• Taking advantage of GIS to handle spatial operations
  • Do vector-based spatial analysis today with lab next Monday
  • Do raster-based spatial analysis next Wednesday will lab the following week
  • Then a test during lab and a guest lecture
  • First half of class (11.205) ends here
  • Second half includes web mapping, field data collection, and mapping APIs using R
  • That's all we have time for in lectures and lab exercises (some terrain analysis and 3D modeling at end)
  • Much of second-half (11.520) is the (small) individual project
  • For project: start thinking about topic of interest, spatial component, and possible data source 

• Spatial 'Join' issues:
  • Combine data from different sources based on location & proximity
  • Present results so that they can by visualized and/or encoded and saved for further use

• Examples of spatial 'joins' - we have already seen some of the tools for these examples

  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... Can we add census data to the sales attribute table? (not just 'select' the census block group containing the sale)
  Which blockgroups are in which Mass Town? Polygon-in-polygon operation Tricky because polygon boundaries may not line up (sliver & gap issues) How do we create a cross-reference table (for blockgroups and towns) Which overlap criterion: intersect, completely within, centroid within,...
  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? What happens to attributes of split road segments (keep names, divide length,...)
  Which shopping centers are close to major highways How do we represent shopping centers (point or polygon) How do we represent highways? (line, polygon, or just the exits) Where is this exit? (the perimeter of the Wrenthan outlet mall is > 1 mile)



• There are many interesting Proximity questions
  • ...and, we now have the background to utilize much of the QGIS Toolbox on our own

• Equals – are the geometries the same?
• Disjoint – do the geometries share at least a common point?
• Dissolve – Eliminate boundaries with same attribute value on each side.
• Intersect – Create all the combined pieces when features are overlaid
• Touch – do the geometries intersect at their boundaries?

(1) Site Selection for Low Cost Grocery Store Chain

• Conceptual Model
• Brainstorm criteria for "good" locations
• Case Study Example
• Factors used in actual Commercial Shopping Center Site Selection
• Powerpoint slides used by commercial firm to market site selection tools
  by Edens & Avant and RPM consulting  
• Think about these marketing slides?
• Is the methodology or analytic scope overstated?
• What considerations are omitted, shortchanged, badly measured?
• From whose point of view is the siting service helpful or hurtful?

• 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
  • 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).

• 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.

• 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:
  • 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, point-in-polygon is used to handle difficult polygon-on-polygon operations such as the blockgroup-in-town issue in HW#2
    • Variations along boundaries create slivers and make polygon-on-polygon overlay unsatisfactory
    • Compute centroids of blockgroups first, or extend one boundary with a small buffer and then intersect, ...
    • Illustrate point-in-polygon operations using blockgroup centroids to see which ones fall inside which towns
  • ArcGIS operation
    • Find the right tool in ArcToolbox (use the 'search' tab if necessary)
    • Use 'spatial join' operation in order to add polygon attribute information to the point attribute table
    • We have seen this join-by-spatial-location operation when doing join-by-attribute operations
    • This operation - and many others - can be accessed from ArcToolbox (under the 'analysis tools' menu)

• 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
  • Since polygons aren't likely to cross all lines precisely at the ends of the line segments, this operation can be complex since the operation must split lines at the polygon boundaries and assign the appropriate attributes to each new segment.

• Many possible ways of intersecting polygons with one another
  • Intersection: Polygon 1 + polygon 2 = polygon 3 (with common areas)
  • Union: Polygon 1 + polygon 2 = polygon 3 (with both areas)
• Many issues about how to interpret the results
  • Handling slivers due to geometric errors or differences in degree of generalization of boundaries
  • 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)
    • We will do an 'area allocation' to find the fraction of area in a block group that is inside the buffer and use that fraction to estimate the population living inside the buffer

Cut out the industrial area from two block groups near Sherman and Walden St. in Cambridge (near Fresh Pond).  The folder, cambridge_lecture7qgis.zip, in the class data locker contains all the files needed for this example.  To illustrate the process using small datasets, we extracted as separate shapefiles two block groups (from cambbgrp) and all the 'industrial' land in Cambridge (from camb_area_lu_1999).

Cambridge block groups: STCNTRBG = 250173546003 and 250173546001 with Industrial land use (LU21_CODE=16) overlaid across Sherman St.
Open 'Processing / Toolbox' and choose 'Difference' option within 'vector overlay'
Cut industrial land out of the two block groups using the 'difference' tool (Input layer is camb_bg_fresh_pond to get parts of it that are *not* within industrial zones.)
Log tab of 'difference' tool reports output of running python algorithm.
End up with the industrial area cut out from the two block groups NOTE: right-side block group now consists of two polygons.
BEWARE: all attribute values are retained in attribute table - even if no longer correct (e.g., area, perimeter, and landacre). You can recalculate area and perimeter, but the change in landacre is unknown. (Do you understand this?)
Use 'Field Calculator' to recalculate last column ('tmp') or add new field so it shows the new area of each block group.
	Note that the new area of each block groups is now saved in the 'tmp' field.

Example used Color Orthophotos from MassGIS

• LANDUSE_POLY.ZIP = zipped shapefile of 1951-1999 landuse (108 MB):
  http://download.massgis.digital.mass.gov/shapefiles/state/landuse_poly.zip

<< This section illustrates the ERASE operation using ArcGIS software >>

ArcGIS Example using 'ERASE' Operator:

Cut out the industrial area from two block groups near Sherman and Walden St. in Cambridge (near Fresh Pond).  The folder, cambridge_lecture7.zip, in the class data locker contains all the files needed for this example.

Cambridge block groups: STCNTRBG = 250173546003 and 250173546001 with Industrial land use (LU21_CODE=16) overlaid across Sherman St.
Choose the ERASE extract tool in ArcToolbox
Use the industrial land use boundaries to erase the industrial parts of the two block groups
End up with the industrial area cut out from the two block groups
BEWARE: all attribute values are retained in attribute table - even if no longer correct (e.g., area, perimeter, and landacre). You can recalculate area and perimeter, but the change in landacre is unknown. (Do you understand this?)
	Note that the new area of the block groups is reduced by the erased area.