Download Chapter 3 – Base Map Creation and

Chapter 3 – Base Map Creation and Considerations
In This Chapter
Base Map Benefits
3-1
Tax Parcels
History and Current Status
Recommendations
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Orthophotography
History and Current Status
Recommendations
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Street Centerlines
History and Current Status
Recommendations
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Address Points
History and Current Status
Recommendations
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Base Map Benefits
For local governments, the acquisition and integration of an accurate and up-to-date
parcel base map is arguably the critical centerpiece of a County’s GIS effort. The
parcel/ownership (“cadastral”) base data layer represents the source of County revenue,
planning analysis, and ultimately development.
The successful deployment of
comprehensive base map layers requires careful planning and consideration of proposed
and anticipated uses as an integrated GIS. Technological developments and advanced
applications that can affect these considerations include: controlled digital image and
planimetric feature overlays (hydrology, water utility features, pavement outlines,
building footprints, etc.), and emergency vehicle location and routing through real-time
GPS tracking (AVL/AVI). Subsequent attempts to accommodate unanticipated data
and/or application needs, which are dependent upon or associated with the quality and
accuracy of the original base map, can be extraordinarily time-consuming and costprohibitive.
Several immediate and long-lasting benefits normally derived from base map integration
include:
• A comprehensive inventory and accounting of all taxable land
• Comparisons of deeded and calculated (actual) acreage
• Inventory and determination of County-owned lands and conveyances
• Verification of tax rolls & incorporation of County annexations, providing checksand-balances of appraisal entity records
• Provides a strong foundation for control of future growth, land development, and
population forecasting
• Fosters closer cooperation with other local governmental entities
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Among the more tangible and practical benefits emerging from the base map:
• County mailing lists & affected property owner notifications (i.e., zoning changes,
public works notification)
• Improved Sheriff/Fire response and emergency evacuation
• Readily available property ownership information
• Property valuations and County service expansion/prioritization
• Nuisance abatement & building code enforcement
• Creation of County Map Books and Atlases
Tax Parcels
History & Current Status
Spotsylvania County maintains its own parcel map. The base map that includes parcels
is regularly updated by the GIS Division—this component of the base map is developed
in AutoCAD. One of the priorities for the County’s GIS implementation effort will be
migration away from maintenance of the legacy base map managed in AutoCAD towards
the maintenance of parcels as a personal geodatabase. A general recommendation is
provided for the migration to occur over the current year. The GIS Division (GIS
Manager) will need to coordinate the prioritization of data development efforts with this
objective in mind.
At present, several departments, including the Planning and Utilities Departments, utilize
the parcel map data. This data is made available by the GIS Division; the data is also
maintained in a derivative version as a Shapefile. The County expects to continue to
maintain a parcel layer in the ArcGIS environment, but in a geodatabase format utilizing
daily updates from the Commissioner of Revenue’s Office (COR).
The process to create a new parcel includes:
• The parcel (boundary) is created in AutoCAD
• The address points are assigned with the ranges and points applied using an
AutoCAD MSAG script
• A parcel record is created from the real estate database
• This record is associated with the parcel lot and the parent parcel is retired
The original parcel dataset was orthorectified to aerial images in 1997 by MSAG. The
parcel’s spatial components are created entirely in AutoCAD, which link to the attribute
information stored in an AS400 database. The Division is intending to migrate this
process entirely to GIS, ideally in an ArcSDE/SQL environment. This would allow for
topological incorporation of the parcels and their attribute information as well as a
seamless integration with the document management system and Land Records
Information System (LRIS) database, managed by the COR. The GIS Division is
committed to converting the entire GIS system to SDE. At present the files reside in
shape, AutoCAD (.dwg or .dxf), or geodatabase format. It is recommended that the GIS
Division strive to convert all datasets to a geodatabase format, residing in an enterprise
SDE system.
At present, the COR is utilizing a customized NovaLIS application to fulfill it’s data and
records management needs. It is essential that the information developed by the COR’s
Office is able to be integrated with the County’s Cadastral dataset. The COR should
utilize the functionality of the NovaLIS system for this, as NovaLIS is intended to
seamlessly integrate with GIS. If NovaLIS is unable to export the data in a way that can
be integrated, the GIS Division should work with the COR to make this data workflow an
automated process.
