© 2005. materials by Austin Troy. All rights reserved Lecture 4: Intro to the Vector Data Model and to Map Layout Introduction to GIS By Austin Troy, University

© 2005. materials by Austin Troy. All rights reserved

Lecture 4: Intro to the Vector Data Model and to Map Layout

Introduction to GIS

By Austin Troy, University of Vermont


1. Vector Data Model

Introduction to GIS


Intro to Vector• Recall that there are three basic “feature” or

“object” types in the vector data type:– Point – Arc – Polyons

• A given layer holds a given feature type (e.g. “roads” is a line layer, “counties” is a polygon layer, “weather stations” is point)

Introduction to GIS


Intro to Vector• A point layer only consists of a bunch of (x,y)

coordiantes

• In a line (arc) layer, points define lines

• In a polygon layer, lines define areas

• Hence each level of vector features builds on the last

Introduction to GIS


Intro to Vector• Each point has a unique location

• 2 points define a line segment

• One or several line segments define an arc

• The endpoints of an arc are “nodes

• The angle points are “vertices” (sing. Vertex)

• The feature is the arc, not the line

• Two arcs meet at the nodes

Introduction to GIS

Image source: ESRI Arc Info electronic help


Intro to Vector• Several arcs can scribe a polygon

• Polygons are closed regions whose boundaries are made up of line segments connected at many angles.

• Polygons: area of homogenous phenomena

• These phenomena can be described by one or more stored attributes

Introduction to GIS


Vector Representation:lines•Ring: this is a series of line segments (a string) that close upon each other

•It is NOT a polygon!!

•The computer does not know that the area inside “belongs” to that object

Introduction to GIS


Vector Representation:lines•A polygon is encoded differently, because the computer “knows” that the areas within those arcs “belongs” to that polygon, while it does not with a ring

Introduction to GIS


• What is it? Explicit encoding of spatial relationships between objects are encoded: the spatial location of each point, line and polygon is defined in relation to each other

• Two major purposes:

1. Allows for powerful analysis tools

2. Quality control mechanism.

Introduction to GIS

Vector: Topology


• Arc-node and node topology : the way that line features connect to point features

• Polygon topology: the way that neighboring polygons connect and share borders

• Route topology: the way that a line feature of one type (e.g. commuter rail line) shares segments with line features of another type (e.g. Amtrack rail line)

• Regions topology: the way that polygons overlap (e.g. GIS layers with a time component) or when spatially separate polygons are part of the same feature

Introduction to GIS

Types of Vector Topology


• One of the most important functions of topology is ensuring data quality and “logical consistency”

• Why? Because with topology can define complex and nuanced spatial rules.

• Single layer quality control: When you bring in line and polygon data from external sources, you will often find errors such as lines (arcs) that dangle or overshoot, polygons that don’t close, adjacent polygons that show up as not sharing a border (we’ll return to this later in the semester)

Introduction to GIS

Quality control and topology


Vector Topology helps deal with:

Introduction to GIS

overshoots

slivers

dangles

Not sharing border


• A topological structure helps ensure these problems don’t happen because it allows for enforcing of user-defined spatial rules

• ArcGIS 9 includes new tools for defining and validating topology rules

• Mutli-Layer quality control: Topology can also be used for defining spatial rules between layers to minimize errors and ensure logical consistency between them

Introduction to GIS

Quality control


• Say we have the following layers: property lots, sidewalk, building footprints, zoning map

• We can specify topological rules, like:– Lots must be enclosed polygons– Buildings must be entirely within a lot– Sidewalks must be outside a lot polygon and

entirely within the public right of way– Lots must fall entirely within a single zone– All lots must have access to a right of way

Introduction to GIS

Topology rules: Example


Vector Topology TableConsists of four elements

1. Polygon topology table• Lists arcs/links comprising polygon

2. Node topology table• Lists links/arcs that meet at each node

3. Arc, or “link” topology table• Lists the nodes on which each link/arc ends and

polygons to right and left of each link/arc, based on start and finish nodes

4. Table with real world coordinates for each point

Introduction to GIS


Vector Topology Table

Graphical display of arcs, nodes, vertices and lines

Topology table for the ARCs making up the polygons

A table of the polygon topology

Introduction to GIS

Image source: ESRI Arc Info electronic help


Spaghetti Data Model•Just because feature looks like a point, line or polygon does not mean it’s topological

