It is very rare that a user would specify the parameters for the transformation directly. 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). This is almost never a perfect match, so the variance between each GCP location and the location predicted by the functions can be measured and summarized as a
Root-mean-square error (RMSE). A lower RMSE thus means that the transformation formulas closely match the GCPs. Once the function parameters are determined, the transformation functions can be used to transform every pixel of the image to its real-world location. Two options are usually available for making this transformation permanent. One option is to save the parameters themselves as a form of
metadata, either in the header of the image file itself (e.g.,
GeoTIFF), or in a sidecar file stored alongside the image file (e.g., a
world file). 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. ==Software implementations==