Check a known point

Occupy a control point or design coordinate and record the residual ΔN/ΔE/ΔU. It's the one accuracy figure a receiver can't give you.

On this page

A receiver's RTK FIXED ±0.012 m is the receiver's opinion of its own solution. It stays just as confident with a wrong base coordinate, a wrong antenna height, a datum mismatch or a false fix. A check shot is independent: you stand on a point whose coordinate you already know, and the app records how far off your measurement is.

Check in at the start of the day and again at the end. Each check is its own record, and all of them feed the QA report.

Before you start

  • Something known to check against. Check a known point lists only:

    • imported design points, from a CSV, LandXML or other import, or computed in COGO, and
    • points collected as the Control Point type.

    Ordinary collected points aren't offered: checking a point against itself proves nothing. Until one of these exists, the row is greyed out.

  • Ideally, a coordinate system set for the project, so residuals are computed on the grid.

Key in a control point from its datasheet

If your control comes from a datasheet with published northing and easting, enter it as a Control Point:

  1. Set the project's coordinate system to the zone and unit on the datasheet (see Choose a coordinate system).
  2. On the Survey tab, choose Control Point as the type. Use the crosshair to drop a point on the monument's approximate location, or collect one there.
  3. Tap the point, then Edit → Position….
  4. Tap Type, then N/E (EPSG:…).
  5. Enter the published northing and easting in the zone's unit, and the elevation in your display unit if it's known.
  6. Optionally add a reason, such as NGS datasheet AB1234, then tap Save typed coordinate.

The app refuses values that land outside the zone and shows where they would have landed. Keying metres into a ftUS zone, or the neighbouring zone's values, is caught here. The typed position is marked as edited, with unknown horizontal accuracy, and the values you keyed are kept in the position history.

Record a check shot

  1. Go to Tools → Check a known point.
  2. In Check against…, tap the point you're standing on. For a line or area, use the arrows to choose the vertex.
  3. The Check point screen guides you in like stakeout: arrow, distance, ΔN and ΔE.
  4. Centre the pole over the mark, level it, and pick a Tolerance: 0.5 m, 1 m, 2 m or 5 m.
  5. Tap Record check shot.

The result appears under the button:

  • Residual 0.018 m, green if within tolerance, orange if not.
  • ΔN, ΔE and ΔU, always measured − known. A positive ΔN means your measurement is north of the known coordinate.
  • Differenced on the project grid, or a note that it used a local tangent plane because no coordinate system is set.

The button then reads Record another check shot. Each tap records a separate check, so you can take several on one occupation. After more than one, the screen shows 3 check shots this visit, each kept as its own record. The same fix can't be recorded twice: the app waits for a new position (and at least 1.5 seconds) before accepting another tap.

What the verdict means

The pass/fail verdict compares the horizontal residual with the tolerance you picked. A check shot is still recorded if it fails. A failed check is exactly what you want on file.

A large residual usually points at one of these:

  • A wrong base coordinate or correction source on a different datum.
  • A wrong pole or antenna height (look at ΔU).
  • A false or float fix.
  • A mistyped control coordinate, or the wrong monument.

Where check shots go

  • Tools → QA report shows the horizontal RMS of all check shots at the top, how many were outside tolerance, and each check under Observed accuracy — check shots. The QA report row in Tools shows the RMS, or Not verified if there are none.
  • Export bundle (ZIP) includes checks.csv, one row per check with target, measured, kind, delta_north_m, delta_east_m, delta_up_m, horizontal_residual_m, tolerance_m, result (pass or fail), frame (grid or tangent), crs_epsg, vertical_basis, timestamp_utc and notes.
  • Each check also creates a Check Shot asset named Check: <target>, carrying the full position record.
  • Tools → Site localization unlocks after the first check shot, and uses every check as a control pair.