© 2005. materials by austin troy. all rights reserved lecture 4: intro to the vector data model and...
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© 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
© 2005. materials by Austin Troy. All rights reserved
1. Vector Data Model
Introduction to GIS
© 2005. materials by Austin Troy. All rights reserved
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
© 2005. materials by Austin Troy. All rights reserved
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
© 2005. materials by Austin Troy. All rights reserved
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
© 2005. materials by Austin Troy. All rights reserved
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
© 2005. materials by Austin Troy. All rights reserved
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
© 2005. materials by Austin Troy. All rights reserved
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
© 2005. materials by Austin Troy. All rights reserved
• 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
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• 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
© 2005. materials by Austin Troy. All rights reserved
• 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
© 2005. materials by Austin Troy. All rights reserved
Vector Topology helps deal with:
Introduction to GIS
overshoots
slivers
dangles
Not sharing border
© 2005. materials by Austin Troy. All rights reserved
• 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
© 2005. materials by Austin Troy. All rights reserved
• 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
© 2005. materials by Austin Troy. All rights reserved
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
© 2005. materials by Austin Troy. All rights reserved
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
© 2005. materials by Austin Troy. All rights reserved
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
© 2005. materials by Austin Troy. All rights reserved
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
© 2005. materials by Austin Troy. All rights reserved
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
© 2005. materials by Austin Troy. All rights reserved
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
© 2005. materials by Austin Troy. All rights reserved
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
© 2005. materials by Austin Troy. All rights reserved
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
© 2005. materials by Austin Troy. All rights reserved
2. Map Layouts and Cartographic Representation
Introduction to GIS
© 2005. materials by Austin Troy. All rights reserved
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
© 2005. materials by Austin Troy. All rights reserved
Layouts• Example
layout (from lab 6)
Introduction to GIS
legend
North arrow
Scale bar
title
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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
© 2005. materials by Austin Troy. All rights reserved
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
© 2005. materials by Austin Troy. All rights reserved
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
© 2005. materials by Austin Troy. All rights reserved
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
© 2005. materials by Austin Troy. All rights reserved
Arc Map: data frame
•Both frames are shown in layout view
Introduction to GIS
Frame 1
Frame 2
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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
© 2005. materials by Austin Troy. All rights reserved
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
© 2005. materials by Austin Troy. All rights reserved
Layer Files • Use layers when you have lots of non-numeric
categories
Introduction to GIS
© 2005. materials by Austin Troy. All rights reserved
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