Using the Helmert (two-point) transformation in Quantum GIS

This quick guide is intended to guide the reader through the Helmert georeferencing
method, which is available in the plugin repository of open source desktop GIS package
Quantum GIS (QGIS)1.
The guide has been written mainly for an archaeological audience, as it is still common
practice to offset plans of archaeological features using no more than two points, usually
the section drawing points, which are surveyed.
The plans would then be scanned and referenced to the site data for digitisation using a
two-point transformation in CAD software.
Until recently open source GIS packages, the use of which is becoming increasingly
popular within archaeology, have not been able to reference raster images using only two
points, but required a minimum of three points for (usually polynomial) georeferencing.
This is undisputedly the most accurate method of georeferencing, however the offsetting
and survey of additional points is time-consuming, which is not ideal on archaeological
sites. Hence, proprietary CAD software still had to be regularly used for two-point
transformations of archaeological site plans.
Now the excellent open source GIS package QGIS contains this tool within its
georeferencing plugin and thus provides another reason not to use proprietary CAD
software for archaeological spatial data.