gw_derive_setback

Deriving Setback areas from cadastral data.

Each cadastral polygon has a fixed front, side and rear building line setback. These distances from the parcel boundaries are established by the city authorities and no structures can be built within those distances from the parcel boundaries.

In the Calculating the street frontages of cadastral polygons article we discussed a method to define the street frontages of the parcels. In this article we will use the results and some additional procedures to derive the Setback Areas.

We are going to use the original cadastral polygons as well as two derived datasets:

Street front lines
The rest of the parcel boundaries

Original parcels dataset

Street fronts derived here

Other parcel boundaries derived here

All parcel boundaries symbolized by the setback distance

STEP1 : Using the datasets (street fronts and other parcel boundaries) derived as described in Calculating the street frontages of cadastral polygons article. we will assign to each polyline the setback distance defined by the city authorities.

We have a dataset with all the street fronts. We just need to
- add a new field (Buffer_Distance for example)
- calculate all the values to the legal front setback distance
The rest of the parcel boundaries will have two different setback distances - one for the side setbacks and one for the rear setbacks. We need to
- add the new field (Buffer_Distance for example) to this dataset as well.
- Select all the "Side" boundaries of each parcel. These can be defined as the boundaries that intersect the street front lines. Therefore we can select them by using the standard "Select Layer By Location" tool to select all the polylines from the input dataset that intersect the street fronts dataset.
- Now we can calculate the value of the "Buffer_Distance" field for the selection to the legal "Side" setback distance
- The non selected at this stage polylines are the "Rear" boundaries. We can switch the selection using the standard "Select Layer By Attribute" tool. This will unselect the "Side" boundaries and will select the "Rear" boundaries
- Now we can calculate the value of the "Buffer_Distance" field for the selection to the legal "Rear" setback distance
Now we have the legal distances calculated for each polyline. We just need to merge all parcel boundaries together using the Merge tool available in the ToolBox implementation of ET GeoWizards.

All of the above can be modeled in the Model builder. See a sample model here

Buffered parcel boundaries

Buffers clipped with original cadastre polygons

STEP2: Deriving Setback polygons. Two simple steps needed:

Buffer the parcel boundaries using the standard Buffer tool with the FIELD option and the values in the "Buffer_Distance" field calculated in STEP1
Since we are interested only in the areas inside the cadastral polygons we can clip the buffered polylines derived above with the original cadastral polygons. The standard Clip function used

At this stage we have all the setback area defined. The problem is that we have this area for all parcels. We have to continue in order to be able to identify the individual Setback area for each cadastral polygon.

Setback boundaries merged with internal parcel boundaries

Parcels split to Building areas and Setback areas.

STEP 3: Defining the separate Setback areas for each polygon.

For this step we'll employ a techniques described in detail here. Three sub-steps are involved:

Using the Polygon To Polyline function to get the boundaries of the clipped buffers (derived in STEP 2) as polylines.
Merge the buffer boundaries with the internal boundaries of the cadastral polygons
Use the Build Polygons function on the merged dataset to get each parcel polygon split in two:
- Setback area
- Building area
IMPORTANT NOTE: The Clean Polylines option of the function should be used to ensure that topological relations between the polylines before starting the building polygons process.

A sample model of steps 2 and 3 can be viewed here.

We are almost there. This dataset however does not have any attributes.

Label points.

Final Polygons.

Final polygons labeled with the parcel number.

STEP 4: We need to get two important values:

Is a polygon a "Setback" or not.
The number of each original cadastral polygon needs to be assigned to the both polygons:
- Setback polygon
- Building Area polygon

To achieve this we need to go through the following sub-steps:

Add a new field (for example "Setback") to the Clipped Buffers derived in STEP 2 above.
Calculate the value in the new field for all records to a new value (for example [Setback] = "Yes" )
Use the Polygon To Point function with the "Label" option on the split polygons derived in STEP 3 above
Use Spatial Join to join the Clipped Buffers polygons to the label point. In this fashion all points that represent setback polygons will get value "Setback" = 'Yes'. In other words we'll be able to distinguish the Setback polygons from the Building polygons
Use Spatial Join to join the original cadastral polygons to the label points. This will add all original attributes of the parcel polygons to the label points
Use Spatial Join to join the label points back to the Split polygons. This will transfer all the attributes collected in the label points to the Split polygons.

As a result we will have a dataset that will represent the parcels. Each parcels will have two separate polygons:

Setback polygon
Building Area polygon

A sample model of STEP 4 can be viewed here.

Sample Model of the process. The model uses the three sub-models:

Sample model of the entire process.

See ET GeoWizards User Guide for more information

For any comments and enquiries contact: webmaster@ian-ko.com
All ESRI products mentioned are trademarks of Environmental Systems Research Institute, Inc.
Copyright: Ianko Tchoukanski