Topo to Slope to Polygon

[KaitlynBurgess]

Hi I am new to spatial analyst (I have ArcMap 10.1) and raster files 😕  I'm trying to create a percent slope polygon. I have a polyline shapefile of contours with an elevation field. I'm not sure what is the more accurate way to convert that to a Raster, (Topo to Raster, Feature to Raster, or Polyline to Raster) or if it even matters? The results vary.

Once I have that Raster file I am able to calculate slope easily enough using the Spatial Analyst --> Surface-->Slope tool although then I cannot convert this Raster slope to a Polygon (Conversion Tools --> Raster to Polygon).

Thanks! Kait

[XanderBakker · Accepted solution]

Hi I am new to spatial analyst (I have ArcMap 10.1) and raster files 😕  I'm trying to create a percent slope polygon. I have a polyline shapefile of contours with an elevation field. I'm not sure what is the more accurate way to convert that to a Raster, (Topo to Raster, Feature to Raster, or Polyline to Raster) or if it even matters? The results vary.

Once I have that Raster file I am able to calculate slope easily enough using the Spatial Analyst --> Surface-->Slope tool although then I cannot convert this Raster slope to a Polygon (Conversion Tools --> Raster to Polygon).

Thanks! Kait

Hi Kait,

With respect of translating your contourlines to raster; the only correct way to obtain a DEM (Digital Elevatiuon Model) is to interpolate the data. Feature to raster and polyline to raster will not do that. These tools will simply assign a value of the contour to the cell, but in flat areas (where there will be less or no contour lines) this will result in NoData. So use Topo to Raster only for this purpose.

You could read the Help topic on "How Topo to Raster works". It states:

The Topo to Raster tool is an interpolation method specifically designed for the creation of hydrologically correct digital elevation models (DEMs)

I see you were able to create the slope (the method you describe is correct), but were not able to translate the raster (floating grid) to polygons. The reason is that a raster with decimal value (floating) cannot be translated to polygons. Only Integer raster are allowed in this process. Normally the raster is first classified and then converted to polygons. See the topic on "Reclass by ranges of values" which uses the tool "Reclassify (Spatial Analyst)". The result is an integer raster which can be converted to polygons.

In case you don't want to classify the data, you could convert it to integer (it will truncate the values) and then convert to polygons. If you can explain what you wish to obtain, I'm sure we can recommend which way to go.

Kind regards,

Xander

[KaitlynBurgess]

Thanks Xander!

That's exactly what I needed to do. I'm intersecting the slopes ranging between 15 and 25% with erodible soils. So I used the Int tool, then converted to polygon. I tried Reclassify but since I'm running an intersect after, I want the slope polygons to remain as individual cells not larger classified polygons. So after I pulled out the intersected polygons I classified them within symbology.  Thanks again! Kait