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Recommendations
There are four primary recommendations for the management and maintenance of
Spotsylvania County tax parcels:
1. Integration of Geo-Spatial data to a centralized data store (e.g. Geodatabases)
2. Consolidation of tax assessment data with the County’s parcel layer based on
automatic updates from the Commissioner of Revenue’s Office
3. Implementation of standards for all geo-spatial data
4. Implementation of an alternate key for the parcel layer to ensure no duplication
from subdividing
Integration of Geo-Spatial Data to a Centralized Data Store
All Spotsylvania County departments will benefit from access to a single, up-to-date,
and accurate digital tax parcel layer. In addition, other geo-spatial data products
derived from parcel data need to be consolidated and centrally distributed to eliminate
duplicate and disparate versions of the same data (this includes geo-spatial data
maintained for department specific projects such as Planning). In order to seamlessly
distribute tax parcel data to appropriate users, the tax parcel layer, or a copy of the tax
parcel layer, should be maintained in an ESRI ArcGIS enterprise geodatabase. Currently
the parcel layer is stored in an AutoCAD file format and distributed in either AutoCAD
format or as an ESRI Shapefile. The distribution of this layer to other departments,
especially via an Intranet GIS Data browser, will require that the layer and its
corresponding attributes be maintained in a central data store that can be easily read
and accessed by a variety of GIS applications.
An enterprise geodatabase format is the native spatial data format used in ESRI and
ESRI-based applications. This format stores all spatial and attribute data in one
centralized location, within a commercial relational database management system
(RDBMS). The County has existing SQL Servers which could be utilized for establishing
a geodatabase. It should be noted that enterprise geodatabases require ESRI’s ArcSDE
to access spatial and attribute data stored within the RDBMS, thus the County will need
to acquire and maintain ArcSDE as long as the County intends to maintain data in the
geodatabase format.
However, in the near term, personal geodatabases may be developed as an interim step
in establishing centralized data stores. One of the great advantages that the County will
gain in utilization of a geodatabase is the ability to perform high-end queries of attribute
data in an enterprise database system. In addition, much of the attribute data can be
integrated into web applications that do not necessarily need a mapping interface—the
Information Services Department could develop standalone applications utilizing web
development tools.
Consolidation of Tax Assessment Data with the County’s Parcel Layer Based on
Automatic Updates from the Commissioner of Revenue’s Office
As aforementioned, the COR maintains the tax assessment information for Spotsylvania
County.
An automated process should export an up to date database that will
consolidate the County’s tax assessment data with the parcel layer more frequently.
This consolidation will create a comprehensive tax parcel layer that provides more up-todate ownership and assessment information. At present the GIS Division uses a
snapshot extract from the Commissioner of Revenues’ AS400 (legacy) database. This
process does not adhere to a strict schedule.
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As mentioned earlier, it is recommended that the GIS Division/IS Department/ and the
Commissioner of Revenue develop an automated process for extracting applicable tax
assessment information and linking it to the parcel geodatabase. This automated
update should be conducted on a nightly basis.
Not all the information collected by the COR may be appropriate for inclusion in the
parcel geodatabase. The decision as to what information should be included in the
enterprise accessible parcel geodatabase should include additional support and
assistance from the County Attorney and/or County Administrators.
Implementation of Standards for All Geo-Spatial Data
Another critical issue in the County’s GIS efforts is the implementation of geo-spatial
data standards. It is recommended that the GIS Divison develop standards for the
maintenance of the County’s parcel layer. Using a common standard will enable the
County to operate more efficiently by providing a common structure for the layer and its
derivative products, as well as by enabling streamlined data distribution internally and
externally.
The County will need to develop geo-spatial data standards, including layer and
directory naming conventions, directory structures, metadata, and index conventions. It
should be noted that these standards do not apply solely to parcel data and its
derivative products, but rather that these standards will apply and support all enterprise
geo-spatial data.