Spaghetti Model is:•Non-topological data model that looks like vector•collections of line segments and points with no real connection or topology•Only stores features coordinates; there are no relative relationships encoded in this model •each feature has no knowledge of other features that it intersects, is adjacent to, contiguous with or near

Introduction to GIS


Spaghetti Data•Generally have loose ends, nodes not “snapped,” polygons don’t fully close, etc•Polygons defined by coordinates of circumscribing points, so common boundaries between two polygons are often registered twice.•Generally come from CAD files or digitizing•They often look fine to the user, but are useless from the standpoint of spatial analysis•This approach is memory inefficient

Can “clean” these data, using user-defined tolerances

Introduction to GIS


Vector Map representation and Scale

Scale is the ratio of the map distance to the ground distance

Hence, 1:200,000 means 1 cm on the map = 200,000 cm in the real world

The smaller the ratio, the LARGER the scale and the smaller the area depicted

That area is known as the map extent.

Introduction to GIS


Map representation:ScaleScale and Vector representation are closely tied up

On a small scale map (e.g. 1:2,000,000) a city is represented as point, without dimension, while on a large scale map (1:24,000), a city would likely be represented as an area with dimensions

Think of other examples: rivers, roads, buildings: these

The smaller the scale, the more we abstract, and the more we use points and lines

Introduction to GIS


Map representation:ScaleAs we get into larger scales, and demand higher levels

of representation and accuracy we increasingly represent everything with areas, because everything has dimensionality.

Extreme example: if we get large enough scale, a fire hydrant must be represented as an area, not a point

USGS has rules about representation and scale: for instance, on 1:24,000 topo maps, they use lines to represent streams less than 40 feet wide and double lines (areas) to represent larger watercourses.

Introduction to GIS


Map representation:ScaleNotice how here on this topo map, some structures appear as points, while others have shape: this is based on decision rule

Introduction to GIS


2. Map Layouts and Cartographic Representation

Introduction to GIS


Layouts• You can very simply create a map for layout

in Arc GIS by simply clicking View>>Layout view.

• Layouts are designed to cartographically acceptable, which means they must have the key elements of a printed map, such as scale bars, north arrows, legends and titles. These can be added from the Insert menu

Introduction to GIS


Layouts• Example

layout (from lab 6)

Introduction to GIS

legend

North arrow

Scale bar

title


Layouts• Legends are edited in the Legends property window,

which can be accessed by double clicking the legends. Best way to learn about it is try it out

Introduction to GIS

Legends can show layer name as well as intervals for quantitative data and category names for categorical data


Layouts• You can change names of the layers for the sake of

your layout legend (since most layers have pretty unintuitive names) in the layer properties window

Introduction to GIS


Layouts• In layouts you can have detailed and highly formatted

labeling and annotation. You can use an attribute field to label; this is specified in layer properties

Introduction to GIS


Layouts: data frame•You can create a new view or “data frame” within Arc Map with separate data layers, or with the same data layers at a different scale

•Can access properties for the whole data frame by double clicking on “Layers” at the top of the TOC or by right clicking on the data frame

Introduction to GIS


Arc Map: data frame

•Both frames are shown in layout view

Introduction to GIS

Frame 1

Frame 2


MXD Files • You can save your layout, along with all other

preferences and settings by saving an Arc Map Document (MXD) file. However, this is not saving your data, only the settings, including the layout. If you move the MXD, you must move the layers with it. This is one reason why a geodatabase is easier than multiple shapefiles

• To save, just go to File>>save as

Introduction to GIS


Layer Files • Layer (.lyr) files save all your symbology and

settings for one single file. It is primarily for saving legend settings. So, for instance, if I make a layer with 300 land use categories, and I create a legend classification that regroups them into 30 categories, each with a special color or hatching, I can save that as a layer file.

• Once created, opening a layer file will open the data layer with all the preferences saved. You can move the data around without moving the layer file as long as both are on the same system.

Introduction to GIS


Layer Files • Use layers when you have lots of non-numeric

categories

Introduction to GIS


Layer Files • This can be made in Arc Catalog, by right clicking

and clicking “create layer.” Then I can create the legend preferences in Arc Catalog

• Can also be made in Arc Map

Introduction to GIS

Can also import a layer file’s symbology in properties

Documents

© 2005. materials by Austin Troy. All rights reserved Lecture 4: Intro to the Vector Data Model and to Map Layout Introduction to GIS By Austin Troy, University