Friday, October 24, 2008

Business Requirements - Parcel Definitions

The term parcel has different meanings to different departments for different purposes by different people, even though they may have a generally accepted definition.

  1. For eample, by looking at APN location on polygons, we see some adjacent polygons with the same APN. So does each polygon represent a parcel, or the adjacent polygons together constitute a parcel? A close examination shows that they are the same parcel with different lot numbers.

  2. There are disjoint polygons with the same APN. Does this mean that the set of such polygons constitute a single parcel, or is each polygon is parcel, in which case we either assume that a polygon does not necessarily have a unique APN, in which case it is a contradiction in terms.

  3. What about polygons that are not associated with APNs but are NULL instead? Should they be treated as a single or separate parcels?

  4. In a two storey building, there are 2 APNs associated with a given polygon. Here we have a polygon with two possible APNs - how do we distinguish one from the other?

Some reasons for these difficulties:
  1. Generally we like to have a single simple polygon represent a parcel with distinct APN. The majority of parcels appear to satisfy this criteria.
  2. When an APN consists of 2 disjoint polygons, it causes difficulty because to treat is as one feature requires that the feature cannot be simple, but is a compound feature which may cause difficulty in analysis.

The following is suggested as a more complete definition:

  1. We have a set of simple polygons that are the "footprints" made up of line features.
  2. Each simple polygons may be associated with 0 or more APNs.
  3. An APN is a number which may be associated with 0 or more polygons.
  4. Small polygons (small with respect to the scale of parcels are looking are) should be eliminated. The size depends on the accuracy and precision of the underlying data.
  5. The polygons for each APN are merged together into one feature. If the polygons touch one another, the result is a simple polygon. Otherwise the result is a compound feature.
  6. Polygons without APNs (i.e. those with Null APNs) should generally be merged together if they touch one another.

Let us consider what the definition of a parcel does for us:

  1. A parcel is a description of real property.
  2. Each parcel has a distinct legal owner or group of owners at any time.
  3. The components of each parcel are collectively considered a single unit of transaction.
  4. A legal owner or group of owners may own one or more parcels.
  5. An APN is a number provided by the County Assessor's office to uniquely define a parcel. Usually it is to identify a parcel on which to assess taxes.
  6. Some parcels do not have tax assessements. These may not have APNs, but there is still an owner, such as the city or the state.
  7. Some polygons, such as those bounded by right of way lines, may be considered parcels, but keeping track of them may not be the appropriate thing to do in the parcel layer.

Parcel Data - Slide 9

In the AutoCad layer show lines as boundaries, the Parcel Boundaries include the following types:
  1. Parcel Line (A line separating two land parcels)
  2. Private Right of Way Line (separating a land parcel from the Private Right of Way)
  3. Public Right of Way Line (separating a land parcel from Public Right of Way)
  4. Air Parcel Line (a line indicating a boundary between an "Air Parcel" and a land parcel or between two "Air Parcels")
  5. Parcel Split Lines (a Line that splits a parcel into two separate parts, each with a different APN but owned by the same owner, and where the parcels go together)

APN Locations include:

  1. APN Number
  2. Position of the APN, usually at or near the centroid of the bounding perimeter of the polygon

Wednesday, October 15, 2008

Application of Variable Buffer Radius

An example of how the Variable Buffer Radius feature in Geomedia can be applied to visualize data on a map. This was a poster presentation in the GeoSpatial World 2005 conference.

Additional descriptions will be added as time permits. Please add to comments section if you would like to see it sooner rather than later.

Cleaning up layers in a City BaseMap

The description is an expansion of a presentation given at the GeoSpatial 2005 conference held in San Francisco in April 2005. A slideshow of the presentation follows:

Additional descriptions will be added as time permits. Please add to comments section if you would like to see it sooner rather than later.

Thursday, September 25, 2008

Creating a Feature Class of Pairs of Intersecting streets .. Part 2 Limitations

The results are the points at which streets with different names connect to one another. In most cases, this satisfies the obvious meaning of the term cross street. But it could give rise to some strange special cases.

1) Does a T - junction constitute a cross street?
2) Does the point at which a minor street merges into a major street constitute a cross street?
3) Does a single street which changes names constitute a cross street? What if the difference is only the direction, but the name remains the same?
3a) Does a street that takes a sharp bend constitute a cross street?
4) Does a traffic circle with multiple spokes on it constitute a cross street? If so, is it the center point of the circle, or is each junction of the spoke intersecting the circumference of the circle a separate cross street? What is the name of the circumference street of the circle? If it does have a formal name, how do you distinguish the two points generated when a street enters and leaves the circle?
5) How do we handle loops when the street loops upon itself? The above procedure returns as a cross street the point where a street changes name with no other street crossing it. This requires us to have a more formal discussion of the term cross street.
6) How do we define the beginning or ending points of a street? One way is to call the beginning point as the point at which the street number is lowest, and the ending point at which the street number is highest. Another would be to avoid the problem completely by not distinguishing the ends (like a piece of rope that has two ends and no beginnings).
7) What if one road goes over another road (e.g. as at an overpass)? This could show up as a cross street, but would definitely would not meet the vernacular definition.