GCP & PPK Accuracy

PPK gets your photo positions right; ground control points (GCPs) anchor the whole reconstruction to surveyed coordinates; checkpoints prove the result. This guide covers all three. (For the upload flow itself, see Running a Survey.)

Getting control points in

1. In the left menu, click Office › Work › Field › DSP, then the folder, then the project.
2. Upload the photos first: on the mission’s card, click “Upload photos”, and choose “Upload only — then ground control (step 6) and processing (step 7)”.
3. On the mission’s card, click “Add ground control”. The Guided GCP setup opens at “Import your GCP coordinates”. (On the map page the same button reads “Open Guided GCP setup”.)
4. Drop your GCP file (CSV or TXT: a name, then easting/northing or latitude/longitude, then elevation) where it says “Drop a CSV / TXT file here, or browse”.
5. Check the coordinate system it found (change it under “Advanced: change anything” if the points look shifted), pick the “Height reference of your GCP elevations”, and click “Next ›”.

A gcp_list.txt dropped into the upload zone with the photos is also auto-detected.

Marking GCPs in the photos

A GCP only helps once the software knows where it sits in your images. The Guided GCP setup does the placing for you: in “Mark your GCPs” it puts each GCP on its photos from your drone’s RTK position and camera angles.

1. Check the crosshair sits on the centre of the target (drag it there if not).
2. Click “Confirm mark”. Use “Skip photo ›” for a photo where the target can’t be seen, and “Remove mark” to undo one.
3. Repeat until each GCP has at least 3 photos — 5 is better — then click “Next ›”.

Nothing enters your GCP file without your confirmation.

Choosing checkpoints and processing

1. In “Choose your checkpoints”, keep the recommended split or set each point’s role: “Control”, “Checkpoint”, or “Not used”. Hold out about 20–30% of your points, at least 3, spread across the site.
2. Click “Next ›” through “Review processing settings”.
3. In “Ready to process”, click “Run: save GCP setup”. It saves the setup and takes you to the processing page, where you see the price and start the run.

Proving accuracy with checkpoints

Checkpoints are surveyed points held out of processing and measured against the finished surfaces — the honest, independent test.

1. When the run is Ready, go to Office › Work › Field › DSP, open the project, and on the mission’s row click “⋯”, then “View the finished map”. (From the map page, “Open Map” opens the same map.)
2. In the map’s side panel, open “Checkpoint Accuracy”. “Pull NGS Monuments” adds published survey control near your site as extra check points.
3. Click “Run Accuracy Check” (it needs the elevation model). If your checkpoint heights use a different datum, set “Vertical shift (m)” first.

The result reports, in feet:

• Horizontal RMSE and Vertical RMSE
• NSSDA accuracy at 95% confidence for each
• Per-checkpoint residuals, plus a plain-language statement

The same accuracy section appears in the run’s “Deliverable Report” — checkpoint counts, RMSE values, NSSDA 95% figures, vertical bias, and the per-checkpoint residual table — so your client’s PDF carries the proof.

Common questions

GCPs or PPK — do I need both? PPK corrects where the camera was; GCPs pin the model to surveyed ground truth. PPK alone gets you excellent relative accuracy; add GCPs when the deliverable must tie to a survey datum, and checkpoints when you have to prove it.

How many GCPs should I place? Five well-spread points is a common floor for a typical site — corners plus center — with more for large or elevation-varied sites. Keep some surveyed points aside as checkpoints instead of using everything as control.

Why do my checkpoint heights look uniformly off? Usually a vertical-datum mismatch — ellipsoidal vs orthometric heights. Pick the right “Height reference of your GCP elevations” in step 1 of the Guided GCP setup, or use “Vertical shift (m)” on the accuracy check to reconcile a known offset.

Where do checkpoints come from? Two places: the Guided GCP setup’s “Choose your checkpoints” step holds some of your own surveyed points back as checkpoints, and the map’s “Checkpoint Accuracy” section can “Pull NGS Monuments” — published control near your site.