Datum Transformations in ArcGis defined by the user

This paper intends to treat the use of the Spatial Reference Object Model to enhance Projection and Datum Transformations in ArcGis. In the daily education and research experience in our institute ( ITC, The Netherlands) we are often faced with problems about how to combine information from existing paper maps with data from GPS-readings or with scanned maps or satellite imagery. The correct integration of these pieces of information (e.g. in ArcMap, ArcPad and many other GIS tools) requires a correct matching of the data as far as geographic location is concerned. Maps from different sources, from different types and different years do often miss the needed transformation parameters to convert them accurately into a more up-to-date form or format. Projection parameters are often incomplete, let alone the parameters of the Datum transformation and the underlying spheroids. The author will show software solutions that were developed on the demand of GIS users. The users were from different fields: map-production, education and research. The solved questions are related to projections and unknown datum parameters. The solutions require ground-control input and in certain cases a proper geoid-height model if available.