All file names begin with the prefix CITY. Since the Light Rail Case Study involved examination of nine cities, CITY was replaced with an abbreviation (maximum of 4 letters) for each particular city. However, the commands will work perfectly well without modification if your service areas are being created for only one geographical extent.
Coverages:
CITY_lrs: Light rail stations. Derived from a shapefile using shapearc.
CITY_sts: Street network. Derived from a shapefile using shapearc.
Grids:
CITY_sts_g: 50 foot grid with positive numeric values for all street cells and zero values for all non-street cells. Derived from CITY_sts using linegrid.
CITY_source: Also knows as the Source Grid. This grid has positive numeric values for all light rail station
cells and nodata values for all other cells.
Derived from CITY_lrs using pointgrid.
CITY_cost: Also known as the Network Impedance Grid. This grid has an impedance value for all street cells (use ‘1’ for distance based buffers) and an arbitrarily high value for all for all other cells. Derived by running a con() command on city_sts_g
CITY_dist: Also known as the Raw Travel Cost Grid. This grid contains the distance to the nearest station for all network cells and exaggerated values for all off-network cells.
CITY_alloc: Also known as the Allocation to Source Node grid. This grid contains the number of the nearest station for all cells.
CITY_dist_nd: Grid with the distance to the nearest station for all network cells and nodata for all off-network cells.
CITY_dist_int: Grid with the integer distance to the nearest station for all street cells.
CITY_flatgrid: Also known as the Off-Network Impedance Grid. This is a grid with the same value in every cell. For distance based service areas, the value should be ‘1’, for service areas based on travel time, the value should be equivalent to the impedance assigned to network cells. Derived by running a con() command on CITY_sts_g.
CITY_2rd_dist: Grid with the distance to the nearest street cell in all cells.
CITY_rd_dist: Grid with the distance from the nearest road cell to the nearest station for all cells.
CITY_sumdist: grid with the total distance between each cell and the nearest station cell.
CITY_mask: Grid with a value of 1 for all cells within a network distance of 2651 feet of a station cell and a value of zero for all other cells.
CITY_finaloc: Grid with the number of the nearest station for all cells.
CITY_fin_buff: Grid with the number of the nearest station for all cells within 2651 feet of a station and a value of zero for all other cells.
All text in fixed width typewriter style font is a literal rendition of ArcInfo commands. Text in variable width font represents general instructions. Text in bold italics is general commentary, explaining the broad strokes of the algorithim and giving the reasoning behind each step. Text in regular italics is commentary on each particular command, explaining what that command does.
Ensure that the projection has units measured in feet.
If the street network is composed of different files, merge them together into a single unit.
Clip the street network to eliminate extraneous area. The time required for the cost distance command varies according to the size of the grid, so try to clip just beyond the maximum radius of the service areas.
Edit the street network to ensure that all source nodes lie on the network.
Save.
Arc: &workspace [type directory]
Arc: &stat 9999
The
shapefiles containing the street network and source node locations must be
converted to ArcInfo coverages.
Arc: shapearc [station shp
name] CITY_lrs]
Creates a
coverage of the light rail stations.
**Arc: shapearc [street shp name] CITY_sts
Creates a
coverage of the streets.
Arc: linegrid CITY_sts
CITY_sts_g
Cell Size: 50
Convert the Entire
Coverage (Y/N): Y
Background Value: ZERO
Creates a 50
foot grid with positive numeric values for all street cells and zero values for
all non-street cells.
Arc: grid
Grid: setcell 50
Sets the cell
size to 50 so the pointgrid will match the linegrid.
Grid: CITY_source =
pointgrid(CITY_lrs, CITY_lrs#)
Creates a 50
foot grid with positive numeric values for all light rail station cells and
nodata values for all other cells.
A grid
containing the proper off-network impedance for all cells will be required
later. For this example, the service
area will be distance bounded, so the off-network impedance should be the same
as the network impedance.
Grid: CITY_flatgrid =
con(CITY_sts_g gt –1, 1, 1)
Creates a grid
of all 1’s.
The
costdistance() command requires that all source nodes must lie on the
network. The following steps will
ensure that all source nodes do.
Grid: CITY_mult =
CITY_source * CITY_sts_g
Grid: CITY_check =
con(CITY_mult gt 0, 1, 0)
Converts the
CITY_mult grid so that all station cells that are on the network have a value
of one and all station cells that are not on the network are zero. Nodata cells remain nodata.
