so" to fall back on, because on the existing side, your measurement is the plan. A corner that's off because the offset was guessed doesn't get caught against a drawing — it becomes the drawing, and the error surfaces when the new work shows up and doesn't fit the old.
Getting a clean point off a rough corner
The move is the same one that works on new construction, and it matters even more here: capture the corner from a fixed, known reference instead of an eyeballed one, so a rough or weathered edge still yields a repeatable point.
The Precision Corner Offset (PCO-1) seats on the outside corner and holds your prism at a known 1", 2", or 3" offset, so the true corner backs out of fixed geometry even when the edge itself isn't pretty. Because the offset is defined rather than guessed, two different crews surveying the same existing building get the same corner — which is exactly what you need when your as-built is going to become someone's design reference.
The takeaway from the dirt
Existing corners are unforgiving twice over: the geometry is imperfect, and your number becomes the source of truth everything new is built against. Guess the offset on a weathered corner and you've written the mistake into the design. Fix the offset to a known value and even a rough old corner gives you a point you can stand behind.
The Precision Corner Offset locates outside corners — new or existing, clean or weathered — at a selectable, known offset on a standard 5/8-11 mini-prism mount. ASA frame, 6061-T6 aluminum corner plates, made in the USA. See the specs →