Flow Concentration

[NazmulAhasan]

HI,

I have a raster layer of a roof (imported from Sketchup then converted to raster by Multipatch tool). I want to find out flow concentration on the roof if rain is falling vertically on the roof. Can you tell the process to do this.

Thanks in advance

[XanderBakker · Accepted solution]

It is actually called Flow Accumulation (part of Spatial Analyst). The Help can be found here: ArcGIS Help (10.2, 10.2.1, and 10.2.2)

You should use a cell size small enough to simulate the accumulation. What is the extent of the roof and the cell size you used when converting from the multipatch?

[NazmulAhasan]

I used a larger cell size 0.5 x 0.5 thought it will help to reduce calculation time. But the raster looked coarse. what cell size will be good for this case anyway?

[XanderBakker]

This depends on the detail (precision) you are trying to obtain and the precision of the input data (combined with the extent of your roof). If you provide some more info then I can advise you better...

[NazmulAhasan]

the roof is approx~23 ft in long direction and ~12ft in width.

after all thank's for your effort.

[XanderBakker]

If the roof is that "small" (not an industrial building" or something very large) you could use a resolution that is very high (a cell size much smaller than 0.5m).

I have had some problems in the past with this funcionality when using a cell size smaller than 1 (or using a decimal cell size). At that time I didn't have the time to investigate why and I don't know if the problem is still present in the current version. Since your roof has a small surface, using a small cell size will not resulting in long processing times.

What precision do you need when it comes to the resulting accumulated flow? Interesting case.

[NazmulAhasan]

actually there was no precision boundary. I was asked to produced a 3D plot of flow concentration of the roof. I have this by converting flow accumulation raster to ASCII format than importing to MATLAB asd plotting as surface. just like this.

[JayantaPoddar]

Hi Nazmul,

Try the following

While conversion of Multipatch to Raster, give the output cell size as low as about 0.01 m

Also Go to Environment > XY Resolution and Tolerance

XY Resolution: 0.01 meters

XY Tolerance: 0.001 meters

Then go with flow accumulation as mentioned by Xander Bakker‌

Thanks,

Jay