GEOGRAPHICAL INFORMATION SYSTEM
Abstract- Most of the discussion of time in GIS fits into the general topic of developing a useful model of geographic data. Data models, at their most abstract level, describe objects, relationships, and a system of constraints or axioms. So far, most GIS research posits universal axioms with a strong geometric basis. Time is usually spatialized. Models of GIS should develop to include more than the data, since an operating GIS must be connected to its context in social, economic and administrative life. While time might be reasonably represented as an axis in data space, as a technology, GIS develops in a complex, multi-thread system of events. Understanding the historical nature of the participants in the GIS can help sort out the diversity of data models and the inability to develop common understandings.
KEY WORDS: GIS, Raster, Cartography, Spatial.
I.INTRODUCTION
A Geographic Information System is also called as Geographical Information System or Geospatial Information System. It is a system designed to capture, store, manipulate, analyze, manage and present all types of geographically referenced data. In simple terms GIS is called as the merging of Cartography.GIS describes any information system that integrates, stores, edits, analyzes, shares and displays geographic information for informing decision making. Modern GIS technologies use digital information, for which various digitized data creation methods are used. Land Surveyors have been able to provide a high level of positional accuracy utilizing the GPS derived positions. A GIS can also convert existing digital information, which may not yet be in map form, into forms it can recognize, employ for its data analysis processes, and use in forming mapping output. Map information in a GIS must be manipulated so that it registers, or fits, with information gathered from other maps.
II.GIS TECHNIQUES
The most common method of data creation is digitization, where a hard copy map or survey plan is transferred into a digital medium through the use of a computer-aided design (CAD) program, and geo-referencing capabilities. With the wide availability of ortho-rectified imagery. Heads-up digitizing is becoming the main avenue through which geographic data is extracted. Heads-up digitizing involves the tracing of geographic data directly on top of the aerial imagery instead of by the traditional method of tracing the geographic form on a separate digitizing tablet. The key characteristic of GIS has begun to open new avenues of scientific inquiry into behaviors and patterns of previously considered unrelated real- world information.
III.GIS REPRESENTATION
GIS data represents real objects (such as roads, land use, elevation, etc.) with digital data determining the mix. Real objects can be divided into two abstractions: Discrete objects and Continuous fields.Traditionally, there are two broad methods used to store data in a GIS for both kinds of abstractions mapping references: Raster images and Vector. Points, lines, and polygons are the stuff of mapped location attribute references. A raster data type is, in essence, anytype of digital image represented by reducible and enlargeable grids. Anyone who is familiar with digital photography will recognize the Raster graphics pixel as the smallest individual grid unit building block of an image, usually not readily identified as an artifact shape until an image is produced on a very large scale. Raster data can be images with each pixel containing a color value. In a GIS, geographical features are often expressed as vectors, by considering those features as geometrical shapes. A simple vector map, using each of the vector elements: points, lines, and polygons. No measurements are possible with point features. Line features can measure distance. Polygon features can measure perimeter and area. Vector data can also be used to represent continuously varying phenomena. Additional non-spatial data can also be stored along with the spatial data represented by the coordinates of a vector geometry or the position of a raster cell. In vector data, the additional data contains attributes of the feature. Raster data is stored in various formats; from a standard file-based structure of TIF, JPEG, etc. to binary large object (BLOB) data stored directly in a relational database management system (RDBMS) similar to other vector-based feature classes. Database storage, when properly indexed, typically allows for quicker retrieval of the raster data but can require storage of millions of significantly sized records.Vector features can be made to respect spatial integrity through the application of topology rules such as 'polygons must not overlap'.
IV. MODELING IN GIS
There are many types of modeling in GIS. Some of them are discussed below.
DATA MODELING:
A GIS, however, can be used to depict two- and three-dimensional characteristics of the Earth's surface, subsurface, and atmosphere from information points. Many sophisticated methods can estimate the characteristics of surfaces from a limited number of point measurements. A two-dimensional contour map created from the surface modeling of rainfall point measurements may be overlaid and analyzed with any other map in a GIS covering the same area. Watersheds can be easily defined for any given reach, by computing all of the areas contiguous and uphill from any given point of interest. Similarly, an expected thalweg of where surface water would want to travel in intermittent and permanent streams can be computed from elevation data in the GIS.
TOPOLOGICAL MODELING:
A GIS can recognize and analyze the spatial relationships that exist within digitally stored spatial data. These topological relationships allow complex spatial modeling and analysis to be performed. Topological relationships between geometric entities traditionally include adjacency (what adjoins what), containment (what encloses what), and proximity (how close something is to something else).
HYDROLOGICAL MODELLING:
GIS hydrological models can provide a spatial element that other hydrological models lack, with the analysis of variables such as slope, aspect and watershed or catchment area. Terrain analysis is fundamental to hydrology, since water always flows down a slope.
CARTOGRAPHIC MODELING:
The term "cartographic modeling" was coined by Dana Tomlin in his PhD dissertation and later in his book which has the term in the title. Cartographic modeling refers to a process where several thematic layers of the same area are produced, processed, and analyzed. Tomlin used raster layers, but the overlay method can be used more generally. Operations on map layers can be combined into algorithms, and eventually into simulation or optimization models.
(An example of use of layers in a GIS application.In this example,the forest cover layer(light green)is at the bottom with a topographic layer over it.Next up is the stream layer,then the boundary laywr,then the road layer.The order is very important in order to properly display the final result.Note that the pond layer was located just below the stream layer,
MAP OVERLAY:
The combination of several spatial datasets creates a new output vector dataset, visually similar to stacking several maps of the same region. These overlays are similar to mathematical Venn diagram overlays. A union overlay combines the geographic features and attribute tables of both inputs into a single new output. An intersect overlay defines the area where both inputs overlap and retains a set of attribute fields for each. A symmetric difference overlay defines an output area that includes the total area of both inputs except for the overlapping area.Data extraction is a GIS process similar to vector overlay, though it can be used in either vector or raster data analysis.
