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MODELING IN GIS HIGHWAY ALINGMENT STUDIES
Vector GIS displays the locations or all objects stored using points and arcs.Attributes and entity types can be displayed by varying colors, line patterns and point symbols (Fig) . Using vector GIS, one may display only a subset of the data. For example, one may select all political boundaries and highways, but only areas that had urban land uses.
Fig 5.5 Boundaries and highways
Relational query is an important concept in vector overlay analysis. Differentsystems use different ways of formulating queries. Structured Query language (SQl) is used by many systems. It provides a "standard" way in querying spatial data bases. Using relational queries, the user can select objects interested in producing map output using colours, symbols, text annotations and so on. Reclassify, dissolve, and merge operations are used frequently in working with area objects. They are used to aggregate areas based on attributes. Consider a soil map in Fig. We wish to produce a map of major soil types from a layer that has polygons based on much more finely defined classification scheme. To do this, we process the data using three steps: (i) Reclassify areas by a single attribute or some combination; for instance reclassify soil areas by soil type only (ii) Dissolve boundaries Lines are broken at each area object boundary to form new line segments andnew attributes created for each output line specifying the area it belongs to Fig.
A triangulated irregular network (TIN) is a digital data structure used in a geographic information system (GIS) for the representation of a surface. A TIN is a vector based representation of the physical land surface or sea bottom, made up of irregularly distributed nodes and lines with three dimensional coordinates (x,y, and z) that are arranged in a network of nonoverlapping triangles. TINs are often derived from the elevation data of a rasterized digital elevation model (DEM). An advantage of using a TIN over a DEM in mapping and analysis is that the points of a TIN are distributed variably based on an algorithm that determines which points are most necessary to an accurate representation of the terrain. Data input is therefore flexible and fewer points need to be stored than in a DEM with regularly distributed points. While a TIN may be less suited than a DEM raster for certain kinds of GIS applications, such as analysis of a surface's slope and aspect, TINs have the advantage of being able to portray terrain in three dimensions.
A TIN comprises a triangular network of vertices, known as mass points, with associated coordinates in three dimensions connected by edges to form a triangular tessellation. Three-dimensional visualizations are readily created by rendering of the triangular facets. In regions where there is little variation in surface height, the points may be widely spaced whereas in areas of more intense variation in height the point density is increased.
Contours should always point downridge along ridges Adjacent contours should always be sequential or equivalent
Contours should never split into two
Contours should never cross or loop
Contours should never spiral
Contours should never stop in the middle of a map
Fig 5.6 Contour Map
A streamlined approach is developed for Land Information System to LIS thatincorporates key data into a land registration structure, subsequently transferring them into a fully automated information system. Under this approach, a land parcel is not stored as a polygon or area in the LIS and hence is not used as the base framework for the related database. Instead, a single point feature representing each property is used as the identifier and geographic locator and are usually termed as, "lots by dots." This is the critical difference from the polygon-based approach which attempts to reconcile geometry and compile all land parcels together into a contiguous map of polygons. This is practically impossible to achieve even in the United States since individual surveys of land parcels or 'lots' are often, not entirely accurate and do not actually reconcile with one another.
This point-based property database can be developed from hardcopy very highresolution satellite imagery or aerial photographs, or through ground-based collection using Global Positioning System (GPS) techniques. As the field person collects the geographic (point) position of each land parcel the property identifier number as well as physical characteristics of the land, such as, land cover, soil condition, and number of structures can also be logged. This effectively allows an LIS database to be built in the field during survey. Additional complementary data can also be integrated into the LIS, such as, scanned property documents.
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