Calculating the surface area of a polygon which has a certain depth of water beneath

Question

Hi there,

I am trying to calculate some statistics for my masters thesis and have hit a brick wall!  I'm not hugely competent with GIS and therefore, please excuse my basic explanation!

I have created a series of polygon shapefiles for sites in the ocean I feel are suitable for the deployment of tidal current turbines.  I have then imported into the GIS a raster bathymetry data set.  I want to work out the surface area of the polygons which have depths of water beneath them that meet a specific requirment.

Let me explain in another way

Polygon 1's surface area = 'x'
What is Polygon's surface area where the depth is over 'y'

Cheers for the help!

Tim

JakubSisak · Answer

I want to work out the surface area of the polygons which have depths of water beneath them that meet a specific requirment.

Assuming I understood correctly; Did you try the Polygon Volume tool? You can control the plane height and calculate stats above or bellow the provided reference plane height. The output file contains 3D surface area.  You would just need to convert raster to TIN.
 "Calculates the volumetric and  surface area between polygons of an input feature class and a terrain dataset or  TIN surface."

DonovanCameron · Answer

This can be done using a statement in the raster calculator using the 'Con' function.

You will need to begin by by converting your lake polygons to rasters, respecting the original cell size of your bathymetry interpolation.
http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#/Polygon_to_Raster/001200000030000000/
http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#//00120000002v000000.htm

Now you can use the Con function to create a new raster where the lakes overlap your required threshold.
http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#//009z00000008000000.htm
http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#//009z00000005000000.htm

Examples of the Con Function:
http://forums.arcgis.com/threads/14493-Raster-Calculator-CON-statement
http://forums.arcgis.com/threads/12826-Conditional-Calculation-of-Three-Rasters
http://forums.arcgis.com/threads/14844-Raster-Calculator-quot-con-quot-statement-issues
http://forums.arcgis.com/threads/15339-raster-calculator-for-conditional-statement-to-compare-two-datasets

Con([LakeGrid] & [DepthGrid] >= 75,1,0)
Results in a grid, where only the depth grid portions deeper than 75 'metres' (or) 'feet' that overlap the lake are coded as 1, if they don't overlap, given 0.
These results can then be exported to a polygon to calculate the geometry in a new field to generate the area.

The above is a grid based method. Your linear results from your polygons boundary will become 'gridded' and can create misleading results.

Another option to keep this vector based, is to generate contours from your bathymetry dataset.
http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#//009z000000ts000000.htm
Then you can select the contour profile line that meets your criteria, eg: Select where Elevation >= 75m and create a seamless polygon from this, by dissolving based on a similar Unique ID (you may have to create a field and code it all to equal 1 then dissolve based on this field).
http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#//00170000005n000000.htm

Once the dissolve is complete, we need to make singlepart features. Start an edit session and select all the features in the dissolved depth polygon and press the Explode button on the advanced editing toolbar. This separates the features into their own record.
http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#//001t0000007p000000.htm

Now you need to identify where these depth polygons overlap your lake polygons, and by how much. This is done with the intersection tool. Your inputs for this tool will be both your Lake depth poly and your lake surface poly. Ranks will also need to be assigned as follows; Lake Poly Rank 5, Depth Poly Rank 1, these ranks will snap the attributes from the lake surface (ie, lake name) while exporting only the overlapping portion of the depth poly.
http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#//00080000000p000000.htm

Open the attribute table for the newly created intersection and create a new integer field, maybe double with an appropriate scale and precision to reflect the units you prefer to use. Right click on the header of this field and select Calculate Geometry, and select your area units you wish to show in the field and press OK.

If you have multiple polygons from the depth poly that intersected the lake poly, these will return multiple measurements of area and will need to be summed. Two ways of doing this. Right click the header in the intersected depth poly that shows which lake it falls on (ie, lake name/code) and select Summarize. In the top options, expand the field that has your Area in it, and put a check beside Sum. This will output a table with the unique id of the lake, and a total for the area of your specific depth of all overlapping depth polygons.

Second method is to dissolve this intersected feature using the unique lake id field, and then generating the area field and calculating the geometry. Either way works.