Following on from exploring the layer combinations, described in the previous post, some final changes were made to the base raster from which the map tiles will be split. Firstly, to facilitate geo-referencing the base grid, an equal vertical and horizontal grid offset was applied at the rendering stage. This produces a map-like view that has the vertical aspect enhanced through the Linear Framework. Secondly, a reduced tonal range was used for the hypsometric tinting; this improves the clarity of the lower elevations, however this needs further work so as to enhance the contrast. The size of the resulting raster for the dataset area was 9055 × 13224 pixels.
The image was geo-referenced using gdal_translate to create a GeoTiff which was imported into MapBox’s TileMill to create an MBTiles SQLite file. This file was then expanded using mbutil to create a directory structure of tiles to be rendered through a client-side mapping library, in the prototype instance, Leaflet was chosen. It should be noted that no generalised elevation datasets were used to create the smaller scale zoom levels. In the case of this prototype, the down-sampling of TileMill is the only process applied. An investigation into generalisation across zoom levels would improve clarity and user experience. The image below links to the prototype map interface, this is not optimally hosted, so some of the tiles may take a few moments to resolve as you move through the zoom levels. This version is purely concerned with terrain browsing, subsequently, no linkage to cultural land-use or human settlement/constructed features is made.
To effectively communicate the form of terrain, a number of visual cues need to be rendered so our brain can construct an internal representation from the image presented to our retina. The aim of the Blue Mountains Revealed project was to achieve this through balancing clarity of depiction with an illustrative/cartographic aesthetic. This post will illustrate the separate cue layers that go into the final rendered Blue Mountains tileset. For a boarder review of relief depictions techniques please see Whelan (2011) or refer to Cartographic Relief Depiction, by Eduard Imhof.
The rendered tileset uses a combination of mass (continuous shading) and line to suggest and delineate features. Surface altitude and slope, both object-based visual cues are represented through a continuous tonal shade whilst a view-dependent line-set is created from detecting edges in a specially rendered z-buffer (described in an earlier post).
Frontal breaks of slope are suggested by the continuous shading of terrain slope, whereas rear-facing breaks of slope are highlighted through the linear cues. A hypsometric weighting orders the surface into a visual hierarchy. No interpolation across the shaded areas was applied and, as previously mentioned, the low resolution data patches have had an impact on the slope rendering. These artefacts in slope can be seen in Fig 6. The various layers are shown below along with a combination of the slope and the linear framework.
Imhof, E. (H. J. Steward (Ed)), 1982, Cartographic Relief Representation, Walter de Gruyter
Whelan J. C., 2011, Web-based Vector Terrain Exploration, Proceedings of SVG Open 2011, Cambridge MA
The activities and thoughts of a data visualization and digital mapping consultant.