55387 Chapter 3b

Chapter 3- Digital Representation and Organization of Geospatial Data Concepts and Techniques of Geographic Information Systems, 2e by C.P Lo and Albert K.W. Yeung

Introduction ► GIS:

Maps are dynamic representations of geospatial features, unlike paper maps.

► Data

data.

Model: A way of representing

Representing Geographic Space ► Features

are either objects or phenomena.

► Two

types of models to represent this: object-based and field-based.

► Object-based

modeling treats features as discrete objects    

Must have identifiable boundaries Relevant to intended application Has attribute data Can include either exact objects or fuzzy objects

Figure 3.3

► Field-based

models: spatial phenomenon that vary continuously across the landscape

► Can

be obtained directly or indirectly

► Direct

means it is derived from remote sensing.

► Indirect

means it was processed from a primary data source, i.e.. interpolated surfaces, DEMs, etc.

Figure 3.4

Data Classification ► Geospatial

data typically collected and stored as categorical.

►A

classification scheme includes:

 Descriptive names  Definitions

Non-categorical Data

Categorical Data

Tessellations ► Dive

continuous space into discrete units of unique possible values

► Called

quantizing space

► Less

exact location than vector, but you have more control on spatial uniformity.

► Any

shape possible….but square is normal.

Figure 3.17

Figure 3.19

Table 3.2

Digital Elevation Model (DEM)

Digital Ortho Quad (DOQQ)

Digital Line (Raster) Graphic (DRG)

NLCD Land Cover

STATSGO: Soils

Where do you find all of this stuff? ► TNRIS

: Texas Natural Resource Information System

In Class Skill Builder ► ► ► ► ► ► ► ► ►

Pair-up Download A 30 meter Digital Elevation Model Unzip it to an file on the C:\ (NOT THE DESKTOP) View it in ArcCatalog Open the red toolbox icon Go to Conversion Tools/To Raster/DEM to Raster Convert your DEM to a GRID Load your newly created DEM into ArcMap Try downloading and adding more data from TNRIS to your project.

Vector Data ► Object

–oriented approach to representation of real-world features and is best used to represent discrete objects.

► All

vector data built on two concepts:

 Decomposition of spatial objects into basic elements (points, lines, polygons).  Use of topology (spatial relationships)

Nature and Characteristics of Vector Data ► Basic

Graphical Elements are represented by at least one pair of coordinate points.  Points: One pair of coordinate points  Lines or Arcs: string of coordinates that begin and end with a “node.”  Polygons: closed group of coordinates

► Spaghetti

Data: vector data that has been collected but not structured

Figure 3.20

Structured Vector Data ► Have:

 Topological Relationships : A set of rules and behaviors that model how points, lines, and polygons share geometry. For example, adjacent features, such as two counties, will share a common edge.  Linked to descriptive data in geospatial databases.

Why Topology? ► Manage

shared geometry. For example, adjacent polygons, such as parcels, share edges; adjacent soil polygons share edges. ► Define and enforce data integrity rules (no gaps should exist between parcels, no overlap, etc.) ► Support topological relationship queries and navigation (have the ability to identify adjacent and connected features, find shared edges, etc.)

Why Topology? ► Support

sophisticated editing tools that enforce the topological constraints of the data model (e.g. ability to edit a shared edge and update all the features that share the common edge). ► Construct features from unstructured geometry (restructure spaghetti)

The Other Definition of Topology ► The

branch of mathematics that studies those properties of geometric figures that are unchanged when the shape of a figure is twisted, stretched, shrunk, or otherwise distorted without breaking. ► Math concerned with spatial relationships ► This is how we think, or how we perceive geography using our mental maps (not coordinates)

Figure 3.21

Figure 3.22

Table 3.3

Representation of Vector Data ► Governed

by scale ► Cartographic Generalization: line and area objects are represented by more coordinates at a larger scale than at a smaller scale. ► Cartographic Symbolization: vector data are represented by different symbols that serve to visually distinguish them from one another when the data are displayed.

Basic Elements of Topology ► Adjacency:

information about the neighborhood among spatial objects. ► Containment: information about inclusion of one spatial object within another spatial object ► Connectivity: information about linkages among spatial objects.

Figure 3.23

Figure 3.28

Geodatabase ► Introduced

with ArcGIS 8.0 ► Term has several meanings

 A common data access and management framework for ArcGIS software products that enables folks to use GIS across computer platforms  A generic GIS data model for defining all types of geospatial data by using a rich set of data types and sophisticated referential integrity and topological rules  A combined system of databases that are enhanced by added geospatial data layers and built-in map displays, feature editing and spatial analysis functions  All data sets use the same data tables and metadata and all is cross-indexed.

Figure 3.37

Table 3.4