Topography Slope Rise and Run Limits

[JimmySimpson]

I am running to a calculation problem that is totally racking my brain. I will try to explain to best that I can. What I have is raster dataset with elevation data for site (topography raster). What I am needing to do is identify areas that are less than 5% slope, between 5% and 12%, and 12% or greater. The compliance document I am following prescribes that development cannot occur on slopes greater than 12%, and special permit is required for slopes between 5% and 12%, so I need to call these out separately, and basically ignore the slopes less than 5%. However, there is a stipulation in the document that the slope requirements only applies when the slopes exist for a run of over 50 feet or if the rise is over 6 feet. This is where I am getting lost. What I have done so far is run the raster slope tool on my topography raster, resampled the raster to convert the slopes to integers, run Raster to Polygons, then dissolved based on the integer slope value, lastly symbolized the polygons into three classes for the 3 slope intervals listed above. So my end results is a polygon feature class with three areas of slope classification (see attached, blue is <5%, green 5-12%, orange >12%). Most of the areas from manually measuring do not meet the rise/run stipulation listed above, and aside from manually going through this and measuring and deleting stuff, I am at a loss of how to apply these requirements to the data. Does anyone have any ideas on methodology of how to apply these two requirements to that data and export a polygon feature class of slopes that only contains what I need? If i can provide any additional information or data that could help, please let me know. Thanks!

[DanPatterson_Retired]

When you stipulate the run and that distance differs from the cell size (ie width) then slope isn't the only thing you are going to have to calculate.

A slope map cannot be used as the sole designator because you can have a cell size of 2 m and you may get an elevation difference of 2 meters within the 6x6 meter window used to calculate slope.  that would be a pretty chunky slope... in the real world, that is a mere inconvenience to the bulldozer.

You can use the slope map as a guide, but you are still going to have to extract information on a site by site basie.

[JimmySimpson]

Not 100% sure I understand what you are saying, but the original topographic raster I have has a cell size of 1 foot x 1 foot, and for all my subsequent raster processing, I kept the same cell size. So based on what I know it is calculating slope for 3'x3' areas. I thought about adjusting the cell size to something larger to facilitate the rise/run stipulations, but couldn't wrap my brain around it enough to decide what to do. What I am thinking at this point is to somehow get GIS to tell me for areas of the same continuous slope category, to measure the entire run (length) of the slope category (any area before it changes to a different category) and measure the change in elevation across that run. That would give me two fields to base my sorting on. But I have no idea how to go about those measurements.

[DanPatterson_Retired]

If you have an area that you need to work in, then running profiles upslope is going to be more informative than any actual slope values.  The slope calculation is localized as you know, but as soon as you start incorporating the 'run' aspect into things to assess the maximum rise, then you need profiles.

Increasing the cell size will just make things worse, your slopes will decrease (where developers and planners differ on the appropriate data scale )

You could calculate on a block basis (ie a 10m * 10m block, for instance) the minimum and maximum elevations from which you could assess the worst case possible slope.  A 2 m rise in a 10 m block, is 20%, but realistically, it is a trivial difference (about 6' in 35').  So the block summary (block statistics, not focal statistics in spatial analyst) would be someplace to get an overall view of the elevational changes and possible rise/run calculations prior to focussing in on slope which is too fine-grained for what you want.