In land planning there are two property based layers I work with: zoning and parcel. Zoning provides information on allowable use (such as low or high density residential, commercial, industrial etc) while parcel data provides information on property tenure. They are contained in separate layers and sometimes I need to add the zoning information to the parcels. There are a number of methods of joining the attributes, each with their own drawbacks depending on the particularities of the datasets. Below I summarize the challenge of achieving this and suggest a solution.
At the core of the problem, zoning and parcel lines to not line up. Zoning tends to consist of large polygons, while parcels are generally smaller in size. While zoning is initially created based on parcel lines, overtime parcel lines are adjusted as property mapping is made more accurate from the incorporation of more precise land surveys. This creates a small mismatch between property and zoning boundaries. Furthermore, zoning does not always follow property lines. Zoning will sometimes extend a certain distance from the centreline of a road or watercourse resulting in some parcels that have more than one zone.
There are three methods I can think of to assign zoning attributes to parcels (1) union, (2) spatial join based on parcel centroid and (3) tabulate area. The union method provides exact overlay of the zoning on the parcel layer. The benefit of the union method is that it shows where there is more than one zone on a parcel. However, the union splits the parcels into multiple pieces with the same assessment data so care needs to be taken that the attributes are not counted twice when generating statistics for the assessment data by zone. The largest drawback of the union is that it leaves many slivers where the zoning and parcel data do not align perfectly. It is very time consuming to remove the slivers.
The second method requires the conversion of the parcel polygons to points/centroids and then performing a spatial join to assign a zone to each parcel. This methods provides a single zone for each parcel so some zoning information will be lost. On the other hand this method will not result in any anomalous small silvers that need to be considered. This is a fairly quick method that is useful if accuracy isn’t an issue.
The third method makes use of the tabulate area tool to determine the percent area of a parcels that is made up of the different zones. This method is preferable to the others as it doesn’t split the parcels resulting in the slivers that need to be edited out, and it gives information on the different types of zones on a parcel. It does require some processing to then code the data, but overall it strikes a ballence by being accurate and not too time consuming.