Layer Naming Convention:
LD_<SCALE>_<LAYER NAME>_<VERSION>_<REVISION>
Example:
LD_24000_FIRESTATIONS_2_1
Directory Naming Convention:
<DEPARTMENT>
<THEME A>
<THEME B>
<SUBTHEME A>
<SUBTHEME B>
Example:
FIRE
FIRE_STATIONS
RESPONSE_AREAS
COUNTY_AREAS
WILDLAND_AREAS
Layer and Directory Naming Conventions
Implementation of an Alternate Key for the Parcel Layer
Given the lag in time between the approval of a parcel map and the assignment of a
parcel ID (GPIN) either from planning or the County, it is recommended that the County
implement an alternate key for the management of the parcel layer. At present, the
GPIN is relied upon as the primary key; the addition of a secondary key, will allow the
GIS Division to assign a unique internal ID. In doing so, the GIS Division will facilitate
the tracking and oversight of changes to the parcel map, without requiring a GPIN that
may not yet have been assigned or is subject to change. This alternate key can be
referenced within the layer itself (object).
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Orthophotography
History and Current Status
Digital orthophotography (aerial photographs) can be useful as a backdrop to a GIS,
with overlays of parcel, street centerline, or other thematic data.
Digital
orthophotography is a valuable asset to an organization such as Spotsylvania County.
Digital orthophotography is inherently space-consuming and process-intensive. These
files can be difficult to manipulate and they have the potential to slow down networks
significantly. The most up-to-date orthophotography available to Spotsylvania County is
black and white digital orthophotography, acquired from the Commonwealth of Virginia
in 2002. The next delivery is currently scheduled for 2006.
Spotsylvania County Orthophotography Data Sets
Year
2002
Color/B&W
24-bit Color
Resolution
1’ pixel
Format(s)
TIFF
Extent
Spotsylvania County limits, 407 square miles.
Recommendations
It is recommended that the County continue acquiring high-resolution orthophotos from
the state. The next release will be in 2006. Although the data storage requirements are
large for such data, it is recommended that the County acquire the County Limits from
the 2006 orthophotography as scheduled. The acquisition of higher resolution imagery
will provide a more accurate and detail set of base data from which other layers can be
produced.
Figure 3-2: Example of color orthophotography
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It is recommended that the County continue to utilize this type of spatial data to derive
and generate new GIS data layers, including additional planimetrics data layers (see
Figure 3-3 below). In addition, it is recommended that the County explore the
possibility of performing change detection analysis using chronologically sequential
digital orthophotgraphs (see Figure 3-4 below). Change detection analysis can be used
to assess and evaluate physical changes with regard to physical geography and land
use.
Alley
Bridge
Building
Cemetery
Construction Area
Culvert
Culvert Inlet
Dam
Drainage
Driveway
Electrical Pole
Fire Hydrant
Golf Course
Gravel Parking
Gravel Road
Heliport
Lake
Mobile Home
Paved Parking
Paved Road
Playground
Pool
Railroad
Railroad Signal
Recreation Area
Recreation Court
Recreation Path
Sidewalk
Sign
Slab
Stream
Street Light
Traffic Light
Trail
Trees
Wall
Figure 3-3: Typical Planimetrics Derived from Digital Orthophotography
Past
Present
Figure 3-4: Example of change detection using sequential orthophotography
It is recommended that Spotsylvania County acquire their next digital orthophotography
in a compressed format (MrSID). Traditional digital orthophotography formats, such as
TIFF, are inherently large and take up significant disk space.
Compressed
orthophotography aids the end user in the quick retrieval of information. In addition,
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the use of the digitally re-sampled orthophotography enables quicker printing and
plotting. The display of compressed orthophotography can be configured to be scaledependent.
Using this configuration, end users view a map that displays the
appropriate, higher-resolution imagery at a certain scale threshold. By maintaining
orthophotography in a compressed format, the County will be able to distribute its digital
orthophotography to remote sites more efficiently.
Street Centerlines
History and Current Status
Spotsylvania County currently has a digital street centerline layer with associated
address range attributes. The dataset is maintained and updated by the GIS Specialist.
The process for creating new street centerlines for the County includes:
•
•
•
Compiling new streets, initially from the plat and then from the site plan
Verifying the road names from site plans
Recognizing the official road name, apply policy terms and approve name
New streets are added based on new plat development. The new plats are provided in
AutoCAD as required by third-party engineering firms. For the new platted streets the
AutoCAD files are used to develop the Street Centerlines through an overlay and headsup digitization process. The updated Street Centerline dataset is exported from CAD to
an ESRI Shapefile. A version of the file is kept in AutoCAD for special tasks. The Street
Centerline process is scheduled to progress to an ArcGIS process, complete with MSAG
validation. This dataset is crucial because it is the base layer for the County’s 911
dispatch center.
