• Examine the location patterns of Cambridge stores by using the 'spatial join' tools to tag store location data (bookstores, ice cream shops, record stores) with the demographic characteristics of their neighborhood. (This is a 'point-in-polygon' operation.)
• Create a 1 km buffer around Ames St.
• Estimate the number of young kids living near Ames Street by:
  • Intersecting the Ames street buffer with the Cambridge blockgroup data
  • Apportioning kids in each blockgroup that is split by the buffer in proportion to the block group area in the buffer

Lab #6 Issues:

• Buffering Ames St.: similar to previous buffering of Mass Ave; pay attention to setting map and display units (so you buffer at 1 km)
• Apportioning Cambridge kids to the parts of each block group that falls within the buffer
  • Union and Intersect operations retain the old polygon areas
  • You need to compute the new-polygon areas
  • The Field/Calculate-geometry option simplifies the area calculations (but Lab #6 shows you how to do it with VBA script)

General ArcMap Tips:

• Folder, shapefile, and table names:
  • Use only letters and number with NO spaces, special characters, etc.
  • Start all names with a letter
• Save early and often
  • Always have a recent unused copy of any MXD document (the ArcMap document mymapfile.mxd)
  • Every time you start an ArcMap session from a saved document, save it right away into an mxd document with a new name (version2, ver3, ...)

Leftover from last Wednesday's lecture:

Before starting the lab, I will finish my demonstration from last Wednesday of the steps needed to determine which block groups are in which towns in the 5-town area around Cambridge.

• Strategy:
  • Can't intersect the block group layer with the town boundaries because of 'sliver' problems at the edges
  • Create a 'point' layer of the centroids of all the blockgroups in the area
    • Select blockgroup in and around the 5 towns and save to a new shapefile
    • add X,Y fields to the attribute table (as double precision numbers)
    • Use this VBA script to enter the centroid X,Y values into the new fields

    Dim dblX As Double
    Dim pArea As IArea
    Set pArea = [Shape]
    dblX = pArea.Centroid.X

    • Create a new point shapefile of these X,Y centroid points using Tools/Add-X-Y-data
      • We ended the lecture at this point on Wednesday
      • You need to export the attribute table with XYs and add it as a separate table in ArcMap before you can create the new point shapefile. Export only the XY points and identifiers to simplify the table.
  • Now do a 'spatial join' to tag each centroid with the town that it is in
    • Once you have the centroids saved as a shapefile, it is easy to highlight those blockgroups that are in the 5 towns.
      • Select the 5 towns from matowns00.shp and use the Select-by-location option to highlight those blockgroup centroids that fall inside the 5 towns.
      • HOWEVER, this is not enough. We want to add a column to the blockgroup attribute table with the name of the town that contains that blockgroup.
    • Adding town names to the blockgroup attribute table:
      • You have to add them to the blockgroup centroid attribute table first and then join that table to the original block group table.
      • Adding the MATOWN00.SHP attrbiute information (including town name) to the blockgroup centroid attribute table can be done as a spatial join. However, ArcMap must know the coordinate system of each layer that is being combined. Worse yet, if the coordinate system for your centroid shapefile has not been set, you will have to close ArcMap and use ArcCatalog to set it. Then, finally, you can join the matown00.shp layer into the block group centroid attribute table and then join (by attribute) that table to the original blockgroup table in order to associate town with blockgroup ID.
      • Because this took so many steps, we did not include it in the homework #1 assignment! Happily, ArcMap now provides a spatial join operator that allows direct polygon-on-polygon overlay based on the location of the centroids of one layer.

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Last modified 29 October 2007 by Joe Ferreira.