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UG6thSem_major_GIS Data Structures.pptx DR P DAS.1.pptx

This document discusses different types of spatial data and data structures used in Geographic Information Systems (GIS). It describes how geographical features can be represented digitally using spatial data that indicates location and attribute data that specifies characteristics. Spatial data can be stored using vector data models with discrete features or raster data models with continuous cell values. Common data types include points, lines, polygons, and raster grids. The key aspects covered are vector versus raster data structures, and the use of digital elevation models (DEMs) and triangulated irregular networks (TINs) to represent terrain surfaces.

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DR PRASENJIT DAS
How do we describe geographical features?  by recognizing two types of data: ◦ Spatial data which describes location (where) ◦ Attribute data which specifies characteristics at that location (what, how much, and when) How do we represent these digitally in a GIS?  by grouping into layers based on similar characteristics (e.g hydrography, elevation, water lines, sewer lines, grocery sales) and using either: ◦ vector data model (coverage in ARC/INFO, shapefile in ArcView) ◦ raster data model (GRID or Image in ARC/INFO & ArcView)
DATA USED IN GIS Community Resource Mapping
 continuous: elevation, rainfall, ocean salinity  areas: ◦ unbounded: landuse, market areas, soils, rock type ◦ bounded: city/county/state boundaries, ownership parcels, zoning ◦ moving: air masses, animal herds, schools of fish  networks: roads, transmission lines, streams  points: ◦ fixed: wells, street lamps, addresses ◦ moving: cars, fish, deer
Categorical (name): ◦ nominal  no inherent ordering  land use types, county names ◦ ordinal  inherent order  road class; stream class  often coded to numbers eg SSN but can’t do arithmetic Numerical Known difference between values ◦ interval  No natural zero  can’t say ‘twice as much’  temperature (Celsius or Fahrenheit) ◦ ratio  natural zero  ratios make sense (e.g. twice as much)  income, age, rainfall  may be expressed as integer [whole number] or floating point [decimal fraction] Attribute data tables can contain locational information, such as addresses or a list of X,Y coordinates.
Attribute table “Flat File” with columns and rows Row = geographic feature record Column = attribute field (item of information about a feature) Attribute Data Structure
Contain Tables or feature classes in which: ◦ rows: entities, records, observations, features:  ‘all’ information about one occurrence of a feature ◦ columns: attributes, fields, data elements, variables, items (ArcInfo)  one type of information for all features The key field is an attribute whose values uniquely identify each row Parcel Table Parcel # Address Block $ Value 8 501 N Hi 1 105,450 9 590 N Hi 2 89,780 36 1001 W. Main 4 101,500 75 1175 W. 1st 12 98,000 entity Attribute Key field
 Raster data model ◦ location is referenced by a grid cell in a rectangular array (matrix) ◦ attribute is represented as a single value for that cell ◦ much data comes in this form  images from remote sensing (LANDSAT, SPOT)  scanned maps  elevation data from USGS ◦ best for continuous features:  elevation  temperature  soil type  land use  Vector data model ◦ location referenced by x,y coordinates, which can be linked to form lines and polygons ◦ attributes referenced through unique ID number to tables ◦ much data comes in this form  DIME and TIGER files from US Census  DLG from USGS for streams, roads, etc  census data (tabular) ◦ best for features with discrete boundaries  property lines  political boundaries  transportation
Vector Data Structure lines polygons
In vector data layers, the feature layer is linked to an attribute table. Every individual feature corresponds to one record (row) in the attribute table. Vector Data Structure
Raster Data Structure (Grid)
A raster grid can store values that represent categories, for example, vegetation type The basic grid attribute table has a value and count field The value field has a code or some real number representing information about the grid cell. In this case it is a code for vegetation. The count field shows how many grid cells have that same value. Raster Data Structure
Grids can also store continuous values like elevation Raster Data Structure
Elevation grid for area north of Kirkuk, Iraq From space shuttle radar topography mission (SRTM) Zoom in and you see the grid cells These are called: Digital Elevation Models (DEM) Raster Data Structure
So 2 ways of representing elevation: Vector contour lines Raster grid Raster Data Structure
Manas National Park Satellite Imagery of Manas National Park IRS LISS III Image Bansbari Range Office Mathanguri I.B. (FCC)
