Import GPR depth slices (georadar)

3DSC ingests ground penetrating radar depth slices and places them under the survey area at the depth they were measured at, so that a buried anomaly can be read in the same space as the photogrammetric model of the ground above it.

See also

GPR slices documents every field of the panel; Representing GPR depth slices in 3DSC explains why a slice becomes a textured surface, how the drape works, and where the importer deliberately stops.

When to use this recipe

Reach for it when you have processed depth slices — the 2D amplitude maps a GPR suite writes out for successive depth bands — in either of two shapes:

  • One CSV per slice, a complete regular grid in raster order, with the cell size and the depth in the header line. Cells outside the surveyed polygon may carry an empty amplitude field; they become transparent.

  • One image per slice, with the depth in the file name (Slice_-2889mm.jpg, slice_1.900 - 2.000m.png), optionally with an ESRI world file for the extent.

Raw radargrams, traces along profiles and vendor-native acquisition files are not handled: they are sparse data along lines, and must be processed into depth slices first.

Prerequisites

  • The .blend file is saved on disk, so the cache and any SHIFT.txt land in a predictable place.

  • You know the origin of the survey grid in the project CRS, or a world file beside the rasters carries it. CSV grids usually start at 0, 0 and carry no georeference at all.

  • For draping: the photogrammetric model of the ground surface is in the scene, already positioned in the same shifted frame as the rest of the project.

Step 1 — Point 3DSC at the slices

In the 3D Viewport sidebar (N) switch to the 3DSC tab and expand GPR slices. Use the folder button to select the folder holding the slices; 3DSC reports how many it found and the depth range they cover.

Set CSV grids or Images to match your input.

Step 2 — Build the raster cache (CSV only)

Choose a Colormap and a Normalise mode — leave Whole stack unless you specifically want to chase faint deep anomalies — then press Build raster cache.

The conversion writes one PNG per slice plus a gpr_stack.json manifest. It is modal, so the interface stays alive and Esc cancels. As an order of magnitude: forty slices of 87 MB each convert in about half a minute and produce a cache of some 40 MB, against 3.3 GB of source CSV.

Remember

This is a one-off. Later imports read the manifest and are instantaneous. Keep the cache folder: it is a compact, archivable form of the stack, and it is the same shape as a stack that arrives already rasterised.

For an image stack, press Read image stack instead. Nothing is converted; only the manifest is written. Give the Cell size when no world file sits beside the images.

Step 3 — Georeference and shift

If a world file was found, the origin is already filled in. Otherwise type the CRS coordinates of the local (0, 0) cell into E and N, and set Grid azimuth if the survey grid is not axis-aligned.

Press Import GPR stack.

When the dataset turns out to be in absolute coordinates and the scene has no shift yet, 3DSC stops and asks (Fig. 1). The dialog shows where the data actually is, what the files say about the CRS, and a proposed shift rounded to a memorable figure. Edit it if you want a different origin, leave Write into the scene shift (GSV) enabled so the rest of the project lands on the same origin, and confirm.

../../_images/GPR_shift_dialog.png

Fig. 1 3DSC asking before placing a dataset that sits hundreds of kilometres from the world origin

If the scene already carries a shift, the import goes ahead without asking — unless the EPSG code of the dataset disagrees with the scene’s, in which case you are warned and the scene shift is left untouched.

Step 4 — Hang the slices under the real ground

A slice is a reading taken below the surface the instrument walked on. If that surface is not flat, a flat slice at −1 m is not one metre below the ground anywhere except by accident.

  1. Select the photogrammetric ground mesh and press Declare as photogrammetric ground. 3DSC records the object and its face count, so the drape stays traceable to the document it came from.

  2. Choose a depth reference. Grid on ground is the normal choice: a regular grid over the slice extent, dropped onto the declared surface. Set Grid to 128 unless the terrain is unusually broken. Use Ground geometry only when the terrain’s own topology matters — a breakline a regular grid would cut across — and set a Max triangles budget.

  3. Import again. Each slice now follows the terrain, offset downwards by its own depth.

../../_images/GPR_drape_viewport.png

Fig. 2 Seven slices of the CS07 Tivoli stack draped on an undulating walking surface, sharing one mesh between them

Remember

The drape mesh is shared by the whole stack, so a deep stack under a heavy photogrammetric model costs one surface, not one per slice. The declared ground object is never modified: 3DSC always works on a copy.

Step 5 — Read the stack

Switch the stack display to One or Down to and step through the Slice field. Hidden slices release their pixel buffers, so a stack of several hundred slices stays affordable; with every slice visible at once, six hundred of them will occupy several gigabytes.

Step 6 — Record what you read

When you draw a proxy over an anomaly, select it and press Tag selection as read from GPR. 3DSC writes onto the proxy which slice documents were on screen, how many, the depth span they cover, and the acquisition they belong to.

It writes nothing else. Deciding that an amplitude blob is a wall, and declaring the stratigraphic unit that says so, stays with you — see Representing GPR depth slices in 3DSC.

Troubleshooting

  • “No gpr_stack.json in …” — the cache has not been built yet, or the Cache field points somewhere else. Build it, or correct the path.

  • The stack lands hundreds of kilometres away — the origin is absolute but the scene shift is zero, or vice versa. Check the read-only GSV line under Apply 3DSC shift, and remember that 3DSC only subtracts the shift from an origin that is genuinely absolute.

  • The stack sits at the scene origin and a warning says so — the dataset is local and no origin was typed. Fill in E and N.

  • “the slice extent does not overlap …” — the drape found no ground under the survey area. The georeferencing origin is wrong, or the ground mesh is somewhere else entirely. Fix the origin before draping.

  • Part of the drape is flat while the rest follows the terrain — the report counts grid points that missed the ground; those fall back to the mean elevation. Extend the ground model, or reduce the slice extent.

  • The decimated drape has more faces than the budget — the budget counts triangles. A quad mesh has roughly twice as many triangles as faces.

  • Blender slows to a crawl on a deep stack — every visible slice decodes its raster. Switch the display to One.