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How to Georeference a Map Online and Export GeoJSON

This guide shows how to use MyMapData to upload a scanned map or PDF, align it to a basemap using ground control points (GCPs), digitize features (points, lines, polygons), and export your data for use in QGIS, ArcGIS Pro, or Python.

Works best for: historical maps, field sketches, PDFs, and scanned atlases where you need quick GIS-ready data.
Projection: basemap uses Web Mercator (EPSG:3857). Exported coordinates are WGS84 latitude/longitude (EPSG:4326).
Accuracy: depends on map quality and your control points. Verify results for critical use cases.

Contents

  1. Before you start
  2. Step 1 — Upload your map
  3. Step 2 — Add ground control points (GCPs)
  4. Step 3 — Compute the transform and check alignment
  5. Step 4 — Digitize points, lines, and polygons
  6. Step 5 — Export your data (GeoJSON)
  7. Tips for better accuracy
  8. FAQ

Before you start

You’ll get the best results if your map has a few clearly identifiable locations you can match to the real world (intersections, bridges, sharp coastline corners, building corners, survey monuments, etc.).

  • Input: PDF or scanned map image.
  • Basemap: OpenStreetMap tiles for reference.
  • Georeferencing method: Affine transform (recommended for MVP speed and reliability).
Privacy note: MyMapData is designed to process your map and edits in your browser. Payment entitlements are tracked server-side to allow exporting.

Step 1 — Upload your map

Open the app and upload your PDF or scanned map. After upload, you’ll see the map render on the canvas and the basemap beneath it.

  1. Go to mymapdata.com/app.html.
  2. Click the upload button and select your map file.
  3. If your map is large, give it a moment to render.
MyMapData upload map interface showing a PDF loaded and the basemap visible underneath.
Upload a map (PDF or scan). The basemap loads automatically so you can align your map.
Tip: If your map is hard to see over the basemap, adjust opacity sliders so you can compare features clearly.

Step 2 — Add ground control points (GCPs)

Ground control points connect locations on your scanned map to real-world coordinates. You’ll add a GCP on the map image and enter the corresponding latitude/longitude (for example from Google Maps).

Minimum points

  • Affine transform: minimum 3 GCPs (more is usually better).

How to choose good points

  • Use crisp, unambiguous features (intersection corners, bridge endpoints, sharp corners).
  • Spread points across the entire map (avoid clustering them in one corner).
  • Avoid points on vague features (trees, fuzzy shorelines, soft curves).
GCP placement view showing control points on the scanned map with latitude and longitude input fields.
Add GCPs by selecting a location on the map, then entering latitude/longitude.
Input format: You can paste coordinates as lat, lon. MyMapData accepts latitude first, longitude second (Google Maps-friendly).

Step 3 — Compute the transform and check alignment

After adding enough GCPs, compute the transform. Your scanned map will be warped into place and overlaid on the basemap. Use opacity controls to check that roads, shorelines, or boundaries line up.

  1. Click Compute Transform.
  2. Adjust the map overlay opacity to compare against the basemap.
  3. If alignment is off, add more GCPs or replace the worst ones.
Georeferenced map overlay aligned on top of an OpenStreetMap basemap with opacity adjusted.
After computing the transform, check alignment by toggling opacity and comparing to the basemap.
Accuracy disclaimer: Reported error metrics and alignment are approximations and depend on your source map quality, distortion, and GCP placement. Verify exported data for any critical or legal use cases.

Step 4 — Digitize points, lines, and polygons

Once your map is aligned, you can digitize features directly on the georeferenced overlay. Use the drawing tools in the top bar. Only one tool is active at a time.

Points (⦿)

  • Select the ⦿ tool.
  • Click anywhere on the map to add a point.
  • Points remain visible even after you exit drawing mode.

Lines (／)

  • Select the ／ tool.
  • Click to add vertices along the feature you’re tracing.
  • Double-click to finish the line.

Polygons (▱)

Polygons follow the same idea as lines, but close into an area boundary. (If polygons are enabled in your version, select the ▱ tool, click to add vertices, then finish to close the shape.)

Digitizing mode with point and line tools visible in the top bar and a traced feature on the map.
Digitize features directly on the georeferenced map. Use points for locations, lines for paths, and polygons for boundaries.
Tip: Use more vertices for curved features. Use fewer vertices for straight segments to keep your data clean.

Step 5 — Export your data (GeoJSON)

When you’re ready, export your digitized features for use in GIS tools. GeoJSON works well with QGIS, ArcGIS Pro, PostGIS workflows, and Python (GeoPandas).

  1. Click Export Data.
  2. If export access is locked, you’ll be prompted to pay to unlock downloads for your session.
  3. Download the exported file(s) and import them into your GIS workflow.
Export button and confirmation message indicating export is unlocked and files are downloading.
Export your digitized features as GeoJSON. (Optional: points can also be exported as CSV if enabled.)
Coordinate systems: The basemap uses Web Mercator (EPSG:3857). Exported coordinates are provided as WGS84 latitude/longitude (EPSG:4326), which is the standard for GeoJSON.

Tips for better accuracy

  • Spread GCPs out: place points near corners and across the map, not clustered in one area.
  • Use sharp features: intersections and corners are better than vague shapes.
  • Replace outliers: if one control point has high error, remove it and choose a better match nearby.
  • Check multiple areas: verify alignment in several locations, not just where you placed points.
  • Expect distortion: old scans and folded maps often have local warping that any simple transform can’t fully remove.

FAQ

Why does the map use Web Mercator but exports latitude/longitude?

Web maps (like OpenStreetMap) are rendered in Web Mercator (EPSG:3857) for fast tiled display. GeoJSON exports typically use WGS84 latitude/longitude (EPSG:4326) for compatibility with GIS tools and standard data pipelines.

Can I use MyMapData for property boundaries or legal surveys?

No. MyMapData is not intended for legal, cadastral, or survey-grade work. Exported results should be independently verified for any critical use.

My export downloaded only one file. Is that normal?

Your browser may block multiple downloads by default. If MyMapData exports multiple files (for example GeoJSON + CSV), your browser may ask you to allow additional downloads for this site.

Where can I get help?

Email support at support@mymapdata.com.