PDF Underlays
PDF Underlays
Ribbon: Draw → PDF Attach Command: PDFATTACH, PA
Attaches a PDF page as a raster underlay you can snap to. This is how you trace a recorded plat, a utility as-built, or a GIS export without redrawing it first.
Attaching a PDF
PDFATTACH — select PDF file and options in the dialog, then place the underlay.
The PDF Attach dialog:
| Section | Controls |
|---|---|
| PDF File | Path and Browse… |
| Page Selection | Thumbnails of every page — %d page(s). Click the page you want |
| Placement | Specify on screen, or type Insertion X:, Insertion Y:, Scale:, Rotation °: |
| Snap Recognition | Which features to detect for snapping |
| Raster DPI: | Rasterisation resolution, 72–300 |
Then press Attach.
- Specify on screen —
PDFATTACH — click in viewport to set insertion point. ESC to cancel.Click to place, then set the scale. - Direct insert — enter the insertion point, scale, and rotation numerically and it is placed without a click.
While the page is being converted the dialog shows Rasterizing... and the log says
PDFATTACH — rasterizing, please wait.... On success: PDFATTACH — underlay placed.
Raster DPI
Higher DPI means sharper geometry and more snap targets, at the cost of memory and rasterisation time. 150 is a reasonable default for a plan sheet; go to 300 for a dense GIS export where the lines are thin.
Background handling
Both light-background PDFs (white paper) and dark-background PDFs (CAD exports) work. The background is detected automatically and made transparent, so the underlay floats cleanly over your drawing at any opacity.
The Background toggle on the PDF Underlay ribbon section switches this:
| State | Tooltip |
|---|---|
| Background ON | lines visible, paper transparent. Click to show paper. |
| Background OFF | full raster image visible. Click to hide paper. |
The PDF Underlay ribbon section
Appears when a PDF attachment is selected.
| Control | What it does |
|---|---|
| Background | Toggles paper transparency, as above |
| Vectorize | Vectorize Lines — add PDF snap-line geometry as drawing entities on the current layer |
| Fade | 0–100% opacity slider |
| Snap: L / C / T | Per-underlay snap toggles for Lines, Circles, Text |
Vectorize
Vectorize converts the snap-lines GoSurvey detected in the raster into real drawing entities
on the current layer. It is a bulk trace: instead of drawing over every line by hand, you get
geometry you can then trim, join, and clean up with OVERKILL.
Set the current layer before you press it — everything lands there.
Snapping to a PDF
Snap targets are found by analysing the rendered raster image, not the internal PDF path structure. This matters: GIS exports, scanned drawings, and dense CAD PDFs routinely contain path objects that do not correspond to any visible line. Reading the image is what makes snap land on what you can actually see.
| Target | How it is recognised |
|---|---|
| Endpoints | Line ends and stroke terminals — one foreground neighbour in the raster topology |
| Corners | Bends and junctions where two lines meet at an angle — two non-opposite neighbours |
| Junctions | T-intersections and crossings — three or more neighbours |
| Midpoints | On detected PDF line segments |
| Perpendicular | On detected PDF line segments |
Candidates are filtered through a visibility mask so only points in areas with visible content are offered, and a spatial grid keeps lookup fast even on PDFs with thousands of targets.
Object snap must be on (F3) and the underlay's own Lines / Circles / Text toggles decide
what it offers. See Object Snaps.
Multiple underlays
Several PDFs can be attached at once. Each has independent position, scale, rotation, opacity, and snap toggles.
Selecting several at once gives a bulk Fade control in the Properties panel: "N PDF underlays selected."
Properties
Select an underlay and the Properties panel shows:
| Field |
|---|
| File name and page number |
| Insert X · Insert Y |
| Scale · Rotation (deg) |
| Fade |
| Object Snap — Lines · Circles |
A tracing workflow
Goal: get a recorded plat into a drawing at real-world coordinates.
PAand attach the plat PDF at, say, 200 DPI. Place it roughly.- Set Fade so you can see your own linework against it.
- Turn OSNAP on (
F3) and check the underlay's Lines snap toggle. - Draw the boundary with
LINEorPOLYLINE, snapping to the PDF corners. Or press Vectorize to convert the detected lines in bulk, then clean up. - Run
OVERKILLto remove duplicates and merge collinear segments. - If you have control coordinates for two monuments, run
ALIGNto move the traced geometry onto state plane — see Coordinate Alignment. - Delete or fade the underlay once you are done.
Common problems
| Problem | Cause | Solution |
|---|---|---|
Unable to open PDF. |
The file is corrupt, encrypted, or not a PDF | Re-export it, or open and re-save it from a PDF reader |
PDFATTACH — failed to rasterize page. |
The page could not be rendered | Try a lower Raster DPI, or a different page |
| Nothing to snap to | The underlay's Lines toggle is off, or OSNAP is off | Check both; press F3 |
| Snap targets are sparse or blobby | The raster DPI is too low for the line weight | Re-attach at a higher DPI |
| The underlay hides the drawing | Fade is at 0% and Background is on | Raise Fade, or turn Background off |
| The traced geometry is the wrong size | The insertion scale was a guess | SCALE with R (reference length) across a known dimension, or ALIGN with control pairs |
| Vectorize produced a mess of short segments | Raster tracing always does, on a busy sheet | OVERKILL, then JOIN |
Related
Object Snaps · Coordinate Alignment · Modify Tools · Import and Export