A map represents geographic features or other spatial phenomena by graphically conveying information about locations and attributes. Locational information describes the position of particular geographic features on the Earth’s surface, as well as the spatial relationship between features, such as the shortest path from a fire station to a library, the proximity of competing businesses, and so on. Attribute information describes characteristics of the geographic features represented, such as the feature type, its name or number and quantitative information such as its area or length.
Locational information is usually represented by points for features such as wells and telephone pole locations, lines for features such as streams, pipelines and contour lines and areas for features such as lakes, counties and census tracts.
A point feature represents as single location. It defines a map object too small to show as a line or area feature.
A special symbol of label usually depicts a point location.
A line feature is a set of connected, ordered coordinates representing the linear shape of a map object that may be too narrow to display as an area such as a road or feature with no width such as a contour line.
An area feature is a closed figure whose boundary encloses a homogeneous area, such as a state country soil type or lake.
In addition to feature locations and their attributes, the other technical characteristics that define maps and their use includes:
- Map Scale
- Map Accuracy
- Map Extent and
- Data Base Extent
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.
1 inch = 2.000 feet => 1 inch = 24.000 inches => 1:24.000
The latter is known as a representative fraction (RF) because the amounts on either side of the colon are equivalent: that is 1:24.000 means 1inch equals 24.000 inches or1 foot equals 24.000 feet or 1 meter equals 24.000 meters and so on.
Map scale indicates how much the given area has been reduced. For the same size map, features on a small-scale map (1:1,000,0000) will be smaller than those on a large-scale map (1:1,200).
A map with less detail is said to be of a smaller scale than one with more detail. Cartographers often divide scales into three different categories.
- Small-scale maps have scales smaller than 1 : 1,000,000 and are used for maps of wide areas where not much detail is required.
- Medium-scale maps have scales between 1 : 75,000 and 1 : 1,000,000.
- Large-scale maps have scales larger than 1 : 75,000. They are used in applications where detailed map features are required.
So each scale represents a different tradeoff. With a small-scale map, you’ll be able to show a large area without much detail. On a large-scale map, you’ll be able to show a lot of detail but not for a large area. The small-scale map can show a large area because it reduces the area so much that the large-scale map can only show a portion of one street, but in such detail that you can see shapes of the houses.
To convert this statement to a representative fraction, the units of measure on both the sides being compared must be the same. For this example, both measurements will be in meters.
To do this:
1. Convert 1.6 inches into meters
1.6 inches x 0.0254 meters/inch = 0.04 meters
2. Let us suppose that
0.04 units on the map = 10,000 units on the ground
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:
0.04 / 0.04 = 1 units on the map = 10,000 / 0.04 units on the ground
1 unit on the map = 250,000 units on the ground
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 featuresbecause 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.
Absolute accuracy of a map refers to the relationship between a geographic position on a map (a street corner, for instance) and its real-world position measured on the surface of the earth. Absolute accuracy is primarily important for complex data requirements such as those for surveying and engineering-based applications.
Relative accuracy refers to the displacement between two points on a map (both distance and angle), compared to the displacement of those same points in the real world. Relative accuracy is often more important and easier to obtain than absolute accuracy because users rarely need to know absolute positions. More often, they need to find a position relative to some known landmark, which is what relative accuracy provides. Users with simple data requirements generally need only relative accuracy.
Attribute accuracy refers to the precision of the attribute database linked to the map’s features. For example, if the map shows road classifications, are they correct? If it shows street addresses, how accurate are they? Attribute accuracy is most important to users with complex data requirements.
A map’s Currency refers to how up-to-date it is. Currency is usually expressed in terms of a revision date, but this information is not always easy to find.
A map is Complete if it includes all the features a user would expect it to contain. For example, does a street map contain all the streets? Completeness and currency usually are related because a map becomes less complete as it gets older.
