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  • What is Overlapping Data in a Single Map Layer?

    In relatively rare but important cases, there can be overlapping data in one map layer. Special techniques are needed to understand and identify map features for layers that contain overlapping data. There may be more mapped in the layer than what you "see" on the map. While overlapping map features within a single map layer are not common, they are a necessity for certain layers.

    Examples of overlapping data in a single map layer

    • As an example, consider property improvement plans. One improvement plan might be for streets. Another plan might be for grading over some or all of the street plan area. As years pass, there may be another plan for sewer improvements in the same area or a plan for re-paving of the same streets.

    • While most examples of overlapping data are polygonal or areas, line and point layers can have overlaps too. Bus routes are a good example of a line layer that could have overlapping data. Without overlapping data, a bus route layer could only show streets that have at least one bus route. Without overlapping data, only the bus route street network can be shown. It couldn't show all the different numbered routes that traverse street segments with more than one route such as Speedway Blvd. for example. With overlapping bus routes, the route lines would overlap each other where two or more routes are on the same street segment.

    • Other examples of layers with overlapping data might include wildlife study areas, property rights, and constructions plans.

    A relatively small number of Pima County parcels overlap other parcels. Examples include certain condominiums, buildings with different building floor ownership, and subsurface rights with unique Assessor parcel codes.

    A few overlapping parcels may still be shown with old-style incorrect geometry until we re-draw them with true overlapping geometry. The  old method used "tricks" to show there is more than one parcel in the same area. For instance, If there were two condominium parcels on two floors, the single mapped area was split into two smaller areas to map both. A small parcel area of drilling rights was sometimes shown as a circle within the surrounding parcel. 

    Key Differences With Overlapping Map Features in One Map Layer

    Methods we use for most layers don't work for overlapping map features in one layer because:

    • Individual map features in one layer can't be turned on and off. Either the whole layer with all features is on, or they're all off.

    • Map features that overlap in one layer may be in any order. For map features that overlap in one layer, the overlap order has no meaning. We can't rely on the one we're interested in being "on top".
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