Building a QGIS Helper Application to Overcome the Challenges of Cassini to UTM Coordinate System Conversions in Kenya

Abstract

Three major coordinate systems have been in use in Kenya. They include; Cassini-Soldner, East African War system and UTM coordinate systems. The existence of these multiple systems have triggered the necessity for regular conversions particularly from Cassini to UTM. This is mainly because in Kenya, cadastral surveys are based on Cassini while topographic maps are based on UTM. Thus, it becomes a challenge for users to merge data sets in these two systems. The core objective of this study was to build a helper application to overcome the challenges of Cassini and UTM coordinate system conversions. The methodology used involved carrying out a desk study, presentation and discussion of results from the study and the design of a QGIS helper application that converts coordinates between Cassini and UTM. The desk study involved carrying out a review of existing information on methods currently applied in Kenya to facilitate the conversions. This was aimed at justifying a suitable host application for the plug-in; with QGIS and ArcGIS as the main options. The results from the study were that conversions between Cassini and UTM in Kenya are done using: a calculator, GIS Software, online hosted applications, and Excel Spreadsheet templates. It was determined that QGIS is the best host application since it is an open source software among other reasons. This information was the basis for the design of a QGIS plug-in that converts between Cassini and UTM coordinate systems. The application was developed using Python Programming language. The method used involved computing 6 transformation parameters: scale, rotation, 2 translation elements and 2 other unknowns and compiling them into a csv table (acts as the plug-in database). The developed plug-in works for Cassini and UTM coordinates extending from Longitude 36.5°, Latitude -1.0° to Longitude 36.75°, Latitude -1.25°. It is extendable to work for the whole country without any modifications to the main software code. This can be done by populating the csv table (plug-in database) to include transformation parameters for the rest of the country.