Maps activities or events ® Lines (Arcs) are a

Maps are graphic representation of our perception of the world around us. They represent cartographic interpretation and simplification of reality.

Maps provide two types of information

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·         Locational information

·         Spatial Relationships

 

 

Maps contains features such as Point, Line, Area and Surface.

 

·         Point Features :- wells, control points, sample sites, fire stations

·         Line Features :- roads, hydro lines, rivers, contour lines,

·         Area Features:-urban areas, water bodies, soil/rock units, forest areas

 

Point Features

®     Spatially distributed entities, activities or events

®     Points have a single geographic coordinate such as:

o   Tree

o   Traffic accident

o   Lamp post

 Line Features

®     Spatially distributed entities, activities or events

®     Lines (Arcs) are a series of geographic coordinates joined to form a line such as:

o   Road

o   Stream

o   Railway

 

Area Features

®     Spatially distributed entities, activities or

®     Events

®     Areas (Polygons) are a series of geographic coordinates joined together to form a boundary such as:

o   Lake

o   Soil types

 

There are two formats used by GIS systems to store and retrieve geographical data:

Raster

Format-

•        Data are divided into cell, pixels, or elements

•        Cells are organized in arrays

•        Each cell has a single value

•        Row and Column Numbers are used to identify the location of the cell within the array.

•        Perhaps the most common example of raster data is a digital image.

Data-

·         A grid (or raster) system stores data as a string of characters in which each character represents a location.

·         The basic data unit is a cell or Pixel Each cell/Pixel is assigned only one value

·         An array of Pixels form the entity-Point, Line, Area and surface

·         The shape and size of the array determines the basic Resolution

·         Polygons can be formed indicating areas of homogeneous characteristics

 

Vector

Format-

•        Data are associated with points, lines, or boundaries enclosing areas

•        Points are located by coordinates

•        Lines are described by a series of connecting vectors (line segments described by the coordinates of the start of the vector, its direction, and magnitude or length).

•        Areas or polygons are described by a series of vectors enclosing the area.

•        Any number of factors or attributes can be associated with a point line or polygon.

•        Data are stored in two files: – a file containing location information – a file containing information on the attributes

•        A third file contains information needed to link positional data with their attributes.

Data-

•        A vector system usually stores data as coordinates.

•        For example: each uniform area is surrounded by a set of straight line segments called vectors.

•        In a vector based system every point is recorded by a pair of x and Y coordinates.

•        Straight line segments called vectors are displayed to indicate line based data ( roads rivers wells)

•        The x-y coordinates at the end of each vector can be digitized and stored.

•        Most spatial features can be displayed as: – Points Line- Polygons

 

Raster formats are efficient when comparing information among arrays with the same cell size.

Raster files are generally very large because each cell occupies a separate line of data, only one attribute can be assigned to each cell, and cell sizes are relatively small. Raster representations are relatively coarse and imprecise Vector

 

Vector formats are efficient when comparing information whose geographical shapes and sizes are different.

Vector files are much smaller because a relatively small number of vectors can precisely describe large areas and many attributes can be ascribed to these areas.

 Vector representations of shapes can be very precise.

 

Most GIS software can display both raster and vector data. Only a limited number of programs can analyze both types of data or make raster type analyses in vector formats.

 

Attributes can be numeric or alfa-numeric data. A point, line or area data that is assigned to spatial features

Examples are Stand ID, Compartment No., Vegetation type, Name of the Forest Block, Types of Road, VSS code etc.