Recommendations
Along with the parcel layer, the street centerline layer comprises one of the most
important datasets for the County. As aforementioned, the layer is used and relied upon
by law enforcement and emergency services. It is recommended that Spotsylvania
County continue to develop, maintain and improve its existing digital street centerline
layer using its current development methodology augmented with the following
recommendations.
There are two primary and two optional recommendations for the development and
maintenance of the County’s street centerline layer
1.
2.
3.
4.
Integration of Geo-Spatial Data to a Centralized Data Store
Population of Attributes to Support Routing and Address Geocoding
Requiring GIS Shapefile delivery of new plats (optional)
Development of an address range updating tool (optional)
Integration of Geo-Spatial Data to a Centralized Data Store
As with the parcel layer, all Spotsylvania County departments will benefit from access to
a single, up-to-date, and accurate street centerline layer. In order to facilitate this
access, it will be necessary to integrate the street centerline layer to a centralized data
store. The layer should be integrated to a centralized data store using the same method
as for the parcel layer. However, this workflow could utilize a third party application in
conjunction with a geodatabase to easily incorporate the GIS and CADD layer, the
maintenance of a CADD layer with a parallel version in Shapefile format, or the
maintenance of a CADD layer with a composite version distributed between SQL Server
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(attributes) and Shapefile (geometries). If the fourth recommendation in this section
can not be enforced; delivery of all new plats in Shapefile or SDE layer format. As
illustrated below (Figure 3-5) the acquisition of high-resolution orthophotography will
enhance the accuracy of the Street Centerline layer when used as a georeferenced
overlay during the heads-up digitization process.
Figure 3-5: Example of Street Centerlines Overlaying Aerial Photography
Additionally, a comprehensive set of standards, specifications, and procedures for
maintenance of the street centerline layer will support the development of an accurate
and up-to-date base map layer.
The Commonwealth of Virginia developed such
standards that may be useful guides for Spotsylvania County. The documents are
stored on the Virginal Geographic Information Network (VGIN) web site
(http://www.vgin.virginia.gov/documents/guidelines-standards/guidelines-standards.html).
These documents should provide Spotsylvania County with industry standard GIS data
maintenance and management processes for, but not limited to, the County’s street
centerline layer. Additionally, as recommended for the parcel layer, a unique field (key)
by which all Spotsylvania County street segments can be tracked should be developed
when the Street Centerline layer is consolidated into a central data store. This key
should be utilized to track any internal changes made to the street centerline layer.
Population of Attributes to Support Routing and Address Geocoding
An example of a street centerline data model includes the following attributes (fields):
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This set of attributes is adequate for use of the street centerline layer for geocoding and
routing. Although the County has managed to maintain most of this information, it is
recommended that the GIS Specialist work on population and update of all attributes of
this layer (TravelDirection and PrefixDir will need to be populated for routing). In doing
so, the County will have ownership of a valuable street centerline layer that can be used
for accurate routing. This routing in turn will support a variety of GIS needs and
applications, including generation of optimal routes for a variety of needs, including
emergency/incident response, work orders, and AVL.
A street centerline layer is necessary for many GIS analysis tasks; however, it does have
some limitations. An address search on a street centerline layer will only locate the
interpolated position of that address in relation to the centerline – in the middle of the
street or at a designated offset.
The interpolated location will not show the actual
location of the address. Addresses with multiple units or structures may be ambiguously
located, as a point may be used to represent a complex of buildings (e.g., a college
campus). Additionally, actual addresses may vary from their interpolated location based
on address ranges; in turn, address ranges may not be correct or may be outdated.
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A solution for preventing and addressing such discrepancies is to create an address point
layer. Several local governments have created or plan on creating an address point
layer to yield more accurate address search results. An address point can be placed in
the center of each structure in the County, or at the entrance of the primary driveway.
Structures with multiple units such as apartment complexes and condominiums can have
one address point for each unit (see the Address Point section below).
