What is Georeferencing?
Georeferencing is the process of associating maps or raster images to spatial locations or physical space. For RoS, this means assigning map coordinates from the OS map to pixels of raster images or scans of maps and deeds (in image formats such as jpeg, tiff).
Georeferencing tools and software use a series of Ground Control Points (GCP) to apply mathematical transformations to achieve alignment between the un-georeferenced images and an underlying map or datum (e.g. OSGB36 / British National Grid - EPSG:27700).
What are Ground Control Points?
Ground Control Points (GCPs) in georeferencing are known locations on the Earth that are identified both in a dataset (like an image or raster) and in real-world coordinates. They act as reference points to accurately position the dataset in a geographic coordinate system, e.g. OSGB36. Think of them as anchors that tie a digital map to the real world.
In RoS, control points are features that can be clearly identified on both the OS map and on the deed plan image to be georeferenced. Common examples include road junctions, walls/fences or stream intersections, the mouth of a stream, rock outcrops, or the corners of buildings.
The control points chosen to create a transformation will eventually shift the deed plan image to the correct spatial location via the associated link between the control points. The number of control points required will depend entirely on the complexity of the map image and the type of transformation selected. Adding extra control points does not necessarily make the transformation better and plans officers should always be guided by the transformation and the results.
Generally, placing control points at the four corners of the deed plan image and then spreading a remainder of GCPs throughout the image will have a better effect than concentrating in one area, although that will be dependent on the overlap between the map image and the area which is to be transformed.
Transformation types
There are two types of transformation currently available in the Plan Creator georeferencer tool.
Affine transformation
The Affine transformation is a geometric transformation that preserves lines and parallelism but not necessarily distances or angles. It is a type of bilinear transformation that can be used to correct for geometric distortions in images, among other applications. Common examples include translation, scaling, rotation, and shift along the X and Y axes, all of which can be combined into a single affine transformation, which is what is available in Plan Creator.
In Plan Creator, a minimum of four control points is required for the Affine transformation. The control points should ideally be located surrounding the subjects, and not within the subjects or only in corner of the plan as this will adversely affect the accuracy of the transformation. Further points can be added to improve the transformation if the results are not as accurate as expected.
When to use:
The Affine transformation in Plan Creator is the default option as it should suit the majority of good plans based on the OS map. This is a good transformation to use when the deed plan image matches the OS map e.g. the deed plan shows an edition of the OS map at the correct scale (both current and not distorted historic OS mapping). This is because there is no inherent warping of the image, and it is simply adjusted to fit the OS map, and because they are the same "thing" it should overlay accurately.
Warp transformation
The Warp transformation is a polynomial transformation and is a good transformation method generally used to interpret and approximate complex curves and shapes. Warp transformation involves aligning a raster image (like a scanned map) with real-world coordinates by applying a spatial transformation. This process stretches and warps the image to match known locations (control points) on a geographically referenced dataset. This means they are a very good choice when the original image to be georeferenced is known to be distorted or curved.
In Plan Creator, a minimum of 8 control points is required to use the Warp transformation. By adding more control points (although the residual error may increase in some instances) the overall accuracy of the transformation will increase. However, it is important to use care when applying control points to polynomials as one misplaced point could have a great (negative) impact on the transformation overall.
The minimum of 8 control points should ideally be located surrounding the subjects, and not within the subjects to be plotted or only in corner of the plan as this will adversely affect the accuracy of the transformation. Further points can be added to improve the transformation if the results are not as accurate as expected.
When to use:
Warp is the alternative transformation option when Affine does not produce acceptable results and it is a good transformation to use when the deed plan image is either known to be distorted (e.g. distorted photo copies of the county series map), or is not based on the OS Map but has sufficient common features between the plan and the OS Map to use as the 8 control points. This is because there is not a direct relationship between the plan and OS map so the spatial relationship of control points will not be the same.