GROUND CONTROLComing soon
Stream Capture reads your ground control points, finds the painted targets in the photos itself, lets you correct any mark in a GCP editor and keeps check points strictly apart from the points that position the model. The error it reports is therefore independent proof, not a self-assessment.
Stream Capture is in development and not released yet. At launch it will come in the same download as its sister program Stream Terrain; today's download contains Stream Terrain.
Ground control points (GCPs) are marks on the ground — painted targets or surveyed features — whose coordinates were measured with survey equipment. Photogrammetry uses them to position, orient and scale the model in a real coordinate system.
A drone with RTK or PPK already writes good positions into its photos, but a systematic offset can remain between the drone's frame and your ground survey — most often in height. A few control points remove it.
One line per point: name, easting, northing, height, an optional role and an optional horizontal and vertical accuracy. Exports from common survey and photogrammetry software are read directly, and decimal commas are accepted.
A point with role K is a check point: it never enters the adjustment, so its error is independent evidence. A point you do not fully trust can simply be made a check point.
If the file has no photo marks, the engine finds the painted targets itself and writes what it found to a separate file; your file is never changed. Marks you place by hand are always kept.
The GCP Editor (G) lists the frames that see each point and shows how consistent the marks are, in pixels. Correct marks, change roles, or let Auto-Mark Missing fill only the frames that have none.
If the points fall outside the flight area, the engine tries the region's other projection systems and suggests the one that fits. The GCP file can carry its own coordinate system and heights as ellipsoidal or geoid-referenced; they are converted when processing starts.
When control points are applied and some contradict the rest, the engine reports them by name, in the log and in the report, so you can re-check the measurement instead of trusting a bent model.
The model is positioned on the target marks, but the DSM is built from ground texture, and the two can differ by a few centimetres. With at least seven control points, and a difference larger than 1 cm and 1.5 times its standard error, the DSM and DTM are shifted vertically by that amount, up to 10 cm. Cloud, mesh and orthophoto are not shifted, and the measurement is written to the report.
Plain text, UTF-8, one point per line: name, easting, northing, height, and optionally K for a check point. Coordinates are in the output coordinate system unless you declare the file's own.
In the wizard's Location step choose Add GCP file, or No GCP — trust RTK. Set the file's coordinate system and default accuracies in the Location & GCP section if they differ.
System lets the engine find the painted targets; Me lets you place every mark by hand in the GCP Editor.
Run the project. Warnings about the file — skipped lines, unknown roles, photo names not in the folder — are listed in the log.
Open the editor with G. Check the frames that see each point and the pixel consistency of the marks.
Open the Quality Report (R). Check RMS, from the independent check points, is the real accuracy evidence; GCP RMS is only the fit at the points used.
There is no fixed minimum for georeferencing an RTK flight. The vertical reconciliation of the surface needs at least seven control points, and an accuracy class statement under the ASPRS standard asks for at least 30 independent check points; with fewer the report still states the class but warns that the test was thin.
RTK gives strong horizontal positions, but a systematic height offset against your ground survey can remain, and control points remove it. If your deliverable is height-critical, add control points, or at least check points to measure the offset.
A GCP is used in the adjustment and positions the model. A check point is not used: it is compared with the finished model and gives independent proof of accuracy.
For automatic marking the painted disk should be at least about seven times the ground sampling distance across — for example 0.35 to 0.5 m at 4.7 cm GSD. Smaller disks are still found, but their marks are noisier.
The coordinate system of the GCP file is most likely wrong. The engine says so and suggests the projection that fits; correct it in the Location & GCP section.
Yes. Choose Me as who places the marks, or edit any automatic mark in the GCP Editor. Hand-placed marks are always kept.
Stream Capture is in development and not released yet. At launch it will come in the same download as its sister program Stream Terrain; today's download contains Stream Terrain.
About Stream Capture