Best practice guidelines for the design, production and presentation of vacuum formed tactile maps

Some basic theoretical information

Tactile maps are read by touch in a similar way to braille and moon text. Visually impaired people use the fleshy pads of their fingertips to explore and read raised images. Temporary deformation of the skin, i.e. alteration of its usual contours through contact with a surface, provides stimuli to nerve endings in the skin [10]. The skin on the palmar surface of hands is hairless and is differentiated into two layers, the epidermis or outer layer and the dermis or inner layer. The boundary between them is characterised by finger-like protrusions, known as dermal pegs, which extend from one skin layer into the other. Four types of nerve ending are found in the dermis close to these dermal pegs (see Figure 1,[11]).

When the skin on the fingertips is moved, as when a tactile map is explored, the nerve endings associated with the dermal pegs are displaced. When those nerve endings are moved, signals are transmitted to the brain via connecting nerve fibres, and the reader gains an insight into the structure of the surface being felt [12]. Lederman [13] suggests when someone explores a surface by touch, three aspects of skin deformation help them to discover the structure of that surface. Firstly, the depth to which part of the fingertip descends into a groove, secondly, the cross-sectional area of the part of the fingertip lying within a groove, and lastly the cross-sectional area of the skin on the finger tip that has been moved from its usual resting position.

Katz [14] observes that people must move their fingers across a surface in order to discern its surface qualities and to recognise what the material is; recognition improves with experience. He suggests that using five fingers is more effective than using a single finger, and that people sense the tactile properties of a surface through vibrations transmitted via their fingertips. During tactile exploration, a person senses changes in the surface of an object, rather than changes in the surface of their skin [15]. When people pass their fingers sideways over textured surfaces, tactile stimulation is across the ridges on the skin of their fingertips, which enables the detection of smaller textural elements than when the direction of stimulation is parallel to the skin ridges [16]. This phenomenon may have a bearing not only on the way tactile map readers use their fingers but also on the orientation of area symbols.

Tactile perceptions are also derived from information gained via tissues lying underneath the skin [17]. Gibson [18] suggests that ‘joint position sense’ registers the perception of space and movement, including the relative positions of different parts of a person’s exploring hands. Readers gain an idea of the form and properties of a raised image when they move their fingertips across a tactile map or diagram; it is essential that a consistently light finger pressure is used and continuous contact between skin and image surface is maintained. However, those readers will only be able to understand what is represented if they are able to feel each part of a raised image, and discriminate between them. Tactile discriminability depends on the quality of symbol and map designs, particularly the spacing between each part of a map or diagram.

Map scale, the complexity of the information portrayed and the production technique employed, govern spacing between symbols [19]. Nolan and Morris [20]suggest that 3.0mm is the minimum inter-symbol distance that will ensure readability of a tactile map, and Edman [21] and Amick et al [22] advise that symbols separated by a space less than 3.0mm are unlikely to be individually discernible.

It has not been made clear in previous guidelines whether suggested inter-symbol spacing refers to finished maps or to the moulds for vacuum formed maps. In addition, no mention is made of the alteration to spacing that results from using different gauges of PVC foil as the reproduction material.

When vacuum forming techniques are used for maps, the thickness of the PVC foil has to be taken into consideration when calculating distances between symbols. For example, when 250 microns (0.25mm) thick PVC foil is used, a raised line 2.0mm wide on the map mould will be at least 2.5mm wide in the finished product (incomplete evacuation of the air between mould and PVC foil may further increase the width of a finished line). Therefore, if there is a space of 3.0mm between symbols on a map mould there is likely to be a final gap of no more than 2.5mm between those symbols. This is less than the recommended minimum of 3.0mm. Consequently, extra spacing must be included in the base map design to accommodate a double thickness of the reproduction medium between raised symbols. Spacing of 5.0mm between symbols on a map mould will ensure adequate spacing in a finished map.