The paper presents open source software tools developed by the author to facilitate <i>in-situ</i> documentation of architectural and archæological heritage. The design choices are exposed and related to a general issue in conservation and documentation: <i>taking decisions</i> about a <i>valuable</i> object under <i>threat</i> . The questions of level of objectivity is central to the three steps of this process. It is our belief that <i>in-situ</i> documentation has to be favoured in this demanding context, full of potential discoveries. The very powerful surveying techniques in rapid development nowadays enhance our vision but often tend to bring back a critical part of the documentation process to the office. The software presented facilitate a direct treatment of the data on the site. Emphasis is given to flexibility, interoperability and simplicity. Key features of the software are listed and illustrated with examples (3D model of Gothic vaults, analysis of the shape of a column, deformation of a wall, direct interaction with AutoCAD).