Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Lecture 4: The Vector Data Model, Spatial Joining and Geoprocessing Written.

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Lecture 4: The Vector Data Model, Spatial Joining and Geoprocessing Written by Austin Troy, Brian Voigt and Weiqi Zhou, University of Vermont © 2010

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Intro to Vector Recall: –Points –Arcs –Polygons Given layer holds a given feature type (e.g. “roads” is a line layer, “counties” is a polygon layer, “weather stations” is point) Each level of vector features builds on the last

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Point Feature A point layer: a collection of records with (x,y) coordinates Image modified from ESRI Arc Info electronic help 0123456 0 1 2 3 4 5 6 2,2 6,3 5,5 3,6 1 2 3 4 IDX,Y Coordinates 12,2 23,6 35,5 46,3 … 104,1 10

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted 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

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Line (Arc) Feature Image modified from ESRI Arc Info electronic help Line segment Node Vertices Node Feature 1 Feature 2

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Polygon Feature Area of homogenous phenomena In a polygon layer, lines (arcs) define areas Closed region – first and last coordinate pairs are in the same location Line segments bound the polygon Computer “knows” that interior belongs to shape Lines (Arcs) Points

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Vector Representation:lines Ring: this is a series of line segments (a string) that close upon each other NOT a polygon!!

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Definition1: Explicit encoding of spatial relationships between objects: the spatial location of each point, line and polygon is defined in relation to each other Definition2: Topology is a collection of rules and relationships that enables the geodatabase to more accurately model geometric relationships found in the world. Two major purposes: 1.Allows for powerful spatial analysis 2.Quality control mechanism. Vector: Topology

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted 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 Types of Vector Topology

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Ensuring data quality and “logical consistency” Defining complex and nuanced spatial rules. Single layer quality control: –dangles –overshoots –polygons that don’t close –adjacent polygons that show up as not sharing a border Quality control and topology

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Mutli-Layer quality control: Defining spatial rules between layers Quality control and topology – Polygon rules: e.g. Must Be Covered by Feature Class of Define and validate topology rules in Arc Catalog and Arc Map (see http://webhelp.esri.com/arcgisdesktop/9.3/index.cfm?TopicName= Designing_a_geodatabase_topology ) http://webhelp.esri.com/arcgisdesktop/9.3/index.cfm?TopicName= Designing_a_geodatabase_topology – Line rules: e.g. Must not Self Intersect – Point rules: e.g. Must be Properly Inside Polygons

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted 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 –Lots must fall entirely within a single zone –Lots must either share a border with another lot or with city land, including streets and sidewalks. –In a low-density zone, no more than 20 lots can be touching We can’t do this yet, but will be able to shortly Topology rules: Example

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Vector Topology Table Consists 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

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted 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

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Spaghetti Data Model Non-topological data model that looks like vector collections of line segments and points with no real connection or topology No relative relationships encoded in this model Each feature “unaware” of other features that it intersects, is adjacent to, contiguous with or near

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted 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. Use of points and lines vs polygons depends on scale 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.

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Map representation:Scale Notice how here on this topo map, some structures appear as points, while others have shape USGS has a set of standards for representation based on scale

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Let’s say we want get information about all the houses in four sample neighborhoods and see which ones overlay fire hazard zones Selecting By Location

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Now with “sample houses” active, we click select by theme and tell it to choose features that intersect the features of fire hazard zone Layer to be selected Selection Method Selection rule Selection overlay theme Selecting By Location

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Now we run statistics on the selection, selecting the Price field. This tells us that 1955 houses overlay fire zones it also tells us that the mean price for these properties is $467,551! Selecting By Location

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted If we do an inverse selection by clicking the button, we see that non fire zone houses are worth less on average!Only $246,752 Selecting By Location

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Now, say we want to select features from layer A that are within a distance of features in layer B. In this case we’ll select houses in our sample neighborhoods that are within 1 mile of a Starbucks Selecting By Location:Distance

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted This time we use a different selection method with different parameters Selecting By Location :Distance Note how we can specify the distance for selection

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Now if we run statistics on price again… Selecting By Location :Distance Those within a mile of a Starbucks have a mean value of $504,972 Those not within a mile of a Starbucks have a mean value of $273,866! By the way, these are real data, I’m not making this up!!

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted For that same selection we could get statistics on a different variable—here we’ll look at lot size Selecting By Location :Distance Those within a mile of a Starbucks have a mean size of 8776 square feet Those not within a mile of a Starbucks have a mean lot size of 10,024 sq feet. Why might that be?

