# ArcGIS 3D Analyst Slope Tool



## PaintPeelinPbody (Feb 3, 2004)

Anyone have any experience with this? 

I used it to determine slope percentages of the property I'm building a trail on. Apparently IMBA has used this before as well, but I'd like to bounce some questions off someone who knows more about the process.


----------



## thumpduster (Nov 19, 2008)

I've got a little bit of experience with this but am no expert. Post or shoot me a PM with your questions and I'll see if I can help.


----------



## PaintPeelinPbody (Feb 3, 2004)

What is the accuracy of using contours to produce a raster via SlopeTool? 

I'd basically like to avoid using a clinometer for anything other than checking if the GIS tool is accurate.


----------



## Harold (Dec 23, 2003)

PaintPeelinPbody said:


> What is the accuracy of using contours to produce a raster via SlopeTool?
> 
> I'd basically like to avoid using a clinometer for anything other than checking if the GIS tool is accurate.


That's going to depend on where the contours came from and what the contour interval is. I've only used this with a DEM, which typically has a 30m resolution (I believe 10m resolution is beginning to become available, but not cheaply). A bit of fine detail in the topography is lost there, so you'll still have to ground check for at least short lengths of trail (especially ones that may be on the borderline of the slope criteria cutoff for the standards you are using). However, this process should give you a pretty good idea for longer, sustained slopes.

At this point, I'd primarily use it for helping to streamline the process.

If you have the processing power to crunch 1ft contours out of a DEM (LOTS of extrapolation, so LOTS of processing power is necessary), you'll be able to get some better numbers out of the tool.

It would be very interesting to see if a good Python or ArcObjects programmer could put together an Extension for 3D Analyst that could take a set of criteria (half rule, maximum local slope, maximum sustained slope, maximum distance between grade reversals, positive and negative control points, etc) and spit out some suggested trail routes.


----------



## thumpduster (Nov 19, 2008)

The biggest factor is the accuracy of your contours. Where did you get your contours from? Did you create them from a DEM? If so, what was your DEM source and resolution (raster cell size)?

How large of an area are we talking about? A few acres, hundreds of acres, thousands of acres?

Realize that anytime a primary data source (DEM or TIN) is used to create contours you will have a reduction in both accuracy and precision. To turn around and take a contour to create a DEM raster only further degrades the data quality. Your best bet is to find the highest resolution raw DEM for your parcel you can find (probably 1/3 arcsecond), perform a basic DEM smoothing, and then use that for your slope tool.


----------



## thumpduster (Nov 19, 2008)

NateHawk said:


> It would be very interesting to see if a good Python or ArcObjects programmer could put together an Extension for 3D Analyst that could take a set of criteria (half rule, maximum local slope, maximum sustained slope, maximum distance between grade reversals, positive and negative control points, etc) and spit out some suggested trail routes.


Yeah, that would be very cool. Way outside my skill set!

The problem remains that we're always limited by our original raw data. While the 10 meter (1/3 arcesond) and even some 3m 1/9 arcsecond DEMs are availble from seamless.usgs.gov, the cell sizes keep reducing by a factor of 9 but the actual elevation resolution has not gotten markebly better. Some of those DEM sets are looking at +/- 20%elevation resolution or worse. No matter how much processing power you have GI=GO. Any contour interval below the resolution of your raw DEM is only a calculated guess.

I think where the OP is going with this would be most useful on larger tracts of land, with the usefullness reducing proportionatly as the parcel gets smaller. Nothing can replace boots and brains on the ground, which is more practical the smaller the area you're working in.


----------



## Harold (Dec 23, 2003)

thumpduster said:


> Yeah, that would be very cool. Way outside my skill set!
> 
> The problem remains that we're always limited by our original raw data. While the 10 meter (1/3 arcesond) and even some 3m 1/9 arcsecond DEMs are availble from seamless.usgs.gov, the cell sizes keep reducing by a factor of 9 but the actual elevation resolution has not gotten markebly better. Some of those DEM sets are looking at +/- 20%elevation resolution or worse. No matter how much processing power you have GI=GO. Any contour interval below the resolution of your raw DEM is only a calculated guess.
> 
> I think where the OP is going with this would be most useful on larger tracts of land, with the usefullness reducing proportionatly as the parcel gets smaller. Nothing can replace boots and brains on the ground, which is more practical the smaller the area you're working in.


To be honest, it's not like we're wanting to publish any of this sort of thing. I'd take calculated guesses, personally. However, the usefulness of those calculated guesses is going to depend on the topography of your area. If the slopes are generally pretty consistent and sustained, calculated guesses are more likely to be worthwhile. However, if your terrain is pretty flat and there's a lot of space between contours, you're probably more likely to get a bunk output from the calculated guessing.

It is all just to help guide the workers on the ground after all. While that would just be great if a computer could entirely calculate an entire trail network so that a GPS-enabled SWECO could operate remotely and build trail for us, that sort of capability is a long way off. But sometimes, it would be nice to get the computer to highlight areas that we could use as control points for routing a trail. I think in this case, it's doable keeping in mind the limitations of the original data.



> While the 10 meter (1/3 arcesond) and even some 3m 1/9 arcsecond DEMs are availble from seamless.usgs.gov, the cell sizes keep reducing by a factor of 9 but the actual elevation resolution has not gotten markebly better. Some of those DEM sets are looking at +/- 20%elevation resolution or worse.


Why do you say this? The higher spatial resolution on the newer DEM datasets is because the data is obtained from more recent satellite imagery from the newer satellites that are capable of higher spatial resolution. Are you commenting more about the method used for calculating the elevation of a particular cell?


----------



## fishbum (Aug 8, 2007)

Actually I believe LIDAR gives you the elevation, not imagery?


----------



## Harold (Dec 23, 2003)

fishbum said:


> Actually I believe LIDAR gives you the elevation, not imagery?


It does. It can also give you canopy height, midstory height, and stuff like that. But I do not believe that the widely available free DEM data has been obtained via LIDAR.

http://eros.usgs.gov/#/Guides/dem



> The majority of the 1-degree DEMs are produced by DMA from cartographic and photographic sources. Elevation data from cartographic sources are collected from any map series 7.5-minutes through 1-degree (1:24,000-scale through 1:250,000-scale). The topographic features (e.g., contours, drain lines, ridge lines, lakes, and spot elevations) are first digitized and then processed into the required matrix form and interval spacing.


LIDAR data would probably be perfect for the task discussed here.


----------



## PaintPeelinPbody (Feb 3, 2004)

I messed with LIDAR a little, but at the time I had access to 3D Analyst I didn't think to make any higher resolution contours than the 5ft contours. 

Also, because I'm not working on a workstation with ArcGIS Extension, messing with this stuff is difficult. I'm lucky to have a slope raster that I can work with (transfered from a school computer).


----------



## Harold (Dec 23, 2003)

PaintPeelinPbody said:


> I messed with LIDAR a little, but at the time I had access to 3D Analyst I didn't think to make any higher resolution contours than the 5ft contours.
> 
> Also, because I'm not working on a workstation with ArcGIS Extension, messing with this stuff is difficult. I'm lucky to have a slope raster that I can work with (transfered from a school computer).


It's a real pain, true.

However, look into slope analysis with QGIS.


----------

