Cartographic Portrayal of Terrain in Oblique Parallel Projection

1. Cartographic Portrayal of Terrain in Oblique Parallel Projection Pyry Kettunen, Tapani Sarjakoski, L. Tiina Sarjakoski and Juha Oksanen ICC 20.11.2009

2. Contents • Introduction • Background • Oblique parallel projection • Hypsographic coloring and contours • Equilateral and equiangular grid • Discussion and conclusions

4. …introduction • cartography • scale-down • generalization • abstraction • 3D rendering • mostly perspective • mostly photorealistic • 3D cartography • increasing need for efficient abstraction of 3D space

5. Research objectives • development of a cartographic 3D view • intuitive and measurable • oblique parallel projection • visualization of terrain dimensions • directions, distances, heights • special attention to terrain elevation • assistance for a useful mental model of the terrain

6. Previous research • Häberling et al. (2008) • design principles for 3D maps • Cartwright (2007) • user experiment: details can make 3D view confusing • Döllner (2007) • non-photorealistic 3D geovisualization • Jenny and Patterson (2007) • plan oblique relief

7. Resources • data • digital elevation model • based on a high-resolution aerial laser-scanning • Nuuksio National Park area with few constructions • 1 m mesh, 2 dm height accuracy • software • Natural Scene Designer Pro 5.0 • ray trace rendering • oblique parallel projection

8. Oblique parallel projection • parallel projection • parallel lines in 3D → parallel lines in 2D • scale is preserved on planes || projection plane • oblique • angle between projection plane and rays < 90° • decrease in viewing angle→ increase in visibility of verticality • application to cartography • projection plane || ground • planar horizontal and vertical scales are preserved

9. OPP – Geometry projection plane Projection of co-ordinates:

10. OPP – Viewing • horizontal viewing angle = 180° (to up, north) • vertical viewing angle → 23° • occlusion is important for the 3D impression 45° 23° 15°

11. OPP – Lighting • horizontal lighting angle → SE • according to terrain and viewing direction • vertical lighting angle → 25° • mostly affects the detail depiction SW 35° SE 25° SE 15°

12. Hypsographic coloring and contours • green-to-brown equal interval coloring • 10 m interval → 5 height classes • scene-wide comparison of heights • prevention of the relief inversion fallacy • 2 m contours • more precise height level depiction

13. Equilateral and equiangular grid • equal length of segments, equal angles between→ square, triangle, hexagon

14. …equilateral and equiangular grid • deformation of a draped grid illustrates the projection • georeference frame • prevention of the relief inversion fallacy • selection: triangular (50 m) • height profiles(no redundancy) • six lines per node(path distances)

15. Discussion and conclusions • multifunctional illustration of the terrain dimensions • only direction-dependent 3D view→ simple production and use • high graphical load of elevation visualizations→ additional symbology needs visual space

16. …discussion and conclusions • terrain-dependency of the OPP set-up • occlusion is essential but hides terrain • relief inversion fallacy mostly avoided • light sources might be many and locally adapted • grid usage needs consideration • shape: triangle has benefits • segment length must be adapted to scale • grid color must not obscure

17. Future work • additional data into the view • interactive viewing angle • usability studies

18. Aknowledgements • Multi-publishing in supporting outdoor leisure activities (MenoMaps) • Finnish Funding Agency for Technology and Innovation • Ubiquitous Spatial Communication (UbiMap) • Academy of Finland • Motive-programme (2009-2012)