The most important issue to remember about map accuracy is that the more accurate the map, the more it costs in time and money to develop. For example, digital maps with coordinate accuracy of about 100 feet can be purchased inexpensively. If 1-foot accuracy is required, a custom survey is often the only way to get it, which drives up data-acquisition costs by many orders of magnitude and can significantly delay project implementation – by months or even years.
Therefore, too much accuracy can be as detrimental to the success of a GIS project as too little. Rather than focusing on the project’s benefits, a sponsoring organization may focus on the costs that result from a level of accuracy not justified for the project. Project support inevitably erodes when its original objectives are forgotten in a flurry of cost analyses.
A far better strategy is to start the project with whatever data is readily available and sufficient to support initial objectives. Once the GIS is up and running, producing useful results, project scope can be expanded. The quality of its data can be improved as required.
Even though no maps are entirely accurate, they are still useful for decision-making and analysis. How ever, it is important to consider map accuracy to ensure that your data is not used inappropriately.
Any number of factors can cause error. Note these sources can have at cumulative effect.
E = f(f) + f(1) + f(e) + f(d) + f(a) + f(m) + f(rms) + f(mp) + u
f = flattening the round Earth onto a two – dimensional surface (transformation from spherical to planar geometry) .
I = accurately measuring location on Earth (correct project and datum information) .
c = cartographic interpretation (correct interpretation of features) .
d = drafting error (accuracy in tracing of features and width of drafting pen) .
a = analog to digital conversion (digitizing board calibration) .
m = media stability (warping and stretching, folding. Wrinkling of map) .
p = digitizing processor error (accuracy of cursor placement) .
rms = Root Mean Square (registration accuracy of ties) .
mp = machine precision (coordinate rounding by computer in storing and transforming) .
u = additional unexplained source error.
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.
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:
- Geographic information, which provides the position and shapes of specific geographic features.
- Attribute information, which provides additional non-graphic information about each feature.
- Display information, which describes how the features will appear on the screen.
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.
In 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:
- Point : A single pair of coordinates.
- Line : Two or more points in a specific sequence.
- Polygon : An area enclosed by a line.
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.
Feature Colors and Linetypes. Colors and line representations should be chosen to make the map’s meaning clear. For example, using double-line roads can be quite helpful. Many GIS data sets only include road centerline information. Actual road width is not given. So maps with centerlines only can look like spider webs, which is visually unappealing. Some software and conversion systems can draw roads as double lines, with distance between lines varying according to road type. Centerlines can be included, if necessary. Double-line maps are appropriate for detailed studies of small areas, such as subdivisions, or maps where right-of-way information is important.
Naming Roads. Naming, or labeling, roads are important for proper map interpretation. This information should be legible, positioned in the center of the road or offset from the center, and drawn at intervals suited to the scale of the final map or its purpose.
Landmark Symbols. A good set of symbols should be used to indicate landmarks, such as hospitals, schools, churches, and cemeteries. The symbols should be sized appropriately in relation to map scale.
Polygon Fills. Polygon features, such as lakes or parks, should be filled with an appropriate color or hatch pattern.
Zoom Layer Control. If the GIS software platform permits, map layers should be set up so that detailed, high-density information only appears when the user zooms in for a close-up of part of the map. For example, when a large area is displayed, only the major roads should appear; for a smaller area, both major and minor roads should appear.
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.
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, generalized or schematic.
Process involved in the production of Maps:
- Selection of few features of the real world.
- Classification of selected features in to groups eg. Railway in to different lines. Classification depends upon the purpose.
- Simplification of jiggered lines like the coast lines.
- Exaggeration of features.
- Symbolization to represent different classes of features.
Maps can be broadly classified in to two groups:
- Topographical maps
- Thematic maps
It is a reference map showing the outline of selected man-made and natural features of the earth. It often acts as a frame for other features Topography refers to the shape of surface represented by contours or shading. It also shows lands, railway and other prominent features.
Thematic maps are an important source of GIS information. These are tools to communicate geographical concepts such as Density of population, Climate, movement of goods and people, land use etc. It has many classifications.