Getting the grid and the datum right

Same grid, both surveys

The start surface and the end surface must be in the same projected coordinate system, the same one the rest of your survey uses (your site's MGA zone or local mine grid, for example). If one is on the local grid and the other is in latitude and longitude, they either will not overlap at all or will produce nonsense cut and fill.

EOMeasy does not detect or convert coordinate systems. Two files that look fine on their own will still be wrong together if they were not put on the same grid before they reached EOMeasy.

Height matters as much as position

The Z value is elevation, and it drives the whole volume. Two things trip people up:

• Ellipsoidal versus orthometric height. Raw GNSS gives ellipsoidal height (above the reference ellipsoid). Survey elevations are usually orthometric (above a height datum such as AHD), converted through a geoid model. These differ by tens of metres and the difference is not constant across a site.
• A constant offset cancels; a varying one does not. If both surveys sit on the same datum, any shared offset cancels out when EOMeasy takes the height difference, so the volume is still correct. But if one survey is ellipsoidal and the other orthometric, or they use different geoids, the offset changes across the site and the volume comes out wrong. Keep both surveys on the same height datum.

Drone data is the usual culprit

RTK drone data often comes out of the flight in WGS84 latitude and longitude with ellipsoidal height. Fix it in the processing software, before you export, by setting the output coordinate system to your site grid and height datum:

• DJI Terra: in Output Coordinate System, choose Known Coordinate System, set the horizontal datum by EPSG code, and set the vertical datum and geoid (for example AHD).
• Pix4D: set the project's output and vertical coordinate systems to your grid and datum before you export the point cloud.
• Metashape: set the chunk coordinate system in the Reference pane so the exported cloud carries your grid and datum.

Do this on both flights, and while you are in there, trim each cloud to the area of interest. See Trim a drone point cloud.

If your files are already on different grids

Reproject them in your survey, GIS or CAD software so they share one grid and one datum, then bring them into EOMeasy. EOMeasy is deliberately not a reprojection tool: silently shifting coordinates is exactly the kind of hidden step that produces a wrong number nobody catches. Doing it in the software that owns your coordinate systems keeps the transform visible and checkable.