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted You can also select features in a layer by distance to a linear feature in another layer. Here we’ll find houses in a neighborhood within a mile of a highway Selecting By Location :Distance Note that these smaller roads are in a different layer

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted What if we just want to select those points that are within a distance one just one given feature within a layer? Here we’ll find all homes within 500 meters of Valley Blvd. (let’s say there’s going to be a parade and the city needs to inform all those homeowners near that street). First we must do a single layer query asking for Hwyname = “Valley Blvd” Selecting By Location :Distance

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Once that feature is selected we can do a “select by location” operation Selecting By Location :Distance Notice that this time we check “Use selected features”

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Thus we end up only selecting those houses within 500 m of Valley Blvd, and none within 500 m of other roads Selecting By Location :Distance

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Generally polygons in one layer do not perfectly coincide with those in another. The whole polygon will be selected even if only a small part is coincident, assuming we are using the default selection overlay method, “intersect.” Selecting By Location: Polygons However, there are many other methods we can choose from that will change the number of polygons selected.

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Example: let’s select any census tract that intersects even slightly with a fire zone; here’s the pre-selection map Selecting By Location: Polygons

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Using “that are completely within” method, we get no selected feature. But, with “have their center in” we get Selecting By Location: Polygons

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Likewise, if we select Merced County in the counties layer, activate “highways” in the TOC, and then select by theme, we will only choose those road segments that intersect that county Selecting By Location on Selections

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Note that the resulting selection was made using the “intersect method.” If the “completely within” method is used, a different set of lines will be selected Selecting By Location on Selections

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Note that the resulting selection was made using the “intersect method.” If the “completely within” method is used, a different set of lines will be selected, similar to what we saw in the fire zone- census tract example given previously. Line segments that cross over into next county will not be included Selecting By Location on Selections

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Once a selection has been done using “select by location” you can do all the same things you would do with a normal single-layer selection: –Make a new layer from the selection –Do statistics on it –Make a new field in that layer –Calculate or recalculate a field for a selection What can be done with multi-layer selections?

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Spatial Join Assigns attribute data from features in one layer to spatially coincident features in another Can assign polygon data to a point that overlays Can assign point to point and point to line distances between two layers Simply adds attributes to the DBF table

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Spatial Join We access Spatial Join by right clicking on the “to” layer and clicking Joins and Relates>>join We then specify that we want to join by location and choose which layer we are joining from

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Spatial Join In this case we are going to join tracts to the houses from our sample neighborhoods. Each house inherits all the attributes of the tract in which it falls. This is a great way to assign data from layer to another Note that this creates a new layer

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Spatial Join We can now plot out houses by any of the attributes that were in the tracts database. Here’s a plot of houses graduated by percent unemployment of the tract to which they belong

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Spatial Join:Distance We can also do spatial joins based on distance. Whenever we join a point or line layer to another point or line layer, for each feature in the TO layer it gives us the attributes of the nearest feature in the FROM layer PLUS the distance between those features in whatever map units we specify

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Spatial Join:Distance Let’s say we want to assign as an attribute to our house point layer the name of the nearest major road. The easiest way to do this is with a spatial join.

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Spatial Join:Distance Introduction to GIS Here I have two options: I can either choose to numerically summarize for each point the values of the lines intersecting it, or I can simply assign all attributes from the nearest line. Here we choose the latter My FROM layer

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Spatial Join:Distance Now name of nearest highway is an attribute for each housing point; here I’m plotting out categorically by that attribute after joining

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Spatial Join:Distance Distance from each point to the nearest road feature was also recorded under the attribute “Distance.” Here I’m plotting out distance to nearest major road.

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Spatial Join:Distance We can also do a join to get the distance from a series of points in one layer to a series of points in another: here is distance of houses to nearest Starbucks

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Spatial Join:Polygons Spatial Join is quite intuitive when it comes to assigning attributes to points and lines, but what about when assigning attributes to polygons? Problem: a polygon is layer A may overlay several polygons in layer B, so whose attributes to you give it? Layer A Layer B

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Spatial Join:Polygons Answer: we can do spatial join and summarize (by average, for instance) each polygon in layer A the values of all the overlapping polygons in layer B. Example: Say we have a census tract layer with all sorts of demographic info (population, race, etc) and we have a zip code layer with no demographic info attached to it. Our client is doing a marketing study and needs to have a map showing median age and percent Hispanic by zip code. We have both these attributes in our tract map and need to somehow “transfer” them to the zip code map

