To georeference a scanned map online, you need a tool that lets you assign real-world coordinates to points on your image. For archiving historical maps, dedicated tools like Map Warper work well. For creating a new vector map from a scanned base layer, you need an integrated editor. I built georeferencing into Gratic for this reason: so you can align your scan, trace features with standard symbols, and export a finished OCAD file, all in one browser tab.
Defining the goals for your scanned map.
The first question is always about the end goal. Georeferencing is the technical term for stretching and pinning a flat image—a scan of an old paper map, an aerial photograph, or a field-sketch—onto a coordinate system. Once it's georeferenced, it aligns with other map data. But what you do next determines the right tool for the job. The process involves picking control points on your scanned image and matching them to the same points on a reference map with known coordinates.
Many online tools are built for map librarians and historians. Their goal is to take a static, historical document and make it explorable online. A tool like Map Warper is excellent at this. It focuses entirely on the rectification process: uploading an image, finding control points, and warping the image to fit. The result is a correctly placed image that can be overlaid on a modern web map. This is perfect for making an archive of historical maps accessible to the public or for academic research. The project is the collection of rectified maps itself.
My focus for Gratic is different. I'm a cartographer who needs to produce a new map, often for an orienteering event. The scanned map isn't the final product; it's the starting point. It’s a source, just like satellite imagery or a GPS track. The goal is to extract the information from that scan and create a clean, accurate vector map. This means the georeferencing tool can't be a separate, final step. It has to be the first step in a continuous drawing process. That’s why I built it directly into Gratic. You align your image, and then you immediately start drawing over it with vector pens. The georeferenced scan becomes just another layer in your map file, one you can toggle on and off as you work.
Georeferencing inside a vector editor.
Georeferencing inside a vector editor like Gratic integrates the process into your cartographic workflow. Instead of producing a warped image file to be used elsewhere, you are setting up a temporary tracing layer inside the project where you will build your final map. This integration saves steps and keeps all your work in one place.
When I designed this feature, I wanted it to feel like part of the mapping process, not a preliminary chore. In Gratic, you start a new map by selecting an area. The editor immediately pulls in available open-source data for that location—roads, water features, buildings—and generates contour lines from our elevation data. This gives you a live, accurate vector map from the very beginning. This is your reference map for placing control points.
Here’s how the process unfolds:
- Upload Your Scan: You upload your scanned map or aerial photo directly into your project. It appears as a floating, semi-transparent layer over the base map.
- Add Control Points: You find a feature you can identify on both your scan and the live map—a road intersection, a sharp bend in a river, a distinct building. You click it on your scan, then click the corresponding point on the vector base map. This creates your first control point pair. The live vector data from national mapping services makes finding precise points much easier than using satellite imagery alone.
- Repeat and Rectify: You repeat this process. After three points, Gratic can calculate the transformation and warp the image to fit. The more points you add, the more accurate the fit becomes, especially for scans that have distortions. You can see the image stretch and shift into place in real time.
Once the scanned map is aligned, it sits as a raster layer in your layer stack. Now you can create new vector layers on top. You can trace a forgotten trail using the line pen, map a marshy area using the area symbol for a marsh, or add individual features like boulders. You are using the scan as a guide to create new, native vector objects. These objects conform to ISOM standards because you're using the correct symbol sets from the start. When you're done, you can hide or delete the scanned layer, leaving a clean vector map ready for export as an OCAD file.
Choosing the right tool for your mapping project.
Choosing the right online georeferencing tool depends entirely on whether the scanned map is your final product or your primary source material. It’s a choice between an archival tool and a production tool.
If your task is to take a collection of historical paper maps and make them available for others to view online, overlaid on a modern map, then a dedicated georectification tool is what you need. Map Warper and similar services are built for this purpose. They support collaboration, allowing multiple people to help place control points, and their output is designed to be published and shared as a web map service. The focus is on preserving and presenting the original document in its new, geographic context. The work ends when the map is correctly warped.
However, if you are a cartographer making a new map for navigation, like an orienteering or terrain map, your needs are different. You don't want to just look at the old map; you want to mine it for data. You need to trace its features, update them with your own field survey data, and produce a new, standardized vector file. For this, you need a cartography editor with built-in georeferencing. This is the problem I built Gratic to solve.
The workflow is fundamentally different. You aren't just creating a warped image; you're creating a new dataset. The georeferenced scan is a temporary guide. The real work happens in the vector layers above it. The ability to pull in existing vector data and live contours for your chosen area is critical, as it provides a much more reliable base for placing your control points than a blurry satellite photo. When you're finished, you export an OCAD file, the standard for orienteering cartography. The entire process, from aligning the scan to laying out the print file, happens in one application, in one browser tab.
So, if your project is about publishing a historical map online, look at archival tools. If your project is about creating a new vector map from a scanned source, you need an editor that treats georeferencing as the first step of drawing, not the last step of scanning.
Frequently asked questions
Can I use any image file for georeferencing in Gratic? Yes, you can upload common image formats like JPEG and PNG. The key is that you can identify distinct points on the image that correspond to real-world locations on the reference map Gratic provides.
What happens to the georeferenced image when I'm done? The image remains a raster layer within your Gratic map file. You can hide it to see your clean vector work, or show it to check your tracing. When you export to OCAD, the vector data is the primary output, as that's what's needed for high-quality printing and use.
Is the process the same for aerial photos? Yes, the process is identical. An aerial photograph is just another image you can use as a base layer. Georeferencing it allows you to trace features like vegetation boundaries, paths, and structures accurately onto your vector map.