Digitizing an Old CPT Sounding to GEF: From Scan to Usable File

Almost every foundation design starts with a CPT sounding. But for renovations, extensions or a change of use, that sounding often already exists — as a yellowed scan in the archive, as an appendix to a building permit from 1985, or as a PDF in DINOloket, the Dutch public subsurface archive. Your design software, however, doesn't want a picture but a GEF file with cone resistance per depth. This article explains how to digitize such a scan quickly and verifiably.

Why digitize instead of performing a new CPT?

A new CPT easily costs several hundred euros per test, plus planning and access to the site. For a small extension or a feasibility study, that is often out of proportion. If a usable sounding of the location exists, digitizing is a matter of minutes. When reuse is justified is covered in our article on reusing old CPT soundings.

What do you need?

• A scan of the sounding: JPG, PNG, TIFF or PDF. The higher the resolution, the better.
• The scale of the axes: two known values per axis (for example the grid lines at 0 and 10 m, and at 0 and 10 MPa).
• The ground level (in m NAP, the Dutch vertical datum), usually stated on the sheet as "MV = NAP + …".

Step-by-step

1. Load the scan

Drag the file into the CPT Converter or paste a screenshot with Ctrl+V. Everything runs locally in your browser; the scan is never uploaded — relevant when confidential project documents are involved.

2. Calibrate the axes

Try the automatic axis detection first: the tool looks for the grid and reads the numbers along the axes using text recognition. "NAP" on the depth axis is recognized as the zero point. If that fails (with stained or skewed scans), you click two known points per axis. Choose points that are far apart: a calibration error then spreads over a larger range and becomes relatively smaller.

3. Digitize the cone resistance line

With "Auto-trace" you only click the start and end of the qc line; the algorithm follows the line pixel by pixel and converts it into points. Check the result visually: points can be dragged, deleted or added. Especially with thin peat layers with low qc, it pays to zoom in for a moment.

4. Export to GEF

Enter the sounding number, the coordinates and the ground level, and download the GEF file. By default the data is interpolated to a fixed depth interval of 0.02 m, as design programs expect. Want to know exactly what's inside that file? Read our explanation of the GEF format.

How accurate is the result?

Accuracy is determined by the scan resolution and your calibration, not by the number of clicked points. On an A4 scan at 200 dpi, one pixel corresponds to roughly 0.01–0.03 MPa on the qc axis: more than sufficient for a pile calculation. What matters more is avoiding systematic errors:

• After calibrating, check that the value read out under your cursor matches what you read on the sheet.
• Watch the unit of the depth axis: meters below ground level or meters relative to NAP. Mixing these up is the most common error in digitized CPT soundings.
• Compare the end result as a chart in your design software with the original scan.