Simplification, symbolisation, and spacing
This next stage involves editing source material to include only the information to be reproduced in tactile form. Tactile maps are usually much simpler than visual maps as tactile discrimination between symbols is much coarser than visual discrimination. Figure 14 shows a base map drawn from the source material presented in Figure 13.
Avoid including excessive information (clutter). Using a range of heights for symbols can also reduce clutter in vacuum formed maps.
In standard cartographic practice map symbols are classified into three different types [38], and tactile map symbols may be similarly grouped:
- Point symbols indicate the location of single features but do not necessarily depict dimensions to scale and often bear no relation in size to features represented.
- Line symbols show positions of abstract linear features such as boundaries, and existing linear features such as rivers. Length is usually shown to scale, but width may be grossly accentuated in order to make a symbol readable.
- Area symbols depict plane surfaces, which can be represented to scale in all dimensions.
When a satisfactory version of the base map is ready, the design should be transferred on to the base layer of the map mould. Rigid card, 1.5mm thick, is a suitable base material as it does not distort when symbols are attached with most types of heat resistant adhesive. In addition, it can be drilled to aid venting and is thick enough to enable water symbols to be recessed by cutting out an appropriate shape. The cut-out sections may then be underlaid with corrugated card to represent water features.
Early work on symbol design by Nolan and Morris [39] and James and Armstrong [40] was centred on vacuum formed reproduction and led to the development of the tactile map symbol sets marketed in the UK by RNIB (see Figure 15).
These symbols were intended as a standard symbol set for tactile urban maps. It is possible that a series of symbol sets might be recognised for different applications, for example a set for golf maps and another set for maps of countryside areas (see Tables 1, 2, 3, 4, 5 & 6, in pdf format). However, it is inevitable that any tactile symbol will have more than one meaning across a range of types of map, as there are not enough individually discriminable symbols to allow each to have a unique meaning.
Samples of materials to be used for symbols should be placed on the base map to get some idea of their suitability. These materials should capable of resisting a temperature of around 160 deg. C without shrinkage, distortion or melting. In addition, symbols should not be undercut as this prevents separation of the PVC foil from the mould; a slight outward sloping angle on vertical surfaces eases separation of map from mould (see Figure 16).
Information about symbol dimensions before and after processing, as set out in Tables 1-6, is included as a guide to choosing symbol materials. The choice will be made easier if the following suggestions are considered:
- Point symbols can be solid or outline in form eg nylon finishing washer, small ‘bump-on’ (see Tables 1 and 4).
- Different combinations of width, height and cross-sectional profile give a range of useful variables for point and line symbols eg ‘Corrijoint’ shape, balsa wood, L-shaped RNIB symbol (see Tables 1 and 4 points) and string of beads, square section pine strip (see Tables 2 and 5 lines).
- Line symbols may be continuous or broken; breaks may be raised or recessed and the distance between them can be varied eg square section pine strip, solder wire 18swg, string of beads, milled solder wire, (see Tables 2 and 5).
- Area symbols are more aesthetically pleasing if large expanses feel gentle to the finger tips; a harsh texture can provide contrast for smaller features eg ‘Pebble’ anaglypta wallpaper and open coated abrasive paper grit size p24 (see Table 3).
- Size, shape, arrangement and density of textural elements influence discriminability between adjacent textures eg woodchip wallpaper and abrasive paper grit size p80 (see Table 6).
- Random and regular textural characteristics supply contrast eg ‘Pebble’ anaglypta wallpaper and corrugated card (see Table 3).
- Additional overall height of any symbol will often increase its discriminability eg 150yd and 50yd markers on golf maps (see Table 4).
Some adjustment of the map design might be needed to accommodate sufficient separation between symbols. In addition, it should be remembered that the thickness of the PVC foil will have a significant effect on symbol sizes in the final product relative to the mould. Lines and point symbols should be separated by 5.0mm in the mould, although where there are clear height differences this may not be essential. Point and line symbols may be superimposed on area symbols, as long as there are sufficient height and textural differences. Trials of draft versions of maps with visually impaired people will solve any complexities of spacing and discriminability.
It is possible to design successful maps without using braille labels as long as each symbol used is discriminable from all the others included on the map. (Apart from the title, scale, and key, the author has only used braille labels on street maps. This has not presented a problem with interpretation for any users). A separate braille label sheet can be designed to use as an overlay.
When a designer can identify each component on a finished map mould by touch, it should be given to one or more visually impaired people to test. If the visually impaired tester(s) can discriminate all parts of the design without any problems, the mould can be prepared for vacuum forming. Vent holes 1.0 - 1.3mm diameter must be drilled through the mould at regular intervals, to assist the extraction of air from the space between it and the PVC foil. These holes should be drilled at approximately 25.0mm intervals along edges of line and area symbols, and more frequently around point symbols, to allow full evacuation of air from between the mould and the PVC sheet (see Figure 17). The greater the detail, the more vents are needed to ensure the best possible replication.