where one trace can easily be obliterated by another. Analogue tachograph heads provide no indication to the driver of the need to change the disc. Analogue data is retrieved visually, and can be assisted by manual analysis tools. Analogue discs can also be electronically scanned and analysed by computer, although this analogue to digital conversion process still requires human expert interpretation for best results, due to imperfections in the source disc such as dirt and scratch marks in the wax surface that can be incorrectly read as trace marks. The analogue chart (EU) The analogue chart must be EU type approved. The country of type approval can be found on the rear of the chart, i.e. a mark of E11 would indicate the chart to have been approved in the UK for use in the EU. The chart is manufactured out of heavyweight paper with a black printed face that is thinly coated with a white wax, upon which is printed a number of features. The surface can be scratched or rubbed to reveal the black paper underneath. This enables the traces to be made without the use of ink. The chart features a pear-shaped aperture in the centre, ensuring it is perfectly aligned upon insertion into the tachograph head. There is no facility to prevent it being inserted back to front, where the styli would be prevented from making contact with the wax surface. The centrefield is used by the driver to store certain handwritten information. This includes the drivers name, the date(s) the disc refers to, the start and end odometer readings and the registration mark of the vehicle. Three traces are made in the wax surface by the head. These traces are either made by three separate styli or a single multipurpose stylus. The trace closest to the centrefield is the distance trace. The stylus moves up and down with distance travelled, producing a zig-zag pattern, often referred to as a 'V' trace. A complete deflection is created every 5 kilometres, and therefore each completed 'V' represents 10 kilometres travelled. By counting the zig-zags, the total distance travelled can be calculated and compared against the stated odometer readings in the centrefield. By comparing the end position of the trace for a particular day against the start position for the following day, it can be seen if the vehicle has moved in the intermediate period. The trace in