Sunday, 23 January 2011

Guide: Helmert (two-point) transformation in Quantum GIS

I have written a quick guide on using the Helmert georeferencing method in Quantum GIS. The summary can be read below and you can download it from here.
A big thanks to Conan Parsons (Oxford Archéologie Méditerranée) who pointed this out to me! :)

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.
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.

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