Dense Forest Open Forest Agriculture River Fallow Land Settlement Water Body Typical Tone and Texture of Common Features
Sources of raster data Interpreted satellite imagery, e.g., land cover Conversion of vector to raster data Raster Data Structure
Raster Data Structure
Point Line Polygon Vector Raster Raster data are described by a cell grid, one value per cell Zone of cells
 Advantages  It is a simple data structure  Overlay operations are easily and efficiently implemented  High spatial variability is efficiently represented in a raster format  The raster model is more or less required for efficient manipulation and enhancement of digital images  Disadvantages  The raster data structure is less compact data compression techniques (an often overcome this problem)  Topological relationships are more difficult to represent  The output of graphics is less aesthetically pleasing because appearancerather than the smooth lines of hand-drawn maps. This can be overcome by using a very large number of cells, but may result in unacceptably large files
 Advantages  It provides a more compact data structure than the raster model  It provides efficient encoding of topology and as a result more efficient implementation of operations that require topological information, such as network analysis  The vector model is better suited to supporting graphics that closely approximate hand-drawn maps  Disadvantages  It is a more complex data structure than a simple raster  Overlay operations are more difficult to implement  The representation of high spatial variability is inefficient  Manipulation and enhancement of digital images cannot be effectively done in the vector domain
0 1 2 3 4 5 6 7 8 9 0 R T 1 R T 2 H R 3 R 4 R R 5 R 6 R T T H 7 R T T 8 R 9 R Real World Vector Representation Raster Representation Concept of Vector and Raster line polygon point
Each color represents a different value of an integer variable denoting land cover class Raster representation
Vectorization of Kazironga National Park Boundary Camp Location -Point Feature Drainage - Line Feature Boundary - Polygon Feature Satellite Imagery – Raster Data ,
 a sampled array of elevations (z) that are at regularly spaced intervals in the x and y directions.  two approaches for determining the surface z value of a location between sample points. ◦ In a lattice, each mesh point represents a value on the surface only at the center of the grid cell. The z-value is approximated by interpolation between adjacent sample points; it does not imply an area of constant value. ◦ A surface grid considers each sample as a square cell with a constant surface value. Advantages • Simple conceptual model • Data cheap to obtain • Easy to relate to other raster data • Irregularly spaced set of points can be converted to regular spacing by interpolation Disadvantages • Does not conform to variability of the terrain • Linear features not well represented
Geographic Watershed Information System/ArcHydro – 1 February 2008 What is a Digital Elevation Model?
Digital Elevation Models, con’t.
Digital terrain analyses – DEM principles
Digital Terrain Model (DTM) Digital Elevation Model (DEM) Digital Surface Model (DSM) Three terms (DEM,DSM,DTM) for the same thing?
…DIFFERENCE BETWEEN DSM, DTM AND DEM Digital Surface Model (DSM) is a first surface view of the earth containing both location and elevation information. Digital Terrain Model (DTM), aka "bare earth" as it is often referred, is created by digitally removing all of the cultural features inherent to a DSM by exposing the underlying terrain. A Digital Elevation Model (DEM) is any DIGITAL representation of ground surface topography or terrain. )
Digital Elevation Models, con’t.
10/8/2022 GLY560: GIS and RS  Conversion of contour lines  Photogrammetry  Satellite Stereo  Radar Stereo  Radar Inferometry  Laser Altimetry
DEM principles
 Determine aspects of terrain  Slope, aspect, spot elevations  Source for contour lines  Finding terrain features  Watersheds,drainage networks, stream channels  Modeling of hydrologic functions
A 3rd data structure for representing surfaces: Triangulated Irregular Network (TIN) TIN Data Structure
3 GIS Spatial Data Structure Types
 Advantages ◦ Can capture significant slope features (ridges, etc) ◦ Efficient since require few triangles in flat areas ◦ Easy for certain analyses: slope, aspect, volume  Disadvantages ◦ Analysis involving comparison with other layers difficult a set of adjacent, non- overlapping triangles computed from irregularly spaced points, with x, y horizontal coordinates and z vertical elevations.
Elevation points connected by lines to form polygons that contain topographic information TIN Data Structure
Continuous fields – TIN (Triangular irregular network)
TIN Data Structure
TIN Data Structure