Massachusetts Institute of Technology - Department of Urban Studies and Planning

11.188: Urban Planning and Social Science Laboratory
11.520: A Workshop on Geographic Information Systems

Network Analysis, Advanced Raster, and Web Mapping
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Project Preparation

April 15, 2020

Administrative

  • Monday, April 20 is a holiday (this time without the Boston Marathon)
  • Lab #7 (Part 2, Google Traces): due Wednesday, April 22
  • Homework #3: Raster Analysis and ModelBuilder - No longer required
  • Project proposals: feedback is on Stellar; use breakouts, office hours, and Friday lab time if you have further questions or need help finding or evaluating data.

Feedback from Test

  • Results were quite good with mean=89 and standard deviation = 5
  • Everyone achieved at least a B-level grade.
  • Solutions are posted on the 'Test' page
  • In 2016, Michigan was the closest of the key mid-west states
  • The urban/rural divide continues with a battleground in the suburbs
  • Do you think this political lens can explain some of the current tension between Feds and States regarding pandemic response?

Outline for Today

  • Converting X/Y lat/lon data into point shapefiles
  • Network Analysis
  • Under-the-cover look at Web Mapping

More Covid-19-related Maps


Shapefile from X/Y points
  • In Lab #7 you convert Yelp points of interest into an ArcMap shapefile:
    • Yelp output is a CSV text file (comma separated values) that can be read into Excel
  • Use Excel functions to prepare data for insertion in ArcMap
    • Make sure lat/lon are formated as numbers (see lab notes)
    • In other cases, lat/lon may be in one field & Excel functions can parse them:
      • For example: =MID(G2, 5, 9)  and =MID(G2, 21, 10)
    • ArcMap only reads Excel files saved as *.xls (not the latest *.xlsx format)
      • If they still show up as text in ArcMap (text is left justified), go back to Excel and force coversion (e.g., =1.0*b2...)
  • Add adjusted worksheet from Excel into ArcMap
    • Right-click: Display XY data to convert to points
    • Remember to specify geographic coordinates (lat/lon, North America, NAD83)
    • Results produce an 'events' layer which shows points on map but is not a shapefile
  • Still need to do "Data / export data' into a shapefile before it is saved as a real shapefilef


Network Analysis - Expand discussion of  Network Analysis tools from previous lectures
  • Encoding proximity using a network (or graph) model, facilitates certain types of connectivity analyses
    • Find shortest path along streets from Point A to Point B
    • Find shortest path through N cities (Traveling Salesman problem)
    • How far can you get in 30 minutes
  • Many transportation analyses use network data models
  • Many hydrological analyses use network data models (runoff, flow, ...)
  • Additional information on Network Analyst below


Network Example: using US Census Bureau, TIGER Line Files

  • Geocoding Strategy using TIGER files (from earlier lecture notes)
    • Encode road network as street centerlines links connecting nodes (usually intersections)
    • Attach address information to each street segment
    • Use 'in reverse' to match street address to street segment to get approximate X,Y location
  • TIGER: Topologically Integrated Geographic Encoding and Referencing system
    • Examine attribute table and note columns for to/from information
    • http://www.census.gov/geo/www/tiger/
    • US Census Bureau TIGER line file 2000, technical documentation
      • at Census: http://www.census.gov/geo/www/tiger/rd_2ktiger/tgrrd2k.pdf
      • in class locker: http://mit.edu/www/data/census2k/tiger_tgrrd2k.pdf
  • Illustrative Example
    Street centerline road segments
    Attaching address ranges to road segments

 

What is ArcMap's Network Model?

A network is a system of linear features connected at nodes
E.g, nodes could be where three or more street segments intersect.
The linear feature connecting any given pair of nodes is called an arc, or network link.
Each arc on a network is represented as an ordered pair of nodes, from node i to node j, denoted by (i, j), and thus has direction.
A network representation that is good for transportation modeling may differ from a geographically accurate representation of the physical road (e.g., street centerline, handling exit ramps, 3D overpasses, etc.)
Combining two network models can be hard
    -- How do you handle lanes, exit ramps, and overpasses?

    Other basic elements of a network:

A shortest path is the shortest (or least 'cost' path) from a source node (origin) to a destination node.  In practice, pathfinding seeks the shortest or most efficient way to visit a sequence of locations.

A tour is an enclosed path, that is, the first node and the final node on the path are the same node on the network.

A stop is a location visited in a path or a tour.

Events or locations may be viewed as collection points (e.g., 'origins' or 'destinations' ) where certain resources are supplied or consumed.

A turn on a network is the transition from one arc to another arc at a node (there are 16 ways in which two intersecting (one-lane) roads can allow vehicle flow among the 4 links that 'connect' to the one node).

'Location-allocation' models often use network representation of connected places in order to determine the optimal locations for a given number of facilities (e.g., stores, restaurants, banks, factories, warehouses, libraries, hospitals, post offices, and schools) based on some criteria for assigning people to the the 'nearest' facility.

  • For Lab #7, you use the GoogleWay API to run their shortest path algorithm to compute the walking distance from 'home' to each restaurant (obtained using Yelp points of interest). 


Under-the-cover look at Web Mapping    

  • We have already used:
    • Several "Web Mapping Services" in our ArcMap documents
    • Javascript and Leaflet libraries to draw maps in Lab #7
    • More tutorials are available on DUSPviz: duspviz.mit.edu
  • What is the information exchange between your browser and backend resources?

  • Useful tools for 'publishing' project work

Advanced Raster Analysis (From April 1 lecture notes but not covered)

Summarize grid cells values by fixed geometries using zonal statistics

  • Vector case example: summarize suitability across each census blockgroup
  • Use 'zonal statistics' to average grid cell values within each blockgroup polygon

Finding patches using RegionClass command

  • What if we wanted to summarize by real city blocks and only had road centerlines
  • Create block polygons from road centerlines using 'line coverage to region' RegionClass tool (requires ArcInfo license)
  • Use block IDs to distinguish each 'patch' so zonal sum will compute block average

Moving window analyses

  • Sometimes we don't have a fixed geography
  • Want to summarize clusters of occurrences
  • Example: land use mix (within classic pedestrian 1/4 mile)

Cost-over-Distance measures

  • Some paths are more 'expensive' to traverse than others
    • Simplest form: mask out excluded areas, assume cost per cell is equal elsewhere
    • Alternative: weight distance across cell (or road segment) by cost to traverse
    • E.g. use speed limit to estimate travel time; or penalize steepness, bad soil, flood risk, etc.
    • could be done for raster or vector (street network) models
  • In general, urban grid makes accessibility relatively even
    • But note case of Charles River
  • Online Example: RedFin link shown in previous section to find 30-min job count

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