An introduction and demonstration of the capabilities of the GNOSIS SDK ( http://ecere.ca/gnosis ), high performance 3D geospatial visualization software built atop our open-source Ecere Software Development Kit ( http://ecere.org ). The GNOSIS SDK offers a cross-platform object-oriented API for visualizing geospatial raster imagery, vector data and elevation models in both 3D and cartographic projections. It also features GNOSIS Cartographer, a GIS software allowing to import, visualize, edit, tile, optimize, style and analyze geospatial data. Vector shapefiles, GeoTIFF imagery and ASCII Grid heightmaps are readily supported, while a plug-in system allows to extend support and functionality through additional programming. Through tiling and resampling all supported types of data (raster, vector and elevation) at multiple scales, GNOSIS can handle large planetary scale data sets (up to 1 mm resolution) with consistent performance. Elevation data models, including high resolution point clouds, can drive the terrain rendering system which performs dynamic mesh optimization based on elevation variation. All types of map data can also be draped on top of the terrain, and styles can be modified in real-time. Styling can be applied with cascading style sheets based on the values of associated data records. GNOSIS also supports geo-referencing and rendering 3D models. GNOSIS also provides a map client / server architecture through its own highly efficient protocol, while it will also support serving and accessing data through widely used protocols such as WFS and WMS. Although the GNOSIS SDK itself is currently not open-source, we hope to be in a position to release it under an open-source license in the future and possibly apply for it to become an OSGeo software project.