V. DATA OUTPUT AND CARTOGRAPHY
Cartography is the design and production of maps, or visual representations of spatial data. The vast majority of modern cartography is done with the help of computers, usually using a GIS but production quality cartography is also achieved by importing layers into a design program to refine it. Most GIS software gives the user substantial control over the appearance of the data. Cartographic work serves two major functions:First, it produces graphics on the screen or on paper that convey the results of analysis to the people who make decisions about resources. Wall maps and other graphics can be generated, allowing the viewer to visualize and thereby understand the results of analyses or simulations of potential events. Second, other database information can be generated for further analysis or use.
VI. GRAPHIC DISPLAY TECHNIQUES
Traditional maps are abstractions of the real world, a sampling of important elements portrayed on a sheet of paper with symbols to represent physical objects. People who use maps must interpret these symbols. Topographic maps show the shape of land surface with contour lines or with shaded relief.Today, graphic display techniques such as shading based on altitude in a GIS can make relationships among map elements visible, heightening one's ability to extract and analyze information. For example, two types of data were combined in a GIS to produce a perspective view of a portion of San Mateo County, California.
· The digital elevation model, consisting of surface elevations recorded on a 30-meter horizontal grid, shows high elevations as white and low elevation as black.
· The accompanying Landsat Thematic Mapper image shows a false-color infrared image looking down at the same area in 30-meter pixels, or picture elements, for the same coordinate points, pixel by pixel.
A GIS was used to register and combine the two images to render the three-dimensional perspective view looking down the San Andreas Fault, using the Thematic Mapper image pixels, but shaded using the elevation of the landforms. The GIS display depends on the viewing point of the observer and time of day of the display, to properly render the shadows created by the sun's rays at that latitude, longitude, and time of day.An archeochrome is a new way of displaying spatial data. It is a thematic on a 3D map that is applied to a specific building or a part of a building. It is suited to the visual display of heat loss data.
VII.GIS DEVELOPMENTS
Many disciplines can benefit from GIS technology. An active GIS market has resulted in lower costs and continual improvements in the hardware and software components of GIS. These developments will, in turn, result in a much wider use of the technology throughout science, government, business, and industry, with applications including real estate, public health, crime mapping, sustainable development, natural resources, landscape architecture, archaeology, regional and community planning, transportation and logistics. GIS is also diverging into location-based services (LBS). LBS allows GPS enabled mobile devices to display their location in relation to fixed assets (nearest restaurant, gas station, fire hydrant), mobile assets (friends, children, police car) or to relay their position back to a central server for display or other processing. These services continue to develop with the increased integration of GPS functionality with increasingly powerful mobile electronics (cell phones, PDAs, laptops).GIS technology, as an expansion of cartographic science, has enhanced the efficiency and analytic power of traditional mapping. Now, as the scientific community recognizes the environmental consequences of anthropogenic activities influencing climate change, GIS technology is becoming an essential tool to understand the impacts of this change over time. GIS enables the combination of various sources of data with existing maps and up-to-date information from earth observation satellites along with the outputs of climate change models. This can help in understanding the effects of climate change on complex natural systems. The outputs from a GIS in the form of maps combined with satellite imagery allow researchers to view their subjects in ways that literally never have been seen before.
VIII.ADVANTAGES AND DISADVANTAGES
There are some important advantages and disadvantages to using a raster or vector data model to represent reality:
· Raster datasets record a value for all points in the area covered which may require more storage space than representing data in a vector format that can store data only where needed.
· Raster data allows easy implementation of overlay operations, which are more difficult with vector data.
· Vector data can be displayed as vector graphics used on traditional maps, whereas raster data will appear as an image that may have a blocky appearance for object boundaries. (depending on the resolution of the raster file)
· Vector data can be easier to register, scale, and re-project, which can simplify combining vector layers from different sources.
· Vector data is more compatible with relational database environments, where they can be part of a relational table as a normal column and processed using a multitude of operators.
· Vector file sizes are usually smaller than raster data, which can be tens, hundreds or more times larger than vector data (depending on resolution).
· Vector data is simpler to update and maintain, whereas a raster image will have to be completely reproduced. (Example: a new road is added).
· Vector data allows much more analysis capability, especially for "networks" such as roads, power, rail, telecommunications, etc. (Examples: Best route, largest port, airfields connected to two-lane highways). Raster data will not have all the characteristics of the features it displays.
IX. APPLICATIONS
GIS technology can be used for:
· earth surface-based scientific investigations
· resource management
· reference and projections of a geospatial nature, both man-made and natural
· asset management and location planning
· archaeology
· environmental impact-assessment
· infrastructure assessment and development
X. CONCLUSION
Model of GIS cannot treat time as a sterile, abstract dimension without losing the historical specificity of its context. Modeling of GIS must extend beyond the data stored in the GIS to include the institutions that adopt the technology and the conversions of the industry that manages spatial information. Once these components are included, a model of time must include specific events and historical processes that lead to meanings of geographic phenomena amongst the diverse participants. The field of GIS is beset by cultural expectations about time and progress. These cloud the importance of historical context in the implementation and development of these technological changes. Research in GIS should not stop with models of the data inside the GIS. The technological development process itself has historical roots and progresses in paths not entirely picked for the purest reasons. Even more importantly, the meaning of that data comes from the users and from the context of the uses. The context in turn is heavily influenced by the historical processes that lead to this point. A multi-thread model of historical origins seems much more appropriate than the single axis models that pervade the current thinking.