Check in ArcView using Spatial Analyst Extension
Add CITY_check theme (grid data source)
Check theme table
If there are any cells with a value of 0, some stations are not on the network
If some
stations are 0, quit Grid command (i.e. Arc:
q)
Arc: gridpoint CITY_check
CITY_check_p check
Option: change color to find exact stations
For those stations, DRAW line from station to a street (save edits)
Go back to ** and redo the checklist
Grid: CITY_cost =
con(CITY_sts_g gt 0, 1, 100000)
Converts
CITY_sts_g to a format suitable for using as a cost file in for the
costdistance command. Creates a grid
with ‘1’ values for all street cells and ‘100000’ values for all other cells.
Step 2:
Calculate the Lowest Network Travel Cost
Grid: CITY_dist =
costdistance(CITY_source, CITY_cost, #, CITY_alloc)
Finds the distance from each cell to the nearest
station. The CITY_dist output is a grid
with the distance to the nearest station for all network cells and exaggerated
values for all off-network cells. The
CITY_alloc output is a grid with the number of the nearest station for all
cells. Note that this command can be
exceedingly time consuming, especially with large grids.
Step
3: Create a New Set of Source Cells Consisting of All Network Cells.
In Step 4,
the costdistance() command will be run again to calculate the distance from
each off-network cell to the nearest network cell. Doing this will require transforming the CITY_dist grid into a
form suitable for use as a source grid for costdistance.
Grid: CITY_dist_nd =
con(CITY_dist gt 10000, setnull(0 gt 1), CITY_dist)
Reclassifies
all of the off-network cells (which have very high values thanks to the
artificially high off network impedance) as no data and leaves the values of
all network cells the same. The cut off
point should be greater than the maximum radius of the service area and less
than half the off-network impedance used to make the CITY_cost grid. In this example a value of 10000 is chosen
because it is higher than the maximum service area radius (2651) but less than
half the artificial off-network impedance (50000).
Grid: CITY_dist_int =
int(CITY_dist_nd)
Converts the
values in the CITY_dist file to integers so they can be used as sources in a
costdistance calculation.
Step
4: Calculate the Off-Network Travel Costs
Grid: CITY_2rd_dist =
costdistance( CITY_dist_int, CITY_flatgrid, #, CITY_rd_dist)
Finds the
distance from every non-street cell to the closest street cell. The CITY_2rd_dist output is a grid with the
distance to the nearest street cell in all cells. The CITY_rd_dist output is a grid with the distance from the
nearest road cell to the nearest station for all cells. As with the first costdistance command, this
can take quite a while to run.
Step
5: Calculate the Total Travel Cost
Grid: CITY_sumdist =
CITY_2rd_dist + CITY_rd_dist
Step
6: Trim and Reclassify the Total Travel Cost Grid
Grid: CITY_mask =
reclass(CITY_sumdist, remap, nodata)
Ensure that there’s a copy of remap file in the directory for the city {file called remap}; if not there, copy same file from other city
The remap file
should reclassify all distances between 0 and the desired service area radius
as 1. All other cells will be classed
as nodata.
Step 7: Create
Final Buffers
Grid: CITY_fin_buff =
CITY_mask * CITY_alloc
Creates a grid
with the number of the nearest station for all cells within 2651 feet of a
station and a value of nodata for all other cells.
TO
CLEAN UP HOLES IN GRID:
The resulting
grid may have rather ragged edges. The
following set of commands will smooth the service area boundaries and convert
the service areas into polygons.
Grid: CITY_cleaned =
boundaryclean(CITY_fin_buff)
Grid: CITY_polygon =
gridpoly(CITY_cleaned)
Grid: q
TO DELETE
HANGING CHADS
While the
boundaryclean() command should smooth out the edges of the service areas, there
still may be some ‘hanging chads,’ small one grid cell polygons that are
hanging off the edge of the service area.
The following commands will locate and delete these extraneous polygons.
Arc: ae
Arcedit: ec CITY_polygon
Arcedit: ef poly
Arcedit: de poly
Arcedit: draw
Arcedit: sel all
Arcedit: sel area = 2500
If selection area is 0, skip final steps, just q
The value used
to select should be the area of a single grid cell. In this case, the cells are 50x50, so the area is 2500.
Arcedit: delete
Arcedit: save
Arcedit: q
If the
analysis of the service area polygons is to be performed in some software other
than ArcInfo, the following commands are required to make the coverage
accessible to ArcInfo.
Arc: build CITY_polygon
Arc: export CITY_polygon
Open the
coverage in ArcView and convert it to a shapefile.
TO
MANUALLY REMOVE DONUT HOLES:
Because of the nature of ArcInfo coverages, any ‘donut holes’ within a service area will actually be represented as separate polygons. These polygons are generally undesirable when conducting analysis of the service areas. To get rid of them, delete all polygons which have a Grid Code attribute of –9999.