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# DEM/TOPAZ accuracy

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Dear all,

i am modelling an area about 200 million square miles. there are some roads (its width is about 70 miles, and more than 200 miles of length) and some channels.

i am running the TOPAZ calculation. but, i think i can't get the flow accumulation placed in the channel.

the flow accumulation delineated accross the channel (different with what i predicted/ see in the site).

was that because the width of the channel is very small relative to the total area?

is there any suggestion how to simulate the channel, since i have a lot of channel in the area.

a lot of Basin too.

thank you very much for the attention.

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Dear all,

i am modelling an area about 200 million square miles. there are some roads (its width is about 70 miles, and more than 200 miles of length) and some channels.

i am running the TOPAZ calculation. but, i think i can't get the flow accumulation placed in the channel.

the flow accumulation delineated accross the channel (different with what i predicted/ see in the site).

was that because the width of the channel is very small relative to the total area?

is there any suggestion how to simulate the channel, since i have a lot of channel in the area.

a lot of Basin too.

thank you very much for the attention.

The surface area of the earth is 197 million square miles, so I'm guessing you have your units mixed up. Could the area be 200 million square feet? Also, what's your DEM resolution?

Clark

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• 2 weeks later...

Dear Clark,

Yes you were right. i have my unit mixed up. The area that we are simulating is about 100 square miles.

We delineates the flow accumulation using TOPAZ in DEM module. In the area, we have many channels and roads in it. The road is 30 meters wide, and the channel is about 4-6 meters wide.

The problem is while we run the TOPAZ calculation, sometimes we found that the flow accumulation were delineated across both the roads and the channel. (it supposed to be inclined both with the road and the channel).

What is the DEM cell size that u recommend?

What is the unit for cell width and height in the window as shown in the figure?

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Dear Clark,

Yes you were right. i have my unit mixed up. The area that we are simulating is about 100 square miles.

We delineates the flow accumulation using TOPAZ in DEM module. In the area, we have many channels and roads in it. The road is 30 meters wide, and the channel is about 4-6 meters wide.

The problem is while we run the TOPAZ calculation, sometimes we found that the flow accumulation were delineated across both the roads and the channel. (it supposed to be inclined both with the road and the channel).

What is the DEM cell size that u recommend?

What is the unit for cell width and height in the window?

my DEM resolution was 10 x 10 (cell width = 20 and cell cell height = 20).

thank you very much

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Ida,

The DEM cell size has some impact on where the flow accumulation goes, but you might not be able to get all the roads, channels, and other features to show up on your DEM if you're modeling a watershed that's 100 square miles. If your DEM cell size is 10 meters, it might or might not have the channels and road embankment elevations on your DEM.

I'd recommend going through some tutorials that explain how to handle road embankments or channels that are not represented on your DEM. Here are a couple of links to some tutorials:

Regards,

Chris

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• 2 months later...

yea,

i've tried a different dem cell size (much smaller), and i got it.

thank you very much

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