Appropriate transformations for georeferencing scanned maps in ArcGIS
Helyx customers involved in map production have the need to georeference scanned maps, using ArcGIS for Desktop. In doing so, they must pick a method of image transformation. However, little guidance exists to help users decide which transformation to choose and in what circumstances.
A high quality scan of a well preserved paper map should georeference to the map projection without the need for anything more than Shifting, Scaling and Rotating – all of which are achieved by an Affine Transformation. However, 3rd Order Polynomial Transformations have typically been used because it is recognised that scanned maps can exhibit deformations due to:
• Slippage of the map on the scanner.
• Moisture effects.
• Maps that don’t lie flat on the scanner.
• Due to having been bound in a book.
• Due to the map having been folded.
All these sources of error could cause localised deformations of the resulting map image. A further, more random source of error could come from inaccurate user placement of control / tie points.
This document reports the results of an empirical analysis that tests the ability of the available transformations to correct such errors, without distributing them more widely across the map. An image of a grid was deformed in various ways to simulate the different types of deformation that might occur.