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Georeferencing a PNG format 2D plan in QGIS — step-by-step

Overview

This guide walks through georeferencing a scanned or exported site plan (PNG or TIFF) in QGIS 3.x so it aligns with real-world coordinates. The end result is a GeoTIFF that drops onto your map in the right place, overlaying your basemap.

Before you start

You'll need QGIS 3.x, where the Georeferencer is built in (no plugin to install). Have your plan file ready (PNG or TIFF), and have a reference layer to click matching points against — either an online basemap (OpenStreetMap or Bing, e.g. via the QuickMapServices plugin or an XYZ tile connection) or the background maps function (Tools - Maps) provided by Siradel Planner. You should also know, or be able to identify, a few real-world coordinates for recognizable spots on the plan.

Step 1 — Fix the image orientation

If your image is upside down or rotated 90°, straighten it before georeferencing. Open it in any image editor (or even Windows Photos / macOS Preview), rotate it roughly upright, and save. It doesn't need to be perfect — just not flipped or a quarter-turn off.

A moderate tilt or skew does not need manual correction: with enough well-placed control points and a Polynomial 1 (affine) transformation, QGIS rotates, scales and skews the image automatically to match. So only fix gross orientation by hand; let the georeferencing handle the rest.

Step 2 — Open the Georeferencer and load the plan

Everything from here on is done inside QGIS.

  1. In the main QGIS window, open the tool from the top menu bar: Layer ▸ Georeferencer… (in some older builds it's under Raster ▸ Georeferencer…). It opens as a separate Georeferencer window on top of QGIS.

  2. In that Georeferencer window, click Open Raster — the first button on its toolbar (a file/image icon).

  3. Browse to your plan file (PNG or TIFF), select it, and click Open. The image appears in the Georeferencer canvas.

  4. If QGIS prompts you for a coordinate reference system at this point, accept the default — the real CRS is set later through the control points and the transformation settings (Step 4), not here.

Tip: if you plan to use From Map Canvas to pick coordinates in Step 3, first load your reference basemap into the main QGIS map canvas, so you have something to click against.

Step 3 — Add ground control points (GCPs)

This is the core step: you tie recognizable spots on the plan to real-world coordinates.

  1. Click Add Point, then click a clearly identifiable location on the image (a road intersection, a building corner, a survey mark).

  2. In the dialog, supply the coordinates one of two ways:

    • Type the X/Y directly if you know them.

    • Click From Map Canvas to jump to the main QGIS window and click the matching spot on your reference basemap — usually the easiest and most reliable option.

  3. Repeat for at least 3–4 points, spread toward the corners of the image rather than clustered in the center. More well-spread points give a better fit and let you check the error.

Make sure you enter X = easting/longitude and Y = northing/latitude in the correct fields — swapping them is the usual cause of a mirrored result later.

Reading precise coordinates from Siradel Planner

Instead of QGIS's From Map Canvas, you can read exact coordinates for a reference point directly in Siradel Planner:

  1. In the Scene manager panel, right-click Sites, then Create site ▸ Create site in viewer.

  2. Right-click in the viewer to drop the site precisely on your reference point.

  3. Open the newly created site; its Coordinates section shows that point's exact values — X/Y in Metric, and DMS and Decimal longitude/latitude.

  4. Enter those coordinates for the matching GCP in QGIS.

Make sure the coordinate system used in Siradel Planner matches the target CRS you set in QGIS (Step 4) — otherwise the points won't line up.

Step 4 — Transformation settings and choosing the CRS

Open Settings ▸ Transformation settings and configure:

  • Transformation type: Polynomial 1 (affine) for most flat maps with 3+ points; Thin Plate Spline if the image is warped and you have many points.

  • Resampling: Nearest neighbour for crisp lines, Cubic for smoother imagery.

  • Target CRS: the coordinate system your control-point coordinates are in.

  • Output raster: where the georeferenced GeoTIFF is saved.

  • Tick Load in QGIS when done.

Step 5 — Run and verify

Click Start Georeferencing (the green triangle). QGIS produces a new GeoTIFF and, if you ticked the option, loads it onto your map.

Check the result two ways. First, look at the residuals column in the GCP table — high values flag a badly placed point you should move or delete. Second, confirm the plan overlays the basemap correctly (roads and corners line up). If it's mirrored or clearly offset, the coordinates were likely entered with X/Y or lat/lon swapped, or the wrong target CRS was chosen — go back to Step 3 or 4.

Troubleshooting and tips

  • Mirrored / flipped output: almost always X and Y, or latitude and longitude, swapped when entering a GCP. Check the field order and the target CRS units.

  • Offset by a consistent distance: usually the wrong datum (e.g. WGS84 vs ED50) or the wrong UTM zone. Test the neighbouring zone or datum.

  • Poor fit / warping: add more points and spread them to the corners; watch the residuals and re-place the worst offenders.