GIS Data Models Vector Data Models Vector File Formats Raster Data Models Raster File Formats

Data Abstraction To use GIS the real world must be abstracted into points, lines, polygons, raster cells, and attribute values Class examples may use common object that most people will understand. If you understand how to abstract common objects you will be able to apply the same method to object in your field

What is Vector Data Vector Data uses Points and their (X,Y) coordinates to represent spatial features Points, Lines and Polygons

Points A point is a 0 dimensional object and has only the property of location (x,y) Points can be used to Model features such as a well, building, power, pole, sample location ect. Other name for a point are vertex, node, 0- cell

Lines A line is a one-dimensional object that has the property of length Lines can be used to represent road, streams, faults, dikes, maker beds, boundary, contacts etc. Lines are also called an edge, link, chain, arc, 1- cell In an ArcInfo coverage an arc starts with a node, has zero or more vertices, and ends with a node

Figure 4.3 p58 Bernhardsen

Polygons A polygon is a two-dimensional object with properties of area and perimeter A polygon can represent a city, geologic formation, dike, lake, river, ect. Other name for polygons face, zone 2-cell Scale matters

Data Abstraction Discussion If you do not understand this the rest of GIS will not make sense Scale Matters Intended use Matters

Examples Trees Dikes Roads Rivers

Topology A set of rules on how objects relate to each other Major difference in file formats Higher level objects have special topology rules

Topology Definition The Science of mathematics of relationships used to validate the geometry of vector entities, and for operations such as network tracing and tests of polygon adjacency. The study of geometric properties that do not change when the forms are bent, stretched or under go similar geometric transformations.

Figure 2-9 GIS Fundamentals, Bolstad

Planer Enforcement

Why Topology Matters Error Detection open polygons unlabeled polygons slivers polygons that cannot exist next to each other Network Modeling

Show Placitas Arc Node Topology –Cover# –Lpoly# and Rpoly# –Tnode fnode Label errors

Higher Level Object Regions Networks TIN – Triangulated irregular network Dynamic Segmentation

Regions Overlapping areas with different attributes Fire history Disconnected areas with the same attributes Hawaii

Networks Road systems, power grids, water supply sewerage systems, drainage network Continuous connected networks Rules for displacement in a network Attribute value accumulations due to displacements

TIN Vector Surface Model Triangulated Irregular Network A set of nonoverlapping triangles each with a constant gradient A TIN can honor original input elevations

Dynamic Segmentation Combines a line coverage with a linear reference system Has event tables for point events and linear events

Fig 3.13 p52 Chang

Examples and Demo

Shape Files Shape Files Nontopological Advantages no overhead to process topology Disadvantages polygons are double digitized, no topologic data checking 3 files.shp.shx.dbf

Coverages Original ArcInfo Format Directory With Several Files Database Files are stored in the Info Directory Uses Arc Node Topology –Planer Enforcement –Connectivity –Adjacency

GeoDatabase New GIS Format at ArcGIS 8.0 Three Types –Personal Geodatabase –Microsoft access 2000 database –File Geodatabase –XML based file –SDE GeoDatabase –Multi-user –Can connect to many RDBMS Oracle, SQL server, Informix File are stored in the format native to the RDBMS

Box 3.5 Geographic Information Systems, Chang 04 p. 55

GeoDatabase Shapes are similar to shape files Object-oriented model not a Geo-relational There are 26 topology rules than can be used to relate different layers

Raster Data Model

Figure 3:1 Getting Started with Geographic Information Systems, Clarke (2003) p. 91

Grid Properties Each Grid Cell holds one value even if it is empty. A cell can hold an index standing for an attribute. Cell resolution is given as its size on the ground. Point and Lines move to the center of the cell. Minimum line width is one cell. Rasters are easy to read and write, and easy to draw on the screen.

Raster Pyramids With out pyramids the entire raster must be read for each screen draw Pyramids store reduced resolution dataset files.rrd to increase the speed of screen draws When you add a raster to ArcMap if pyramids do not exist you can create them

Raster Resampling Nearest Neighbor –Closest cell –Continuous and Discrete data Bilinear interpolation –Average of nearest 4 cells –Continuous data only Cubic Convolution –Average of nearest 16 cells –Continuous data only

Quad Tree Compression May be use to get variable resolution for imagery in the National Map

What are Terrains? New Dataset for ArcGIS 9.2 They are a Multi-resolution, Tin-based surface. Comprised of mesurements stored as features in a geodatabase. Terrains live inside Feature Datasets, in a geodatabase.

What are Terrans? Two Main characteristics of Terrrains: –Feature classes participate in a terrain –Rules are established to generate TIN pyramids on-th-fly. They are designed to handle mass volumes of point data in a logical and efficient storage mechanism.

Figure 4.35 Geographic Information Systems and Introduction, Bernhardsen (2001), p. 87

C. Dana Tomlin, Geographic Information Systems and Cartographic Modeling (1990), P. 44 “Yes raster is faster, but raster is vaster, and vector just seems more corrector”

Images are a form of raster data ArcGIS can use many common image formats

Industry Standard Data Models Some Industries have created standard data models It is a good idea to use a standard model to promote sharing of data Some data models can be very complex Complex models require custom tools to be useful

References Getting Started with Geographic Information Systems 4 th Edition, Clark (2003) Geographic Information Systems an Introduction 3 rd Edition, Bernhardsen (2002) Introduction to Geographic Information Systems 2 nd Edition, Chang (2004) GIS Fundamentals, Bolstad (2002) ArcGIS 8.3 Desktop Help Using GRID with ArcInfo version 7 ESRI