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MIT 11 520 - Spatial Analysis

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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 Vector-based Data Models) • Taking advantage of GIS to handle spatial operations • Spatial 'Join' issues: o Combine data from different sources based on location & proximity o Present results so that they can by visualized and/or saved for further use • Examples of spatial 'joins' 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... o Which blockgroups are in which Mass Town?  Polygon-in-polygon operation  Tricky because polygon boundaries may not line up (sliver & gap issues) o 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? o Which shopping centers are close to major highways  How do we represent shopping centers and highways? • 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 the geometric computations can be complex o beware of islands, slivers, etc. o 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 Many mailing lists and GPS readings are 'geocoded' into point files o You need point-in-polygon operation to tag the attributes of the point map layer with characteristics of the places they are 'in' o Sometimes, this is a way of handling difficulty polygon-on-polygon operations such as the blockgroup-in-town issue in HW#2  Compute centroids of blockgroups  Do point-in-polygon operations to see which blockgroup centroids fall inside which towns o 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) o 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)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.


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MIT 11 520 - Spatial Analysis

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