15-01-2013, 02:22 PM
Types of Data Representation
Types of Data.ppt (Size: 3.4 MB / Downloads: 27)
GIS Data Representation
Geographic Information System & Data
Spatial Data - Features that have a known location on earth.
Attribute Data - The information linked to the geographic features (spatial data) describing them
Data Layers - Are the result of combining spatial and attribute data. Essentially adding the attribute database to the spatial location.
Layer Types - A layer type refers to the way spatial and attribute information are connected. There are two major layer types, vector and raster.
Raster Data –
Vector Data -
Topology - How geographic features are related to one another, and where they are in relation to one another.
NON-SPATIAL DATA
Attributes attached to spatial data are referred to as non-spatial data. Whatever spatial data we see in the form of a colorful map on a computer screen is a presentation of information which remains stored in the form attribute tables. Attributes of spatial data must contain unique identifier for each object. There may be other field also containing properties/information related a spatial feature. Attribute table of spatial data also contains ‘x’ and ‘y’ location (i.e. latitude/longitude or easting/northing) of features; however in some GIS software these columns may remain ‘invisible’.For example- if we are doing demographic analysis of villages then attributes of each point (representing a village) must have a unique village ID and other demographic information like total population, number of males & females, number of children etc.In another example- if we are doing some GIS analysis related to road then each road must have its unique Road ID. Other attributes may include like road length, road width, current traffic volume, number of stations etc.
Data input overview
GIS must have capabilities for inputting data; various housekeeping functions to edit, store, and reshape data; analytical operations that manipulate the data into usable information, and output functions.
There are three distinct phases to the data input process:
In the first phase, the design phase, you identify and “conceptually” code all the needed features and attributes for your project.
Second phase is the data acquisition phase, which involves acquiring the needed data from various agencies, storehouses, and organizations, and getting it into a format that your GIS program reads.
Third phase is the data capture phase, you digitize hard-copy maps and data directly into your GIS and transform existing digital data into a format your GIS reads.
GIS output
There is a huge range of products that you can obtain from a GIS, some of which have been illustrated in slides above:
• maps
• 3D visualisations
• tables
• lists
• multimedia
• animated map sequences
Whatever format you use, the output should provide information – new information produced as a result of the manipulation and analysis of spatial data – and help disseminate a clear message that will aid decision making and problem solving.
If you know what output you need from your GIS from the start of a project, it will help you to make decisions about what data to put in, and what analysis you need to perform.
Data Entry
Getting Spatial Data into the computer
Analogue to digital conversion
Modes of Data Input :
Keyboard entry
Coordinate geometry
Manual locating devices (digitizing)
Automated devices (scanning)
Conversion directly from other digital sources
Attributes may have to be coded separately.
Keyboard entry - Keying in data is only appropriate where there is a small amount of data. However, it may be appropriate for student-gathered project data. Keyboard entry may also be appropriate for the entry of text data associated with a map or image.
Keyboard entry, often referred to as key coding, is the entry data into a file at a computer terminal.
This technique is used for attribute data that are available only on paper.
This technique can be mixed with digitizing process for the creation of GIS database
Coordinate system
Geospatial data should be geographically referenced ( called georeferenced or geocoded) in a common coordinate system. The reference points are called tic masks or ground control points. One of the most convenient way of locating points is to use plane orthogonal coordinates with x (horizontal) and y (vertical) axis.
Digitizing
Heads Up Digitizing – After you create a scanned image, you georeference it and use it as a background image within your vector system. Then with the image at its proper geographic location, trace the features that appear on the scanned image.
This process, called “heads-up” digitizing (or on-screen digitizing), is like manual digitizing but without a physical digitizing board.
Digitizing - generally it is of two types:
Manual digitizing &
Automatic digitizing
Digitizers are the most common device for extracting spatial information from maps and photographs.
Manual digitizing - The majority of spatial data entered into a GIS is from manual digitizing.
The digitizing tablet and table allow information to be traced from an existing map or graphic.
Manual digitizing is the most common method of encoding spatial features from paper maps.
It is a process of converting the spatial features on a map into a digital format.
Raster data and vector data
The data model represents a set of guidelines to convert the real world(called entity) to the digitally and logically represented spatial objects consisting of the attributes and geometry.
There are two fundamentally different types of geographic information.
The vector model - information about points, lines, and polygons encoded and stored as a collection of x,y coordinates
The vector model is extremely useful for describing discrete features, but less useful for describing continuously varying features such as soil type or accessibility costs for hospitals.
The location of a point feature, such as a bore hole, can be described by a single x,y coordinate.
Linear features, such as roads and rivers, can be stored as a collection of point coordinates.
Polygonal features, such as sales territories and river catchments, can be stored as a closed loop of coordinates.