Guide 1 · overlay

Overlay a drawing on the aerial

Open this example in Plotlay → The same project, ready to explore. Nothing to install.

Take a scanned plan, pin it to aerial imagery and check it is right, using a real drawing: the main level plan of the Lincoln Memorial from the Historic American Buildings Survey (public domain), on the District of Columbia's 2023 orthophoto at 8 cm a pixel.

4control points (terrace corners)

0.03 mRMS error of the fit

78.39 × 57.25 mterrace measured; drawing says 78.38 × 57.17 m

What you need

The drawing: a PDF, image or DXF. Here, HABS DC-3 sheet 8, Main level plan, a 3840 px scan.
An aerial where you can see the same corners. In Washington DC, choose Map → Washington DC aerial 2023 (8 cm); in the US generally, US aerial (USDA NAIP); in NSW, NSW aerial imagery.

The HABS main level plan of the Lincoln Memorial, as scanned. — Before: the drawing as it comes, with title block, dimension strings and the steps (east) at the bottom of the sheet.

Steps

Open the site. New map project → type Lincoln Memorial or paste a Google Maps link. Switch the base map to the DC aerial.
Add the drawing. Add overlay → Floor plan, drawing or photo and choose the file. It drops in the middle of the view, unplaced.
Work out the orientation first. On this sheet the steps (which face east, toward the Reflecting Pool) are at the bottom, so the drawing is rotated: its top-left corner is the terrace's south-west corner. Plotlay solves the rotation; you only need to click matching corners.
Pin four corners. Pin to map… opens the drawing and the aerial side by side. Click the outer corner of the terrace on the drawing, then the same corner on the aerial. Repeat for all four. Use corners that are crisp on both: the outside of the terrace wall, not a shadow or the roof overhang.
Read the errors. Each pin shows how far it lands from where you clicked. Here every pin is within 0.03 m. If one pin is metres out while the rest are tight, you clicked the wrong corner. Drag it.
Make it readable. In the overlay panel: Crop to the plan (drops the title block), Line colour → Yellow so the linework shows on dark roofs and grass, and dim the base map a little.

The plan pinned on the aerial; every column lands on the real colonnade. — After: every one of the 36 columns lands on the real colonnade; the chambers are traced and measured at 860 m².

Check it against the real thing

A fit can be precise and still wrong (pinned to the wrong corners). So measure something you can check against an independent number. Here that is the dimension strings printed on the drawing itself:

Element	Measured in Plotlay	Drawing says	Difference
Terrace	78.39 × 57.25 m	257'-1¾" × 187'-6¾" = 78.38 × 57.17 m	0.01 m / 0.08 m
Memorial base	61.82 × 40.29 m	202'-1⅞" × 132'-2⅛" = 61.62 × 40.29 m	0.20 m / 0.00 m

Within 20 cm on a building 62 m long, from a scan and a public aerial.

Scanned drawings are rarely perfectly square on the page. With four or more pins Plotlay fits an affine transform, which absorbs a slight skew or stretch in the scan. With two pins it fits position, rotation and scale only.

The same with lower-resolution imagery

Independence Hall first floor plan on NAIP imagery. — Independence Hall, Philadelphia: first-floor plan (HABS PA-1430, sheet 3) on USDA NAIP imagery (0.6 m), pinned at six corners matched to OpenStreetMap footprints, RMS 0.42 m. Coarser imagery gives a coarser fit, which is still plenty for planning.

Sources

Drawings: Historic American Buildings Survey, Lincoln Memorial (HABS DC-3, sheet 8) and Independence Hall (HABS PA-1430, sheet 3), Library of Congress / National Park Service, public domain. Imagery: Orthophoto 2023 © District of Columbia, Open Data DC (CC BY 4.0); USDA NAIP via USGS The National Map (public domain).

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