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

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Lecture 4: Intro to the Vector Data Model and to Map Layout
Lecture 3b Introduction to GIS Lecture 4: Intro to the Vector Data Model and to Map Layout By Austin Troy, University of Vermont © materials by Austin Troy. All rights reserved

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) © materials by Austin Troy. All rights reserved

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 © materials by Austin Troy. All rights reserved

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 © materials by Austin Troy. All rights reserved Image source: ESRI Arc Info electronic help

Introduction to GIS 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 © materials by Austin Troy. All rights reserved

Vector Representation:lines
Lecture 3b 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 © materials by Austin Troy. All rights reserved

Vector Representation:lines
Lecture 3b 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 © materials by Austin Troy. All rights reserved

Introduction to GIS Vector: Topology 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: Allows for powerful analysis tools Quality control mechanism. © materials by Austin Troy. All rights reserved

Types of Vector Topology
Introduction to GIS Types of 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 © materials by Austin Troy. All rights reserved

Quality control and topology
Introduction to GIS Quality control and 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) © materials by Austin Troy. All rights reserved

Vector Topology helps deal with:

Introduction to GIS Quality control 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 © materials by Austin Troy. All rights reserved

Topology rules: Example
Introduction to GIS Topology rules: Example 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 © materials by Austin Troy. All rights reserved

Lecture 3b Introduction to GIS Vector Topology Table Consists of four elements Polygon topology table Lists arcs/links comprising polygon Node topology table Lists links/arcs that meet at each node 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 Table with real world coordinates for each point © materials by Austin Troy. All rights reserved

Introduction to GIS Vector Topology Table A table of the polygon topology Graphical display of arcs, nodes, vertices and lines Topology table for the ARCs making up the polygons © materials by Austin Troy. All rights reserved Image source: ESRI Arc Info electronic help

Lecture 3b Introduction to GIS 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 © materials by Austin Troy. All rights reserved

Lecture 3b 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 © materials by Austin Troy. All rights reserved

Vector Map representation and Scale
Lecture 3b 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. © materials by Austin Troy. All rights reserved

Map representation:Scale
Lecture 3b Introduction to GIS Map representation:Scale Scale 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 © materials by Austin Troy. All rights reserved

Map representation:Scale
Lecture 3b Introduction to GIS Map representation:Scale As 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. © materials by Austin Troy. All rights reserved

Map representation:Scale
Lecture 3b Introduction to GIS Map representation:Scale Notice how here on this topo map, some structures appear as points, while others have shape: this is based on decision rule © materials by Austin Troy. All rights reserved

2. Map Layouts and Cartographic Representation

Lecture 3b 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 © materials by Austin Troy. All rights reserved

Lecture 3b Example layout (from lab 6)
Introduction to GIS Layouts Example layout (from lab 6) title North arrow Scale bar legend © materials by Austin Troy. All rights reserved

Lecture 3b Introduction to GIS 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 Legends can show layer name as well as intervals for quantitative data and category names for categorical data © materials by Austin Troy. All rights reserved

Lecture 3b Introduction to GIS 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 © materials by Austin Troy. All rights reserved

Lecture 3b 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 © materials by Austin Troy. All rights reserved

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 © materials by Austin Troy. All rights reserved

Introduction to GIS Arc Map: data frame Both frames are shown in layout view Frame 1 Frame 2 © materials by Austin Troy. All rights reserved

Lecture 3b Introduction to GIS 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 © materials by Austin Troy. All rights reserved

Lecture 3b 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. © materials by Austin Troy. All rights reserved

Lecture 3b Use layers when you have lots of non-numeric categories

Lecture 3b 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 Can also import a layer file’s symbology in properties © materials by Austin Troy. All rights reserved

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