To show a portion of the Earth's surface on a map, the scale must be sufficiently adjusted to cover the objective. Map scale or the extent of reduction is expressed as a ratio. The unit on the left indicates distance on the map and the number on the right indicates distance on the ground. The following three statements show the same scale.
Then, you can now state the scale as a representative fraction (RF): 0.04:10,000
Though it is a valid statement of scale, most cartographers may find it clumsy. Traditionally, the first number in the representative fraction is made equal to 1:
Scale in Digital Maps
With digital maps, the traditional concept of scale in terms of distance does not apply because digital maps do not remain fixed in size. They can be displayed or plotted at any possible magnification. Yet we still speak of the scale of a digital map.
In digital mapping, the term scale is used to indicate the scale of the materials from which the map was made. For example, if a digital map is said to have a scale of 1:100,000, it was made from a 1:100,000-scale paper map.
However, a digital map's scale still allows you to make some educated guesses about its contents because, generally, digital maps retain the same accuracy and characteristics as their source maps. So it is still true that a large-scale digital map will usually be more accurate and less general than a small-scale digital map.
Because the display size of a computer-based map is not fixed, users are often tempted to blow up maps to very large sizes. For example, a 1:100,000-scale map can easily be plotted at a size of 1:24,000 or even 1:2,000-but it usually is not a good idea to do so. It encourages the user to make measurements that the underlying data does not support. You cannot measure positions to the nearest foot if your map is only accurate to the nearest mile. You will end up looking for information that does not exist.
Map resolution refers to how accurately the location and shape of map features can be depicted for a given map scale. Scale affects resolution. In a larger-scale map, the resolution of features more closely matches real-world features because the extent of reduction from ground to map is less. As map scale decrease, the map resolution diminishes because features must be smoothed and simplified, or not shown at all.
Many factors besides resolution, influence how accurately features can be depicted, including the quality of source data, the map scale, your drafting skill and the width of lines drawn on the ground. A fine drafting pen will draw line's 1/100 of an inch wide. Such a line represents a corridor on the ground, which is almost 53 feet wide.
In addition to this, human drafting errors will occur and can be compounded by the quality of your source maps and materials. A map accurate for one purpose is often inaccurate for others since accuracy is determined by the needs of the project as much as it is by the map itself.
Some measurements of a map's accuracy are discussed below.
The aerial extent of map is the area on the Earth's surface represented on the map. It is the limit of the area covered, usually defined by rectangle just large enough to include all mapped features. The size of the study area depends on the map scale. The smaller the scale the larger the area covered.
A critical first step in building a geographic database is defining its extent. The aerial extent of a database is the limit of the area of interest for your GIS project. This usually includes the areas directly affected by your organization's responsibility (such as assigned administrative units) as well as surrounding areas that either influence or are influenced by relevant activities in the administrative area.
Map features are logically organized into a set of layers or themes of information. A base map can be organized into layers such as streams, soils, wells or boundaries. Map data, regardless of how a spatial database will be applied, is collected, automated and updated as series of adjacent map sheets or aerial photograph. Here each sheet is mounted on the digitizer and digitized, one sheet at a time. In order to be able to combine these smaller sheets into larger units or study areas, the co-ordinates of coverage must be transformed into a single common co-ordinate system. Once in a common co-ordinate system, attributes are associated with features. Then as needed map sheets for layer are edge matched and joined into a single coverage for your study area.
Types of Information in a Digital Map
Any digital map is capable of storing much more information than a paper map of the same area, but it's generally not clear at first glance just what sort of information the map includes. For example, more information is usually available in a digital map than what you see on-screen. And evaluating a given data set simply by looking at the screen can be difficult: What part of the image is contained in the data and what part is created by the GIS program's interpretation of the data? You must understand the types of data in your map so you can use it appropriately.
Three general types of information can be included in digital maps:
Some digital maps do not contain all three types of information. For example, raster maps usually do not include attribute information, and many vector data sources do not include display information.
The geographic information in a digital map provides the position and shape of each map feature. For example, a road map's geographic information is the location of each road on the map.
n a vector map, a feature's position is normally expressed as sets of X, Y pairs or X, Y, Z triples, using the coordinate system defined for the map (see the discussion of coordinate systems, below). Most vector geographic information systems support three fundamental geometric objects:
Some systems also support more complex entities, such as regions, circles, ellipses, arcs, and curves.
Attribute data describes specific map features but is not inherently graphic. For example, an attribute associated with a road might be its name or the date it was last paved. Attributes are often stored in database files kept separately from the graphic portion of the map. Attributes pertain only to vector maps; they are seldom associated with raster images.
GIS software packages maintain internal links tying each graphical map entity to its attribute information. The nature of these links varies widely across systems. In some, the link is implicit, and the user has no control over it. Other systems have explicit links that the user can modify. Links in these systems take the form of database keys. Each map feature has a key value stored with it; the key identifies the specific database record that contains the feature's attribute information.
The display information in a digital-map data set describes how the map is to be displayed or plotted. Common display information includes feature colours, line widths and line types (solid, dashed, dotted, single, or double); how the names of roads and other features are shown on the map; and whether or not lakes, parks, or other area features are colour coded.
However, many users do not consider the quality of display information when they evaluate a data set. Yet map display strongly affects the information you and your audience can obtain from the map - no matter how simple or complex the project. A technically flawless, but unattractive or hard-to-read map will not achieve the goal of conveying information easily to the user.
Clearly, how a map looks - especially if it is being used in a presentation - determines its effectiveness. Appropriate color choices, linetypes, and so on add the professional look you want and make the map easier to interpret. Since display information often is not included in the source data set or is filtered out by conversion software, you may need to add it yourself or purchase the map from a vendor who does it for you. Map display information should convey the meaning of its underlying attribute data.
Various enhancements will increase a map's usefulness and cartographic appeal.
Most GIS software has a system of layers, which can be used to divide a large map into manageable pieces. For example, all roads could be on one layer and all hydrographic features on another. Major layers can be further classified into sub-layers, such as different types of roads - highways, city streets, and so on. Layer names are particularly important in CAD-based mapping and GIS programs, which have excellent tools for handling them.
Some digital maps are layered according to the numeric feature-classification codes found in their source data sets. For example, a major road might be on the 170-201 layer. However, this type of system is not very useful. A well-thought-out layering scheme can make any data set much easier to use because it allows the user to control the features with which you want to work. A good layering standard has layer names that are mnemonic (suggest their meanings) and hierarchical (have a structured classification scheme that makes it easy to choose general or specific classes).
For example, a map could have its roads on a layer called RD, its railroads on a layer called RR, its road bridges on a layer called RD-BRIDGE, and its railroad bridges on a layer called RR-BRIDGE. This scheme is mnemonic because it is easy to tell a layer's contents from its name, and it's hierarchical because the user can easily select all the roads, railroads, bridges, road bridges, or railroad bridges.
Maps and Map Analysis
Computer Aided Mapping has its limitations. Goal of GIS is not only to prepare a good map but also perform map analysis. Maps are the main source of data for GIS. GIS, though an accurate mapping tool, requires error management.
MAP is a representation on a medium of a selected material or abstract material in relation to the surface of the earth (defined by Cartographic association). Maps originated from mathematics. The term Map is often used in mathematics to convey the motion of transferring the information from one form to another just as Cartographers transfer information from the surface of the earth to a sheet of paper. Map is used in a loose fashion to refer to any manual display of information particularly if it is abstract, generalised or schematic.
Process involved in the production of Maps:
Source: GIS Development (http://www.gisdevelopment.net/tutorials/tuman002.htm)