Principles of Bar Code
The use of bar codes can be traced back as far as the 1960s. In the early 1970s bar codes became more and more common, appearing on grocery-shop shelves as the UPC code to automate the process of identifying items of grocery. Today, you can find bar codes just about everywhere; they are used for identification in almost all types of business. Implementation of a bar code system allows procedures to be automated to increase productivity and reduce human error.
As years went by the industry developed several different types of bar code standards for differing purposes; these are called ‘symbologies’. Each defines the printed symbol and how a device such as a bar code-scanner reads and decodes this. Most codes have in common that they represent a graphic image of a number indicated by blanks and blacks, zero and one, thus forming a number in the binary system. A wide black or blank represents two or more zeros or ones.
The main reason for using the bar code system is the accuracy and productivity of the system. In a recent study at Ohio University, several different bar code symbologies were tested to determine their accuracy. A well-trained data-entry operator will usually make a data entry error once every 300 keystrokes. Implementation of even the least accurate symbology thus represents a huge step forward in increasing production and reducing data entry errors. The least accurate symbology, UPC is used in the retail industry. Data Matrix is the most efficient symbology, holding up to 800 characters.

Bar Code and Surveying
In the geomatic industry the use of bar codes is presently restricted to digital levelling. The rod that comes with a digital level instrument is imprinted with a bar code pattern. Unlike the bar code patterns described before, this is not a graphic depiction of a binary number but a unique code pattern all over the length of the rod. The image of the pattern on the rod passes through the objective lens and then via a beam-splitter to a photodetector array, where it is digitised. The microprocessor compares this image to a copy of the bar code and calculates the staff reading, which is displayed and/or stored. The sensitivity of the device is such that single-reading accuracy of 0.2mm to 0.3mm can be achieved and sight lengths can be extended up to 100m. It is obvious that the incidence of misreading and/or wrong notation will fall and therefore the productivity and reliability of the levelling will improve. The system also recognises an upside down rod, which can be useful when performing a double levelling in one run.

The Future
Miniature radio chips that transmit identification data to a receiver at the shop cash desk will replace bar coding in the near future. Besides a further improvement in error-free production and faster transactions, these can also be used as a security device. The levelling rod will, however, survive this next development, as the application of radio transmitters on a rod is hard to imagine. The printing of the bar code on the rod and the sensitivity of the instrument will be improved, thus achieving better accuracy and longer distances.