Georectification in GGP GIS

Georectification allows the GGP GIS to display a scanned image on a secondary screen, register known coordinates on both the map and the image, and then rectify the image so it aligns correctly with the National Grid. Once rectified, custodians can easily add points such as BLPU centroids for new build sites.

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Overview

Georectification is used to align non‑orthorectified site plans (e.g., developer plans, scanned drawings) to the National Grid. Once aligned, the corrected image is saved as an ECW file in the selected raster layer, enabling accurate capture of provisional NGz property seed points until MasterMap updates.

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Example Workflow

Scanned Image

Map – New Build Site for Scheme

Map with Geo‑rectified Scanned Image Inserted

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Prerequisites

Before registering an image:

1. Raster Layer
   Ensure a suitable raster layer exists and is the only raster layer selected for rendering.
2. Supported Image Format
   Image must be BMP, JPG, or PNG. DXF/DWG/PDF files can be converted using IrfanView.
3. Exclusive Access
   Exclusive access to the raster category table is required during registration.
4. Minimum Control Points
   At least four control points, evenly distributed across the image.
5. Visible Map Control Points
   Add small, bright, unfilled symbols (e.g., red circles or squares) at each map control point.

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Georectification Process

Access via: Tools → Register Image

1. Load the Image

Select the image to register. Two dialogs will appear:

• Map Registration Table – Stores map and image coordinate observations and displays residual errors.
• Register Image Dialog – Displays the image and guides you through selecting control points.

2. Capture Map Control Points

Right‑click on each control point on the map. Grid East/North values populate the Map Registration Table.

3. Capture Image Control Points

Click into the X1 column for the first row. Left‑click the corresponding control point on the image.

Repeat for all control points.

Optional: Take multiple image readings using X2/Y2 and X3/Y3 columns.

4. Compute the Transformation

Click Compute. Residual errors will display:

• Acceptable errors → proceed
• Large errors → warning appears
  • Click Ignore only if errors are still acceptable

5. Save and Refresh

Click OK to save the rectified image into the raster layer. Refresh the map to view the correctly aligned image.

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Notes & Tips

Accuracy Considerations

• Map observations:
  Snapping to grid intersections → minimal error. Snapping to MasterMap building corners → ±0.25m to ±0.5m. Manual clicking depends on scale, pixel resolution, and cursor precision.
• Image observations:
  Accuracy depends on image resolution and cursor placement. Larger pixel size increases potential error.
• Gross errors:
  Mis‑identified or out‑of‑order control points produce errors in the tens or hundreds of metres. Do not accept these transformations.

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Additional Tips

• If the Register Image form disappears behind GGP Desktop, click any X/Y cell in the Map Registration Table to bring it forward.
• Selecting a Grid East cell puts GGP into restricted registering mode. Selecting any X/Y cell returns it to waiting mode, allowing zooming, scrolling, or adding symbols.

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Supporting Tools

• IrfanView – Useful for converting DXF/DWG/PDF site plans into BMP/JPG/PNG for registration.