topic Create a frequency plot for raster elevation values with in a polygon in ArcGIS Pro Questions

Create a frequency plot for raster elevation values with in a polygon

DavisTolman — Thu, 05 Oct 2017 15:45:51 GMT

I have a lidar DEM model of the Houston area floodplain with polygons representing areas protected by levees. I would like analyze the elevation within each levee by sampling the elevation values within the polygons and representing them as a histogram or frequency plot. Ideally, the x axis would be binned elevation and the y axis would be the frequency (count).

I've tried the Sample Tool from the spatial analyst toolset, but instead of obtaining the elevation values, I seem to be getting a number between 1 and 256 (...the display value?)

Re: Create a frequency plot for raster elevation values with in a polygon

DanPatterson_Retired — Thu, 05 Oct 2017 15:51:52 GMT

are you sure it was the dem values that were being sampled and not a reclassed version of it?

Check... this link ...to see the format of the table you should have received

Re: Create a frequency plot for raster elevation values with in a polygon

DavisTolman — Thu, 05 Oct 2017 20:15:07 GMT

Thanks for responding. Actually I did try Sample.... but but I'm wanting to compile All of the elevation points within a polygon. What I'm trying to do is to represent the dem elevation values within a polygon in 2-dimensions. Sample is fine for points but I'm looking to represent area in an "elevation histogram" or frequency plot. Any suggestions on what tool to use for this?

Re: Create a frequency plot for raster elevation values with in a polygon

DanPatterson_Retired — Thu, 05 Oct 2017 20:22:08 GMT

oh.. I normally extract the area of the raster that I am interested in, convert it to a numpy array (RasterToNumPyArray in arcpy.da module), then use python/numpy and matplotlib to do graphing. Their graphing tools and handling of raster data for graphing purposes is much easier to use (example of producing a histogram here )

Re: Create a frequency plot for raster elevation values with in a polygon

DavisTolman — Thu, 05 Oct 2017 21:46:32 GMT

Thanks, Python and Matlab are both tools that I would like to learn someday.

Re: Create a frequency plot for raster elevation values with in a polygon

XanderBakker — Thu, 05 Oct 2017 22:23:03 GMT

Could you include a sreenshot of the raster in the TOC? If the data is stored in a Mosaic dataset and if it has a certain renderer defined by default (raster function) you may be looking at the values of the 1st band of the visualization and not the actual elevation values.

Re: Create a frequency plot for raster elevation values with in a polygon

DavisTolman — Thu, 05 Oct 2017 22:27:31 GMT

Is this helpful?

Re: Create a frequency plot for raster elevation values with in a polygon

XanderBakker — Thu, 05 Oct 2017 22:31:32 GMT

Yes it is, but can you expand the "Statistics"part?

Re: Create a frequency plot for raster elevation values with in a polygon

DavisTolman — Thu, 05 Oct 2017 23:27:33 GMT

No statistics have been calculated... do I need to do this?

Re: Create a frequency plot for raster elevation values with in a polygon

DanPatterson_Retired — Thu, 05 Oct 2017 23:55:05 GMT

if you want to see the statistics, you can either calculate them when you bring them in or afterwards using

http://desktop.arcgis.com/en/arcmap/latest/tools/data-management-toolbox/build-pyramids-and-statistics.htm

Re: Create a frequency plot for raster elevation values with in a polygon

DavisTolman — Fri, 06 Oct 2017 02:05:57 GMT

Thanks, I'll calculate statistics and retry.

Re: Create a frequency plot for raster elevation values with in a polygon

DavisTolman — Fri, 06 Oct 2017 16:31:59 GMT

I've tried calculating statistics for this DEM, but the layer properties still says that no statistics have been calculated. Is there something wrong with my raster or am I missing something else.

Below is what my Zonal Histogram table looks like. Rather than showing the rows as feet (elevation), it shows a objectid column instead. I'm pretty sure that the values within the polygons represent the elevation distribution for that polygon, but the units are not apparent.