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Service area analysis - facilities are polygons

SalmanAhmed

Hi,

I want to do a simple 500 meters service area analysis in network analyst. But my facilities are not points but polygons (green parks). I don't want to use making the centroid of each polygon as the facility method because I think the service polygons resulting from that will be somewhat inaccurate especially for larger parks. Is there any other idea to do this?

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CharlotteJ

How do you go about doing a "Select by Location" to select all junctions within a certain distance of the park boundaries? Would this be done through the "Select layer by location" tool? If so then this doesn't seem to allow me enter the network junctions as the input feature layer, as it doesn't allow you to load data from geodatabases, only files. So as the network junctions feature class is within a geodatabase, and I can't seem to copy it into a file to select it from here instead, I'm not sure how to work round this

SalmanAhmed

Yes you are correct. Thank you. I guess at the moment, I would just buffer the park shapes altogether and compensate for loss of accuracy here at some other point in my work.

MelindaMorang

No, this isn't going to work. When you add a location to an NA layer (for instance, you add your park centroid as a Facility in a Service Area layer), the location "locates" on the closest network edge. Although the geographical location of the point remains the same (the center of the park), the network analysis uses the network location of the point when considering how to route to it. So if you erase all the network edges inside the park, the centroid will snap to the closest remaining edge, and the service area will build out from there. If the park is large and has many entrances, this will be pretty inaccurate. It would be more accurate to place a point at every park entrance and calculate service areas around all these points.

SalmanAhmed

I just got another idea. What about using the erase tool to erase parks from the street network. Then the streets will start only from outside the park polygons. Then the resultant service layer using park centroids would be accurate. What do you guys think?

MelindaMorang

That is a possibility, so you just need to experiment a little with the buffer distance until you get what you're looking for. Or if you have one particularly problematic park, manually edit in some entrance points.

MelindaMorang

That doesn't always work well, particularly if your park is very large and rectangular. Then it will have only 4 vertices at the corners.

LukaszRutkowski1

What about using the vertices of the park areas? (Polygon to Line, Feature to Point)

SalmanAhmed

Thanks. But like I mentioned above in this case also there is always a possibility of not all parks intersecting the streets and therefore being ignored in the analysis.

SalmanAhmed

But then there will always be a possibility of some parks not finding any junction within that little distance and therefore being ignored in the analysis. I would rather take the centroid approach and get approximate distances than totally ignore some parks.

FreddieGibson

The only thing that I could add to Melinda's suggestion is that you could automate this workflow with python or modelbuilder. What I'd do is create points for where the park boundaries intersect the edges of the network dataset. This could be done using something as simple as converting your polygons to lines and processing them against the intersect tool to create points or more advanced like using the geometry objects in python to determine to intersecting points. Once you have the intersecting points you could use them as facilities to build your service area. At the end you'd need to merge all of the service areas together to display the result as if you were using the polygon, but you'd also need to handle the overlap between each of the larger impedance values.

MelindaMorang

Do you have your street data (ie, you're not working in ArcGIS Online)? If so, try this:

- Add the network dataset's junction feature class to the map. This is the automatically-generated feature class of network junctions (like intersections) that lives in the same feature dataset as the network.

- Do a Select by Location and select all junctions within some very small distance of the park boundaries.

- Export the selected junctions to a new feature class. These selected junctions can serve as a sort of proxy for park entrances. You can use them as input for your network analysis.

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