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Extract Slope Values from DEM Slope layer

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#1
David25

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Hi all,

I will soon have a slope layer generated from a DEM (possibly LiDar, I'm not sure yet), of which I will need to intersect a line route. But, the only values I care about is the slope percentage of 30% and above. Basically I'll need the areas where slope is 30 or greater and then I can measure the routing against that and see how much passes through that slope.

I'm wondering how to do this as I can't perform a query on the table like I could with vector data.

Any ideas how I can extract/attribute the 30% slope?

Thanks,

#2
Laura Miles

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Do you have the Spatial Analyst extension? If so you should be able to use "Extract by Attributes".
Laura

#3
David25

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Do you have the Spatial Analyst extension? If so you should be able to use "Extract by Attributes".
Laura


Thanks for the quick response. My raster does not have an attribute table so I can't perform an SQL query on it. Perhaps I'm missing something as I know what you are saying, just not sure how I go about it.

#4
David25

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Do you have the Spatial Analyst extension? If so you should be able to use "Extract by Attributes".
Laura


Thanks for the quick response. My raster does not have an attribute table so I can't perform an SQL query on it. Perhaps I'm missing something as I know what you are saying, just not sure how I go about it.


Ignore that actually. I see I have to convert to integer first. But then I don't understand the values in my table. I can I extract above 30% slope?

#5
dsl

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If you are using Spatial Analyst, gererate a slope raster from your DEM. I believe you can set it as percent, otherwise it might have to be an angle. Then using raster calculator select your slope raster and do something like this:

[SLOPE] > 30

This creates a new "temporary" raster. You can right click on it and make it permanent.

In your line shapefile you can add an integer attribute with a value of 1 for all features. Convert this to a raster with the same cell size as your dem/slope raster. Use raster calculator to multiply the two together.

Hope that helps,
David

#6
David25

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If you are using Spatial Analyst, gererate a slope raster from your DEM. I believe you can set it as percent, otherwise it might have to be an angle. Then using raster calculator select your slope raster and do something like this:

[SLOPE] > 30

This creates a new "temporary" raster. You can right click on it and make it permanent.

In your line shapefile you can add an integer attribute with a value of 1 for all features. Convert this to a raster with the same cell size as your dem/slope raster. Use raster calculator to multiply the two together.

Hope that helps,
David


Thanks, I'll try that tomorrow. Sounds like it may work though.

#7
David25

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I think I've got it working and here's how:

Take the DEM
Make the Slope using SA
Convert to Integer
Then, Raster To Features conversion
Then, interest my line with this polygon and the new shapefile will have a field called 'Gridcode'. This is my percentages.

I then query on >=30.

This appears to tally with areas that I thought would be steep.




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