Georeferencing
Georeferencing or georegistration is a type of coordinate transformation that binds a digital raster image or vector database that represents a geographic space (usually a scanned map or aerial photograph) to a spatial reference system, thus locating the digital data in the real world.
The term can refer to the mathematical formulas used to perform the transformation, the metadata stored alongside or within the image file to specify the transformation, or the process of manually or automatically aligning the image to the real world to create such metadata.
A number of mathematical methods are available, but the process typically involves identifying a sample of several ground control points (GCPs) with known locations on the image and the ground, then using curve fitting techniques to generate a parametric (or piecewise parametric) formula to transform the rest of the image.
[3] Once the parameters of the formula are stored, the image may be transformed dynamically at drawing time, or resampled to generate a georeferenced raster GIS file or orthophoto.
[5]: 240 Compared to georeferencing, orthorectification accounts for the Earth's topography, sensor optical distortions, and sometimes other artifacts [6] and is often preferred as a result.
It is thus the extension of the typical task of curve fitting a relationship between two variables to four dimensions.
These formulas allow an image to be moved (the C and F coefficients specify the desired location of the top left corner of the image), scaled (without rotation, the A and E coefficients specify the size of each cell or spatial resolution), and rotated.
[9]: 115 In the last case, if the cell size is r in both the x and y directions, and the image is to be rotated α degrees counter-clockwise, then
Instead, most GIS and remote sensing software provides an interactive environment for visually aligning the image to the destination coordinate system.
The most common method for doing this is to create a series of ground control points (GCP).
When very high accuracy registration is required, it is common to place or paint high-contrast markers on the ground at survey control monuments before the photography is taken, and use GNSS-measured coordinates for the output.
With a minimal set of GCPs, the known coordinates can be entered into the mathematical equations for the desired type of transformation, which can then be solved using linear algebra to determine the coefficients and derive the formulas to use for the entire grid.
A lower RMSE thus means that the transformation formulas closely match the GCPs.
With this metadata, the software can perform the transformation dynamically as it displays the image, so that it appears to align with other data in the desired coordinate system.
The alternative method is rectification, in which the image is resampled to create a new raster grid that is natively tied to the coordinate system.
Rectification was traditionally the only option, until the computing power became available for the intense calculations of dynamic coordinate transformations; even now, drawing and analysis performance is better with a rectified image.
