Glossary

Point cloud - one surveyed object in the project: a seabed area, a scan of a structure. It appears under Point Clouds in the project tree.

Revision - one survey of that object, with a date. A point cloud with three revisions has been surveyed three times. Revisions are what you compare; the first one is labelled Original.

Slot A / Slot B - the two surveys you are comparing. A is the earlier, reference survey; B is the later one you are checking. A is shown in blue, B in orange, everywhere in the panel.

B − A - the direction of every calculation. Positive means B is higher than A, so material was added. Negative means B is lower, so material was removed.

Boundary - the closed area the comparison runs inside. It is an ordinary project shape (a polygon or a rectangle), so it stays in the project and can be reused.

Difference map - the coloured result draped on the seabed: how much higher or lower the newer survey is, cell by cell.

Cell - one square of the grid the comparison is calculated on, 0.5 across by default in the project’s CRS unit - half a metre on a metric CRS. Every number in the panel is a sum over cells.

Observed / not observable - a cell is observed when both surveys have usable data in it, so a difference can be calculated. If only one survey covered it, there is nothing to compare: the cell is left transparent on the map and excluded from every volume.

Common data coverage - the share of your area that is observed. The number to read before any volume.

Deposition - total volume gained, added up over the cells where the seabed rose.

Erosion - total volume lost, over the cells where it fell.

Net change - deposition minus erosion.

Volume mesh - a 3D body showing the shape of the change. A simplified preview; the volume figures come from the full-resolution map.

Profile line - a two-point line you draw across the survey. The Profile tab plots both surveys along it.

Δ (delta) - a difference. On the profile chart, Δ is B minus A at the cursor. In a measurement, ΔL is horizontal distance and ΔH is height difference.

Slope / ruggedness / roughness of the difference - three alternative maps derived from the same comparison. They describe how the change varies from place to place, not the shape of the seabed.

2.5D - the model behind the whole tool: one height per cell, like a heightmap. Right for a seabed, wrong for anything vertical.

GeoTIFF - the standard raster file the map can be exported as, for use in QGIS or other GIS tools.

Two words that get mixed up

A point cloud is the object; a revision is one survey of it. “Compare two point clouds” means comparing one revision from each.