Digitizing a bathymetric chart: from PDF soundings to point data
Old bathymetric charts and survey plots hold bathymetric data, water depths across the seafloor, that often exists nowhere else. To use it, building a seabed surface, comparing it with a newer survey, or estimating a volume, you need the soundings as points with coordinates, not as print on a sheet.
Digitizing them by hand means clicking each sounding and typing its depth. When the chart is a vector PDF, the depths can be read out directly instead.
What ensoundings does
ensoundings reads the depths straight out of the PDF and gives them back as a CSV, one row per sounding:
Example rows, in the real format.
- Easting and northing
- Georeferenced from the coordinate grid printed on the sheet itself, such as border labels like E 875,000 or X=486000. The values stay in that grid's own system and units, whether that's state plane, UTM or a local grid, exactly as labelled. Nothing is converted or reprojected.
- Depth
- The number as printed, in the sheet's own units and relative to its own datum, such as MLLW, MLW, NAVD88 or chart datum. No rounding, no sign change, no conversion.
- Confidence
- How clear the reading was, from 0 to 1. Points that need a second look are flagged for you to check.
If a sheet has no coordinate grid the reader can use, the file says so. It then gives positions on the page instead of real-world coordinates, and never presents those as eastings and northings.
The CSV opens directly in Excel, so this is a PDF-to-Excel step as well, and it imports as points into AutoCAD, Civil 3D, ArcGIS, QGIS or HYPACK.
What to do with the points next
A CSV of easting, northing and depth is the common starting point for most survey, CAD and GIS software: Excel, AutoCAD, Civil 3D, ArcGIS or QGIS. Typical next steps are building a surface, TIN, DEM or DTM from the points, comparing it against another survey, or calculating cut and fill volumes between two surfaces.
Check the sheet's notes for its datum, projection and depth units before you combine it with other data. The CSV keeps the values exactly as the sheet labels them and doesn't convert them for you.
Points, not contours (yet)
ensoundings reads individual soundings: the printed depths. Charts that show depth only as contour lines (isobaths or depth contours) or shaded bands, with no soundings, aren't supported yet. Reading contours is on the roadmap.
Not for navigation charts
Official nautical charts, such as NOAA's electronic navigational charts (ENC), are already published as data, and nothing ensoundings produces is for navigation. It's for survey and engineering sheets, where the depths exist only on the drawing.
Will it work on your sheet?
Honestly, it depends on how the PDF was made. The quickest way to find out is to try it: you see every sounding it found, laid over your sheet, before you pay anything. A sheet it can't read costs nothing.
Most likely to work
- One-page vector PDFs exported from CAD or GIS. These are the “original” PDFs, not scans.
- Depths printed as decimal numbers next to their marks, such as 17.1 or 9.8, whether typed as text or drawn as line work.
- A coordinate grid labelled around the border.
- One sheet at a time. From a plan set with several sheets, you pick the sheet to convert, and you can outline just the soundings on it.
Not supported yet
- Scanned or photographed sheets, or any PDF that is just an image of a drawing.
- Depths shown only as contour lines (isobaths) or shaded bands. On the roadmap.
- Depths written as whole numbers with no decimal. On the roadmap.
- Very large or very dense drawings. If one can't be processed, you're told why and charged nothing.
How it works
- Upload a vector PDF and pick the sheet to convert from the window that opens. To read only part of it, click around the soundings to outline them; otherwise the whole sheet is read.
- Review the result: the number of soundings, the overall confidence, and every flagged point. No account is needed.
- Pay and download once you're happy: $75 per sheet, or less with a monthly plan. You get the CSV immediately and can download it again any time within 18 months.
Your PDF is used to make the conversion and isn't kept afterwards.
Before you rely on it
The CSV is a reading of a drawing, not a new survey. It can only be as accurate as the sheet it came from, and an automatic reader can miss or misread a sounding. That's why every point carries a confidence score and why the review step exists. For estimating and checking, verify against the source sheet. Before using the data for anything where depth is a safety matter, or as the basis of a bid or design, have a licensed surveyor check it.
Try it on your sheet
Upload a sheet and see what it reads before you pay anything.