Cost-distance function

UlisesHuerta

Hello GIS community,

I have developed an accessibility surface to health facilities using the cost-distance function in ArcMap 10 (ArcInfo).
I created 1) a target raster map containing the locations of health facilities, and 2) a friction-surface raster where each pixel contains the estimated time (in seconds) required to cross that pixel (based on different travel speeds according to different land classes that include roads). The resolution of my rasters is 30m by 30m.??? Using the cost-distance function, the friction surface and raster of target locations were then used to compute the travel time from each pixel in the study area to the nearest target location.

My problem is that I am getting really weird values in the outcome raster. The raster displays nicely and accurate in terms of showing accessibility. Higher accessibility can be seen through road networks evident in the raster. However, the values range from 0 - 32, 443, 300 as it can be seen on my attachment.

Can anyone shed some light on the values?

Thank you in advance

Ulises



[ATTACH=CONFIG]17241[/ATTACH][ATTACH=CONFIG]17241[/ATTACH]

Screen Shot 2012-08-26 at 4.50.11 PM.png

JoeNunn_0-1622552111841.png

XanderBakker

Hi Ulises,

I notice that you have a friction-surface that contains the estimated time to cross a pixel:

2) a friction-surface raster where each pixel  contains the estimated time (in seconds) required to cross that pixel  (based on different travel speeds according to different land classes  that include roads). The resolution of my rasters is 30m by 30m.

You should however express the friction in time per unit (sec/m) in your case. See the Help section:
http://resources.arcgis.com/en/help/main/10.1/index.html#/How_the_cost_distance_tools_work/009z00000025000000/

The cost assigned to each cell represents the cost per unit distance for  moving through the cell. The final  value  per cell is the cell size  multiplied by the cost value.  For example, if the cost raster has a  cell size of 30, and a particular cell has a cost value of 10, the final  cost of that cell is 300 units.

To correct this, simply divide your friction raster by the pixelsize (30) and try again.

Good luck!