Isochrone along direction map of watershed

[GiuseppeCipolla]

Hi everybody,

I am trying to generate a isochrone map starting from a map of the times to cross a single cell and a flow direction map. Basically, the final map has to provide, for each cell, the total time to reach the outlet of the watershed, which will be given by the sum of all the values in the cost map (i.e., the time map to cross a single cell) of the cells falling within the flow path provided by the flow direction.

To extract the path from a point to the outlet i can use TraceDownslopeFlowPath and then through RasterCalculator I can sum all the values of my cost map of the cells along the path. However, this procedure should be repeated for all the cells of my raster.

Is there a procedure to follow or a tool that i can use?

Best.

Giuseppe

[jblindsay]

This sounds like a problem for the CostDistance tool, using your outlet and cost rasters as the inputs. In this case, the output Cost Accumulation raster will provide you with your isochrones. Reclassify this surface if you wish, or use the Contour tool to create true isochrones.

[GiuseppeCipolla]

I agree with you in the sense that CostDistance tool may be a great help. However, to compute the cumulated cost, i think it follows the path's directions driven by minimum costs. In my specific case, I need to follow the paths given by the flow direction map generated with the D8Pointer. Indeed, giving as input to the the CostDistance the outlet raster as a source file and the cost raster, the resulting "Output Backlink File" is different from the D8 Pointer flow direction map.
Is there a way to specify the flow direction as an additional input?

[jblindsay]

You could probably make this work using the D8MassFlux tool. In that case, input your DEM and set the Loadings file as your cost raster. You'd need to have dummy efficiency and absorption files filled with constant values.

[GiuseppeCipolla]

Thank you so much for your reply. I tried as you suggested setting the absorption and the efficiency as constant rasters with 0 and 1 values, respectively. The result is a correct cumulated flux downstream, meaning that in the outlet cell i have the sum of all the cells within the costs map, as the same principle according to which the flow accumulation map is generated.

However, i Just Need the accumulation to be done in an upstream mode, so that in correspondence of the most distant from the outlet point (according to the D8), i have the time of concentration of the basin, thus the sum of times of all the cells of the coat map crossed to reach the outlet (not the sum of all the costs map like in the downstream mode). Is there a way to achieve this?

Thanks and best regards.