You georeference a scanned map online by uploading it to a browser-based editor and aligning it with known coordinates. I designed Gratic's georeferencing as a live tool for creating new maps from older sources. You place your scanned image, like a field sketch or an older map, onto a real-world base map. This turns the static image into a traceable layer for creating a new, accurate orienteering or terrain map with my vector pens.
The process of georeferencing in a browser
Georeferencing in a browser involves anchoring a static raster image to a live map by matching points on both. The process transforms your scanned map from a simple picture into a spatially aware layer in your project. In Gratic, this entire workflow happens in a browser tab, with no software to install or license. You begin by creating a new map project, which loads my base data for your chosen area, and then you upload your scanned image. The editor places your image as a floating object over the base map, ready for you to begin the alignment process by identifying control points.
Control points are the core of georeferencing. These are identical, recognizable features that exist on both your scanned map and my base map, such as a road intersection, a building corner, or a distinct bend in a river. You click the feature on your image, then click the corresponding feature on the base map. I recommend at least three such pairs of points, as this is enough for the editor to calculate the necessary transformation. The software then rotates, scales, and skews your image to fit the real-world coordinates of the base map, a process known as an affine transformation that corrects for distortions from the original scanning process.
The key to this workflow is the quality of the underlying base map data I provide. When you open a project, you are not starting with a blank canvas. Gratic automatically pulls in available data, including elevation models I hold tile by tile and vector data from national mapping services. This means you can find reliable anchor points on detailed contour lines, streams, and roads that are already correctly positioned. The entire experience is designed for cartographers, removing the technical overhead of traditional GIS software and letting you focus on the map itself.
Matching a source image to the right base data
The quality of your final map depends on both the source image you start with and the base data you align it to. A good source image for georeferencing might be a high-resolution scan of a historical topographic map, a hand-drawn field sketch from a survey, or an unreferenced aerial photograph. The main requirement is the presence of clear, distinct features that you can confidently identify on the base map. While higher resolution is generally better, even rough field notes can provide the critical initial layout for a new orienteering map when aligned correctly.
I built Gratic to provide reliable base data directly within the editor, simplifying this matching process. As my site states, I publish both halves of what the editor can pull in: my own tiled elevation data and vector data from every national mapping service I can connect to. This integrated approach means you are aligning your scan against verified information. For a terrain map, you can use my detailed contour lines as your ground truth; for an urban map, you can use building footprints and roads imported from official sources. This removes the need to source and import base layers yourself.
My commitment to transparency extends to this data. I provide information on the source of the national mapping data, including its license, and am clear about why a source might be blocked or unavailable. In addition to vector data, you can also use satellite imagery as a base layer, a method I used when creating a map of a half-square-kilometre area in Cyprus. Tracing over satellite imagery is a powerful technique, especially in regions where official vector data is sparse. The choice of base layer depends on your project's specific needs, but the tools to align your source image are consistent across all of them.
From a georeferenced image to a finished map
Once your scanned map is georeferenced, it becomes a template for creating a new, precise vector map using professional cartography tools. The aligned image sits as a semi-transparent layer within the Gratic editor, allowing you to trace its features accurately. This is the primary purpose of georeferencing within my workflow: to enable the creation of new, high-quality vector data from an existing raster source. The original image serves its purpose as a guide and is then superseded by the vector objects you create.
This is where my focus on cartography becomes most apparent. You are not tracing with generic line and shape tools; you are using pens that behave as a cartographer expects and apply standard symbol sets. When you trace a fence from your georeferenced field sketch, you select the fence tool, and Gratic draws it according to the correct ISOM specification. This object-based approach means that if you need to adjust the symbol later, you can change it once and the update applies to every instance of that fence on your map. This ensures consistency and adherence to established mapping standards.
The entire workflow is designed to produce a professional map file. The case study of the Cyprus map, where I traced 400 objects over satellite imagery, shows this process in action for a real-world project. When your drawing is complete, you have several options. You can proceed to lay out the map sheet for printing directly from the browser. For those who use other professional cartography software, Gratic files are designed to open in OCAD. This ensures that the work you do in my editor fits into established professional workflows, allowing for collaboration and finishing in other specialized desktop applications.
Frequently asked questions
Can I use my own base map data? Gratic provides base map data from national mapping services and my own elevation models directly in the editor. While you cannot upload custom vector base maps, you can upload a georeferenced image of your own and use that as a visual guide for tracing new vector features with my tools.
What file formats can I import for georeferencing? You can upload common raster image formats like JPEG and PNG. The feature is designed to help you align a static image that lacks spatial information. The vector data, such as roads and contours, is what you create on top of this layer using my drawing tools, which are built for orienteering and terrain maps.
Does Gratic work with any map projection? Gratic operates on a standard web map projection to ensure that data from my various sources, like elevation tiles and national mapping services, are displayed consistently. When you georeference your scanned map, you are aligning it to this system, which makes your new vector map compatible with these datasets.
Can I export my georeferenced image? The purpose of georeferencing in Gratic is to enable the creation of a new vector map. The primary output is the vector data you draw, not the transformed image. This vector data can be exported as a file that opens in OCAD, fitting into professional cartography workflows. The georeferenced image remains a tracing layer within your Gratic project.