Digitisation

Digitisation is the most important technique of data input and storage in a GIS but is expensive and time consuming

Map making has been one of the most important achievements for humankind as it was because of this humankind was able to spread throughout the globe. New lands were discovered and inhabited and in all these activities the role of maps cannot be underestimated. Thus, for ages maps have been used to portray the surface of the earth on to a paper. But while viewing the maps, one generally tends to forget that each line or point depicted on the map actually represents a considerable area on the surface of earth. Thus, if lines present on the maps are not presented accurately, it means that a large area of land becomes disputed. Now we have an age of digital revolution. Right from digital movies and digital music to digital information, the internet has played a major role in accelerating this digital revolution. Maps have become a part of this digital revolution and internet mapping is the 'in' thing now. The main issue of discussion is the ways and means of depiction of maps in digital form and the probable reasons for the occurrence of such errors in the process.

Behind the success of this mapping technology is a major role played by the technology of capturing data in digital form. Data forms the backbone of the GIS industry. Spatial data is available mostly in analogue form i.e. in the form of maps, imageries, aerial photographs etc. Perhaps the biggest bottleneck in the GIS industry is the proper reproduction of the analogue data into digital form. The process of converting an analogue map into a digital format is known as "Digitisation". Although the most important technique of data input and storage in a GIS, digitisation is also one of the most expensive and time consuming aspects of data input in a GIS. The digital capture of data from the analogue sources viz. maps, imageries, aerial photographs etc. is carried out in two different methods, manual digitisation and heads up digitisation (i.e by raster scanning using optical scanners).

Manual Digitisation
Manual digitisation is carried out on a digitisation table which looks like a drafting table containing a mouse-like device with crosshairs called a 'puck'. The digitisation table works in a sophisticated manner similar to a tracing device electronically recording the positions of points and lines. It is embedded with a mesh of fine wires forming a grid like surface and this is draped or covered with some smooth surface. On this surface is mounted the map or the imagery which is to be digitised. The map is then taped to the surface which prevents the map from any types of movement. The puck is placed on the map and the electronics of the system working on electrostatic system in conjunction with the wire grid picks the signals from the puck and converts the position of the puck into a digital signal. This signal is processed by a software in the computer converting the signal gets into x,y co-ordinates depicting the position of the puck. Thus the features on the map or imagery are literally traced out by the puck. A very important aspect in this regard is the correct choice of the co-ordinate system referred to as projection system, so that the features present on the curved surface of the earth is projected properly on to a flat map. Many new digitisation softwares support a variety of projection systems but while working with a paper map, the projection used by the base map should be known and the software should be set accordingly.

The spatial data stored forms the raw data for a GIS environment. Spatial data from the maps are stored in the form of points, lines and polygons. This means that features on the maps are represented digitally in three different forms i.e. points, lines and polygons. In point mode individual locations (like cities, villages etc. depending upon the scale of the map) are recorded by positioning the puck over the point and generating a single table co-ordinate pair. In a line mode, the lines are digitised by recording the co-ordinate positions of the starting point and the end point of the line segment. This is true for a straight line. However, curved lines are digitised by breaking up the curved line into a series of straight lines and recording the co-ordinates of these series of points. A polygon is a series of closed and interconnected lines. They are generally digitised in the form of a series of straight lines but the last and the first point of this series coincide with each other.

Digitisation results in tracing the map or other analogue feature into digital form in which each feature has a proper locational identity. To ensure that each point is displayed in its original location a few steps have to be maintained while digitising. First, three general points (or reference points) located outside the map area, preferably the corners of the map, are digitised. This helps in defining the position of the map with respect to the table. This ensures that if the map is displaced, then the new corners are to be only digitised and no other changes are to be made within the map area. The next and the most important step is to register and define several control points with a minimum of four points. Control points refer to points on the map whose real geographic co-ordinates are known. This is required for transforming the digital map into a similar projection as the original map such that each feature on the digital map displays its correct geographic location. After these preliminary steps, the features on the map are traced out using the puck in the form of points, lines and polygons with each feature having the correct geographic location.

Heads-Up Digitisation
Another very commonly used method of digitisation is the on-screen digitisation or 'heads-up digitisation'. This method of digitisation is very similar to manual digitisation except that the base map or image is already in a digital raster form i.e. in the form of a digital image. It is known as 'heads-up digitisation' because the attention of the user is focussed up on the computer screen and not on a digitisation tablet. The main idea in this type of digitisation is to convert this digital image into a form usable in the GIS environment i.e. in a form such that each feature on the map has a geographic co-ordinate associated with it. In this method, the first step is to convert the paper maps or imageries into a digital image. This is accomplished using a scanner. A scanner automatically captures map features, text, and symbols in the map as individual cells, or pixels, and produces an automated digital image in raster format. But this raster image lacks any geographic information which has to be inputted manually. For this, the digital image is displayed on the screen and zoomed to a comfortable level such that all the features on the digital images can be easily traced out on the screen itself to create new layers or themes. For this method also choosing the proper projection is necessary. In a similar manner, control points with known geographic locations are identified and marked based on which the geographic co-ordinates of all the features in the map is known. Another improvement in on-screen digitisation is the interactive tracing method which automates the line tracing method in such a way that it traces one line at a time automatically under the guidance of the operator.

Accuracy in Digitisation Process
A very important aspect of digitisation is the accuracy of the digitised products. Spatial accuracy of the features depicted on the map is very important for a good GIS database. But accuracy of the digitised map depends on a number of different types of errors. The most important of which are geodetic, machine, cartographic, manuscript and random errors. Geodetic error crops out of the improper choice of a projection system. This is because maps present the three-dimensional face of the surface into a two-dimensional paper according to a projection system. Digitisation in a different projection system results in inaccurate placement of features such that the map and the digitised image do not overlap each other. Machine error is an inherent error which cannot be removed but can only be minimised. It depends on the accuracy of the digitisation table and maybe introduced while conversion from analogue to digital form of the map. Cartographic errors are errors that are present in the source map itself and which get transferred into the digital form of the map. The source maps may sometimes depict inaccurate results because of incorrect interpretations or because of incorrect drafting of features as seen on the map. Manuscript errors are introduced based on the quality of the source maps. Paper maps are known to shrink with time. Any warping, stretching, folding or wrinkling of the original map will affect the digitisation process as proper co-ordinates of such maps cannot be placed. Apart from this, the quality of digitisation also depends upon the operator who is digitising the maps. An experienced digitiser can digitise the maps with more accuracy and speed as compared to a newly appointed digitiser. Similarly, on-screen digitisation is more accurate as compared to manual digitisation as the images for on-screen digitisation are scanned at higher resolutions such that the operator can zoom the image to the scale of the original raster data and digitise with a higher level of accuracy. But ultimately the quality of digitisation still remains with the operator !