Massachusetts Institute of Technology Department of Urban Studies and Planning 11 520 A Workshop on Geographic Information Systems 11 188 Urban Planning and Social Science Laboratory Lecture 7 Spatial Analysis Vector Models October 19 2005 Joseph Ferreira Jr based in part on notes by Visiting Prof Zhong Ren Peng in Fall 2003 Major Topic Spatial Analysis using Vectorbased Data Models Taking advantage of GIS to handle spatial operations Spatial Join issues o o Combine data from different sources based on location proximity Present results so that they can by visualized and or saved for further use Examples of spatial joins o o o Which Cambridge house sales fall within which census blockgroups Point in polygon operation Because we want to know about the neighborhood surrounding lower price homes Which blockgroups are in which Mass Town Polygon in polygon operation Tricky because polygon boundaries may not line up sliver gap issues Which roads are in the flood plain Line in polygon operation Should you cut up the road segments or list line segments with part 50 inside Which shopping centers are close to major highways How do we represent shopping centers and highways o There are many interesting Proximity questions General spatial operations Equals are the geometries the same Disjoint do the geometries share a common point Intersect do the geometries overlap Touch do the geometries intersect at their boundaries Buffering Vector Data Buffering creates new polygon features by computing which areas are within a certain specified distance of the selected object Buffering may define a new spatial object whose boundary enclosed all the area within the buffer o o o the geometric computations can be complex beware of islands slivers etc involves choices when buffers of selected features overlap dissolve Buffer Input Points lines or polygons Buffer Output Polygon s Buffering Raster Data Classify grid cells according to whether they lie inside or outside the buffer In addition to distance you can also use any function of grid cell values e g travel speed as a buffer criteria The result is a new table of cell values for each grid cell e g a raster map distinguishing all grid cells with say less than 30 minutes travel time from a particular point e g a city center More on raster modeling and analysis after next Monday s vector analysis lab Point in Polygon Whether a point lies inside or outside a polygon In the case of many points and many polygons the task is to assign points to polygons Common operation o o o Many mailing lists and GPS readings are geocoded into point files You need point in polygon operation to tag the attributes of the point map layer with characteristics of the places they are in Sometimes this is a way of handling difficulty polygon on polygon operations such as the blockgroup in town issue in HW 2 o Compute centroids of blockgroups Do point in polygon operations to see which blockgroup centroids fall inside which towns ArcGIS operation intersect the two coverages in order to add polygon attribute information to point attribute table Desktop ArcGIS tools have less sophisticated capabilities than workstation and arcedit tools that cost additional money for the extra tools in ArcToolbox Line in Polygon Used to determine whether a line lies inside or outside a polygon In the case of many lines and many polygons the task is to assign line segments to polygons ArcGIS operation Intersect lines and polygons in order to tag attributes of line segments with polygon information o Since polygons aren t likely to cross the lines at the ends of the line segments this operation can be complex if you wish to split lines at the polygon boundaries and assign the appropriate attributes to each new segment Polygon Overlay Many possible ways of intersecting polygons with one another o Intersection Polygon 1 polygon 2 polygon 3 with common areas o Union Polygon 1 polygon 2 polygon 3 with both areas Many issues about how to interpret the results o Handling slivers due to geometric errors or differences in degree of generalization of boundaries o Handling attributes Some attributes apply to each fragment without change e g in flood plain in city Some attributes need to be allocated to each fragment e g population in each part of blockgroup In next Monday s lab exercise we will have examples e g estimate number of people living within a specified distance of MIT s biology building o More spatial data processing 1 Dissolve features based on an attribute 1 Merge layers together 1 Clip one layer based on another Spatial Analysis Example Locating a Shopping Center 1 Criteria 1 Within two mile of a major highway but no less than one quarter mile away from a major highway 2 At least one mile away from a water body e g rivers lakes 3 Not in the flood zone 4 Close to a major residential area at least 2500 households within 5 miles 5 Have a large piece of vacant land at least 10 ac endif Classic site suitability analysis endif Decompose task into many ArcGIS analysis steps Data Required Highway layer linear feature Vacant land layer polygon feature 1 Rivers layer linear or polygon feature 1 Lakes layer polygon feature Land use layer polygon feature 1 Flood zone layer polygon feature 1 Census layer blockgroup level polygon feature Spatial Analysis Process Select vacant land parcels larger than 10 acres result vacantlg10 Buffer highway layer with 2 mile and 25 mile result hw2 and hwquarter Buffer Rivers and Lakes layers with 1 mile result river1 and lake1 Intersect vacantlg10 with hw2 vacanthw1 useful land Union hwquarter with river1 lake 1 and flood need to do two layers at a time result notusable land not meeting the criteria remove notusable from vacanthw1 result usable intersect usable with census blockgroup layer touch the boundary of result usableblock calculate and reselect usableblock with number of total household larger than 2500 result bigusableblock Additional criterion 1 Consider an additional criterion The median household income of the proximate residential area should be more than 30 000 2 Refine major residential area criterion 3 Require enough people within a buffer area around site 4 Require proximity to dense residential area ArcGIS Example from Homework 2 find which blockgroups are in which town We can do a Spatial Join involving polygon to polygon operations but that won t work well because of the many sliver issues along the edges of the blockgroups and town boundaries Instead