Wednesday, November 27, 2013

Finding Lakeview Lots

I was just asked by an appraiser to make a map of a subdivision so that he could determine which lots have a view of a nearby reservoir.  This area was affected by the Rockport 5 Fire in August of 2013, so a re-appraisal is in order to assess the effects on properties in the area.

Rather than just make a map, I thought I could do some view-shed analysis to select the lots that have a view of the reservoir.

Observer Points

I started out by creating a fishnet grid of the reservoir with the label points option checked.  I used the resulting label points as my observer points for the viewshed tool in the 3D Analyst toolbox.  I exported the reservoir to a stand-alone feature class, then ran the fishnet tool using that feature class as my extent template.  On my first attempt I used a 40' x 40' grid, but I found this was too small - creating thousands of observer points.  My next attempt I used 300' x 300' and that resulted in a little over 500 observer points.
Create Fishnet Results
Points and polygons from Create Fishnet tool.
I then had to get rid of the points outside of the reservoir boundaries.  I did this by selecting the points that intersected with the reservoir, then switch the selection and deleted the selected points.

Smaller DEM

I didn't want to do viewshed analysis for the entire DEM, so I used the Extract by Mask tool in the Spatial Analyst - Extract toolbox.  For the mask, I just created a simple rectangle polygon in a stand-alone feature class.



I used the smaller DEM raster and the label points for input to the viewshed tool.  It took several minutes to process the viewshed even with a smaller DEM.  It would probably be faster with fewer observation points.  The result is another raster with two columns of data.  The VALUE field has the number of observer points visible from that cell.  The COUNT field has the number of cells that can also see that many observer points.

I converted the visibility raster to polygon features and deleted the polygons with a GRIDCODE of 0.  Then I selected the properties within the desired subdivisions that intersected the visibility polygons.  Then unselected those that just touched on the edge, or had the majority of the lot that wasn't inside the visibility layer.

Final Map

The final map showed the subdivisions color coded by each of the phases, the locations of existing buildings, the locations of buildings that burned in the fire, the fire perimeter, and the visibility raster.  All overlayed on recent aerial imagery.  I also created a spreadsheet of lots that were considered view lots.

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