Cadastral polygons (Parcels, Lots, Erfs, Stands, Plots, etc.) are used in many GIS applications. A common task that is required quite often is to derive the street frontage of the cadastral polygons.
A common feature of the cadastral datasets (almost everywhere in the world) is that the streets (street servitudes) are represented as an empty space.
| Typical
cadastral dataset
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A
typical cadastral dataset:
The logic therefore in achieving the task would be:
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| Polygon
boundaries as polylines
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STEP1: Polygon To Polyline function will create polyline dataset from the polygon boundaries. At this stage each polygon is represented by a single polyline and the polylines overlap on the boundaries between the adjacent polygons. |
| Cleaned
polylines
A Polyline dataset representing duplicates
Clean polylines overlaid with the duplicates
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STEP 2: Clean
Polylines function will do the following:
We need also to have a dataset that contains the duplicate boundaries between the polygons. The Clean Polylines function has an option to create such an output The difference between the dataset created by the Clean Polylines function and the one created by the Polygon To Polyline function is that the new dataset does not have duplicate polylines between the adjacent polygons. The Duplicates dataset has a single polyline indicating each duplication. One can notice from the image on the left, that what actually is missing from this dataset are the street frontages of the cadastral polygons (no data duplication there because the street servitudes are empty spaces). The task from here on gets much easier. We just have to eliminate from the Cleaned polylines those that have had duplicates. The image on the left shows the clean polylines dataset (green) overlaid with the dataset indicating the overlaps (red). All we need to do is to remove the red polylines and we will remain with the street frontages. We need a method to do this without removing some of the street frontages as well. |
| Middle points of the
duplicates.
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STEP 3: Polyline
To Point function has three export options - Vertices, Nodes,
Middle Points. Since we want to create entities that will help
us to select all duplicate lines without selecting any other line we
are going to use the "Middle Point" option.
On the left we can see that each red polyline has a corresponding point. None of the points touches the green polylines. Having the middle points we can just use Spatial Selection to select the polylines from the clean dataset that intersect the middle points and delete them. In order to make the process Model Builder friendly we will make an additional step. |
| Selected
Street Fronts
|
Preparation of the point
dataset:
STEP 4: Spatial Join the middle points to the clean polylines using the "Nearest" option and a very small search tolerance (0.01 for example). STEP 5: Select And Export function will export all the street front polylines to a new dataset. As a Where clause use "Duplicate" <> "Yes" STEP 6: Calculate Length function will calculate the length of the street fronts in units of choice. |
| Street fronts labeled with
their length
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On the left is an image
representing the results. We can continue with:
The above tasks can be achieved with very similar techniques. |
|
Sample Model
of the entire process
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See ET GeoWizards User Guide for more information