question on how to create cost-weighted distance?

Discussion created by amstip on Jun 21, 2010
Latest reply on Jul 1, 2010 by amstip
I would appreciate feedback from a seasoned raster guru on the following problem that I need to resolve for my doctorate research. I set out to calculate food store access based on the distance to food stores from residences using bus routes in NYC. I wanted to do it with Cost-weighted Distance in Spatial Analyst. But when I started to generate grids to input into cost-weighted distance tool using I run into a problem. Here is what I have tried:

I selected raster cell size to be 5x5 feet (size I chose somewhat arbitrarily and set explicitly in Spatial Analyst > Options). I then gave bus route shapefile a new field to use when rasterizing and assumed that it takes a bus 6 about minutes to travel each mile. Thus, it would take 0.0057 minute to cross each 5x5' cell on the bus route. I also assumed that a participant will need to walk to the bus stop from the residence at the speed of 30 about minutes per mile. Thus it will take 0.0284 minute to cross each cell by food until he/she gets to the bus stop. Based on this logic, for bus travel model each bus route pixel should have a value of 0.0057 while the rest of pixels covering Brooklyn should have value 0.0284.

I then converted the 2 shapefiles (bus route and surface of NYC) into rasters and tried to combine them in order to use the values in the Value field as friction.  But when I tried to combine these new grids in Field Calculator I get the added value where bus route is (0.0057 + 0.0284). How do i merge these rasters so that all bus routes have cells with values  0.0057 and the rest of the city grid will have value of 0.0284 Not sure how to proceed??? Also, Do I set cells that fall outside of NYC as NoData?

Thank you in advance,