Raw geographic data are available in many different analog or digital forms, such as maps, aerial photographs, satellite images, or tables. A map is a graphic depiction of all or a part of a geographic realm in which the real-world features have been replaced by symbols in their correct spatial location at a reduced scale, serving as a method of visually interpreting the data representing the real-world features. Although the spatial information provided by maps is on paper, they form an important source of data for GIS. But to utilise the data for different types of analysis, one needs to have the data in digital form. The need for this type of digital data has also led maps to become the final stage in GIS work. Maps within the GIS environment can be temporary, designed merely for a quick informative glance, or permanent, for presentation of ideas as a substitute for a picture or a report. There are three, not mutually exclusive ways, to create a digital geographic database: (a) acquiring data in digital form from a data supplier, (b) digitisation of existing analog data, and (c) carrying out one’s own digital survey. However, in all cases, the data must be geometrically registered to a generally accepted and properly defined co-ordinate system. Whether in digital or analog form, the data has to be first converted into a form that is acceptable to the internal database structure of the GIS being used. Once data have been captured, they must be checked for mislocation and value errors. Most GIS provide data editing tools for this work. Once this is over, the data is assigned to several different layers and the entire map can be obtained by viewing all the layers simultaneously.
Map production procedures start with detailed plans and proper research related to the production of the maps. The statistical data, which are related to the maps, have to be documented properly. Choosing the base maps, the source maps and the type of projections required for the production of the new maps are the most important parts of the map-making procedure and therfore these are carefully selected. The cartographer is required to produce the essential data required for production of new maps. These data include the map neatly drafted in fine pencil or ink on Mylar, keyed to stable base map; a description of map units, credits, title, authors, field dates, references in a text file; the name(s) of technical reviewer(s). Along with this, the specifications required for making the new map are clearly defined. The specifications or the map elements refer to the objects that are included on a map or chart, such as the title, legend, colour of the various elements in the map, title, acknowledgements and scale of the required map, composition of maps including font style and size, etc. For remotely sensed images, the minimum descriptive information that should be included on the map is the row, path and date of the image.
The data represented in the map can be manually incorporated to a GIS in four main stages:
- Entering the spatial data,
- Entering the attribute data,
- Spatial and attribute data verification and editing, and
- Where necessary, linking the spatial to the attribute data.
The simplest way of inputting spatial data is to type the coordinates into a file or input programme of a GIS. But this is time-consuming and laborious. This is greatly reduced by the use of hardware devices such as digitisers and scanners. Maps are either digitised by hand on a high-accuracy digitising tablet or scanned by high-resolution scanner with subsequent digitisation. Many GIS have digitisation programmes as part of the software semi-automating the digitising process through a series of dialog box-driven routines. The main purpose of digitising is to encode the map coordinates (y-coordinates) in the digital format. The result is the vector format of the map which further undergoes rasterisation or vice-versa, according to the needs of the user.
Attribute data (also called feature codes) are the descriptive information about the geographic features used to describe characteristics, measurements and other facts or observations about geographic features. Attribute data are put into the GIS database either manually or by importing the data, using standard transfer format like TXT or ASCII.
Once the data has been entered, it is important to check them for errors, inaccuracies, omissions and other problems. Errors in spatial data can be checked by printing the data and comparing the original data with the printed data. Attribute data can be checked either manually or by running a computer programme to check for gross errors.
The final step in manual input of data in GIS involves linking of the spatial and attribute data through identifiers which are common to the records in both the data. The linkage operation provides an ideal chance to verify the quality of both spatial and attribute data. It can also be used to check whether all the links have been properly made or not.
Linking of the spatial and attribute data completes the database after which analysis operations are undertaken, resulting in the creation of a map in a form, understandable to a user. The user can either view the map in the screen itself or receive the output on plotters and printers.
Plotters are output devices for making copies of geographical data on paper or film. In the plotter, two-dimensional line images are created by moving the drawing device containing the coloured pens and the paper horizontally and vertically (x and y directions). All the information are drawn by a series of line-drawing commands given by the software. The plotters are now getting replaced by the printers. Colour printers that can print upto A0 size are now extensively used for obtaining copies of maps. They work with the same principle as the working of a laser printer.
But printers and plotters are used to produce only a few copies of the maps. For making a large number of copies, generally the maps are published. For publishing the maps that have been prepared using GIS softwares, the maps have to be converted into a postscript format. Postscript is a programming language used to make publication-ready output files. In the press, the postscript files are changed into positives. Positives are transparencies that correspond to a particular percentage of a colour. For a colour map, four or more positives are made i.e. cyan, yellow, magenta and black and another shade of the required colour, or in other words, one positive for one colour. The contents of a positive are etched into plates made of aluminium. Thus, for preparing a colour map, four or more plates are required. Then the respective colours, i.e. yellow, cyan, magenta and black are spread over the respective plates and printing is done by passing the map over the plates. Thus, in the map, the first images printed are yellow in colour. The same map is then passed through magenta plate and the images in that colour get printed and so on till the final colour i.e. black gets printed. The combination of different percentages of the four colour results in all the possible colours that we see on the map.
