Land records · Parcel maps · October 7, 2026
Georeferencing a Plat Map in GIS: How It Works
How georeferencing a plat map works: aligning a scanned plat image to real-world coordinates using control points so it overlays GIS parcel layers.
Georeferencing a plat map is the process of aligning a scanned plat or historic map image with real-world coordinates so it can be overlaid on modern GIS layers such as parcels, roads, and aerial imagery. The user identifies control points, features visible on both the plat and a reference map, such as road intersections or section corners, and the software stretches and rotates the image to fit. Georeferenced plats help researchers compare historic subdivisions, roads, and boundaries with today’s landscape. This page is general land-records background, not legal advice.
Basic steps
| Step | Description |
|---|---|
| Scan | Create a high-resolution image of the plat |
| Choose reference | Use orthophotos or parcel layers |
| Pick control points | Match features on both |
| Transform | Apply a fit, such as affine or polynomial |
| Check errors | Review residuals and visual fit |
| Save | Store with coordinate information |
Good control points
Road intersections, section corners, railroad crossings, and permanent structures make good control points. Avoid features that may have moved, such as streams or fences.
Transformations
Simple transformations shift, rotate, and scale. More complex ones bend the image to fit, which can introduce distortion. Use the simplest transformation that fits well.
Accuracy limits
Historic plats may contain drafting errors, paper shrinkage, and different survey methods. Georeferenced results are approximate and not suitable for legal boundaries.
Datums
Make sure the reference layer and project use the correct coordinate system and datum. See what is a geodetic datum.
Uses
- Comparing old subdivisions with current parcels.
- Locating vacated streets and alleys.
- Studying historic land use.
- Supporting title or genealogy research.
Tools
Free GIS software includes georeferencing tools. Some online platforms also offer simple georeferencing.
Example
A researcher scans an 1890 subdivision plat and georeferences it using four street intersections visible on a modern orthophoto. The overlay shows a vacated alley running behind current homes, explaining an odd easement in a deed.
Sharing georeferenced maps
Save georeferenced images with world files or in formats like GeoTIFF so others can use them in GIS. See how to use a county GIS viewer.
Checking your results
After georeferencing, toggle the plat on and off over the reference layer and look at areas between control points. Streets that line up near control points but drift in between suggest the plat has drafting errors or that more points are needed. Look at the residual error reported by the software for each point and remove or replace points with large errors. Recording the control points and settings used makes the work repeatable.
Common sources of mismatch
Older plats may have been drawn from compass surveys with magnetic bearings, and streets may have been built differently than platted. Some platted streets were never built. These differences are part of what georeferencing reveals.
Copyright and use
Recorded plats are public records, but some scanned collections have use terms.
Bottom line
Georeferencing a plat aligns a scanned map with real-world coordinates using control points so it overlays modern GIS layers. It is useful for research but approximate, not a replacement for surveys. Use stable control points and the right datum. Explore plats via the Platbookmapper map.
Georeferencing a plat map FAQ
What is georeferencing?
Aligning a map image with real-world coordinates.
What are control points?
Features visible on both the plat and reference map.
Is a georeferenced plat accurate for boundaries?
No, it is approximate.
What software can georeference plats?
Many GIS programs, including free ones.
What makes a good control point?
Stable features like road intersections and section corners.