Historic maps do not always have the same cartographic accuracy as modern maps, and so it can be difficult to compare them when looking for changes over time. The geo-rectification process allows users to align historic and modern maps by identifying 'match points' between the maps, e.g. a specific intersection. The historic map can then be stretched or warped to align with the modern map. The result is that one map can overlay the other and changes over time can be identified. The National Library of Australia and the Public Record Office Victoria both have geo-rectification projects that encourage public participation.