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Spatial Join:Polygons Unfortunately, the tract boundaries and zip code boundaries do not match up in the slightest. Note that tracts are not nested within zip codes—they cut across

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Spatial Join:Polygons To deal with this we do a spatial join of two polygon layers and choose the “summarize” option (the first radio button). This allows us to choose a statistic by which to summarize the value of all the constituent tract polygons for each zip code polygon. In this case we’ll choose “average” as our statistics

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Spatial Join:Polygons This results in a new output zip code layer with the average of every census tract variable; here median age is plotted; note that ArcGIS calls it Avg_MEDAGE so that we know what statistic this is based on

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Purpose of Geoprocessing Tools for breaking down the size of map features: –Union, Intersect, Clip Tools for increasing the size of map features: –dissolve and merge (indirectly) Arc/Info and Arc Toolbox include various other geoprocessing overlay operations, such as Update and Dissolve Regions Found in Arc Toolbox

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Union Combines features of two theme Each theme is treated the same Goes to extent of largest theme Keeps all line work, creates new polygons Breaks down features into smaller minimum mapping units Can use selected features option too Keeps all attributes Image source: ESRI Arc Info electronic help

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Intersect Yields polygons representing areas that are common to both layers Preserves line work within common extent Usually creates many new, smaller polygons Preserves all attributes from both

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Union vs. Intersection Union is the entirety of two overlapping sets of features and intersection is the common area only Continuous and exhaustive vs. “island” polygons have different ramifications for these tools Layer 1 +Layer 2 Intersect: Layer 1 + Layer 2 Union: “1 AND 2” “1 OR 2”

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Union vs. Intersection: Example Here’s an example. Say we have deer wintering areas in one layer and conserved lands in another.

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Union vs. Intersection: Example Intersect gives us land that is BOTH, and preserves all polygon boundaries within that common extent

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Dissolve : Example Now we have created a county map, and for each county we have an attribute as the sum of population of the constituent zip codes

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Merge Allows you to “join” two adjacent or non- adjacent themes into the same layer Like “tiling” Best when attributes match

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Merge Often when you merge you will want to follow up by dissolving. This is because artificial polygon boundaries were created at the borders by the act of splitting the data up into tiles; merging brings the tiles back together and dissolving joins together polygons that were split up in the process

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Tools: Buffering Buffering is when you draw a polygon around a feature (point, line or polygon); Here we’re buffering a stream

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Tools:Variable Width Buffering Where width of buffer varies with an attribute Often we must recalculate that attribute to make the buffer width meaningful Example from lab: a buffer based on traffic volume

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Geoprocessing vs. select by location By building a buffer of a given distance around a feature, we can then do overlay analysis and select features from another layer that are inside or that intersect the buffer With select by location, we can only do 2 layers at a time. When we combine buffering with geoprocessing, we can ask questions across unlimited numbers of layers. With select by location, have the problem of features with partial overlap; in geoprocessing, combine the two layers to create smallest necessary minimum mapping unit

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Combining Buffering and Geoprocessing: Example From Lab 4: say we made fixed buffers around deer wintering areas and water bodies, and a variable buffer around roads, based on traffic: ( note the lab is a bit different ):

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Combining Buffering and Geoprocessing: Example Then we could, for instance, find areas that are near deer wintering areas and water bodies but far from traffic: First we would intersect the deer wintering and water body buffers, yielding areas that are near both those. Then we would union the result with the traffic buffer and run an attribute query to determine which areas meet the criteria

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Combining Buffering and Geoprocessing: Example The intersection of deer wintering buffers and water buffers would be the area in the red

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Combining Buffering and Geoprocessing: Example Now we can query for polygons that were created from the intersection (met the two good criteria) and for areas that are not within a traffic buffer

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Combining Buffering and Geoprocessing: Example We can then create a layer from that—Note that we have created entirely new polygon boundaries and geometry by cutting and splicing these buffers together.

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Combining Geoprocessing Tools Involve multiple tasks performed in sequence, such as those that clip, buffering, intersect, union, then select datasets. –Step by step –Create and run a script –Build and run a model: Arc Model Builder

Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Lecture 4: The Vector Data Model, Spatial Joining and Geoprocessing Written.

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Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Lecture 4: The Vector Data Model, Spatial Joining and Geoprocessing Written.

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Presentation on theme: "Fundamentals of GIS All lecture materials by Austin Troy © 2010except where noted Lecture 4: The Vector Data Model, Spatial Joining and Geoprocessing Written."— Presentation transcript:

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