Requiring GIS Shapefile delivery of new plats (optional: short-term)
Requiring that developer and engineering firms deliver shapefiles of new plats and
streets could streamline the development process and increase the spatial accuracy of
the layer. However, this process would require concrete standards by which all firms we
be required to adhere. Additionally, a quality assurance process would have to be put in
place prior to the shapefile data being added to the centralized base map layer.
Development of an address range updating tool (optional: long-term)
A future development process that would streamline the layer maintenance process
would be the development or acquisition of a tool that allows for the updating of address
ranges in the ArcGIS environment (see Figure 3-6). Updates to this layer are performed
on a frequent basis by the GIS Specialist; it is expected as the organization grows other
GIS personnel within the GIS Division or at the departmental level will assist with the
effort of updating and rectifying the street centerline layer. Requiring training or access
to the production ready data and using ArcView (flagship) applications should not be
required of all users, especially if their data development responsibilities are narrowly
focused (e.g. street centerlines only). For these types of data custodians an application
developed for a specific task reduces the risk of data corruption and data entry error.
Figure 3-6: Example of a Street Centerline Layer Editing Tool
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Address Points
History and Current Status
Spotsylvania County currently has an address points layer. This layer has been
developed through a combination process of paper-based and database integration. The
GIS Specialist assigns addresses to new locations during the addition of new street
centerlines. The process is initiated by a subdivision and subsequent submission of new
plat development. The new plats are provided in AutoCAD as required by third-party
engineering firms. Working with the Planning Department, the GIS Specialist creates
the address points using the ranges and points applied using an AutoCAD MSAG script.
Recommendations
It is highly recommended that Spotsylvania County streamline the Address Points data
development process. A primary address database should be identified and that
database should be part of the address creation process in the Planning Department.
Addresses in the primary address database should be address-matched to the site
address of each parcel in the parcel layer; records that do not exist in the primary
address database that do exist in the parcel layer should be verified and incorporated as
address points as well. The centroid of each parcel will serve as the preliminary address
point. Addresses that exist in the primary address database that do not exist in the
parcel layer should be verified and reconciled to a valid County address, using plats or
street centerline attributes. Figure 3-7 provides a general overview of the methodology
to create and validate address points.
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Phase I
Assessment & Evaluation
- Identify configuration and database environment
- Assess existing data model
- Evaluate data quality and identify errors, anomalies, and/or inconsistencies
- Determine feasibility for address clean-up
Phase II
Database Analysis and Data Modeling
Table
Table
Table
- Identify required tables
Springbrook tables
- Identify and evaluate ancillary GIS data layers
- Configure links with data translation software
Phase III
Business Logic Design and Development
- Design and develop stored procedures (as needed)
- Develop validation rules and exception handling
- Configure complementary GIS data layers
- Design and create spatial queries and application rules
Table
High
Phase IV
Point Validation and Editing
- Generate address points
- Geo-reference points and validate location
- Spatial validation of attributes and record information
- Edit non-conforming and/or inaccurate points
- Edit non-conforming and/or inaccurate attributes
High
Low
Low
High
Phase V
Sampling, Verification, and Validation
High
Low
Low
- Random sampling of records
- Verification of correct location and attribution
- Validation of business logic
- Database queries to determine exceptions
High
Phase VI
Exception Identification and Recommendations
High
Low
Low
- Aggregate query and validation results
- Identify exceptions to be rectified
- Summarize and report exceptions
- Make recommendations for editing of duplicate and/or extraneous records
Phase VII
Final Verification and Validation
- Random sampling of records
- Database and spatial queries
- Process documentation & training
Figure 3-7: Address Point Creation Methodology
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Once address points are created, they should be rectified to the appropriate location
through use of digital orthophotography (see Figure 3-8) as well as GPS field work (as
needed). The GIS Division, in conjunction with the recommendation of the GIS Steering
Committee, will need to develop business logic for the placement of multi-unit address
points and other special case addresses, such as mobile homes. In addition, QA
procedures will need to be utilized to identify addresses in the primary address database
(and/or parcel layer) that are incomplete, inconsistent, or out of date. The creation of
an address point layer is often an iterative process that requires multiple efforts before a
comprehensive and up-to-date address point layer is created.
Before
After
Figure 3-8: Point Movement
The address point layer should be maintained in a geo-spatial format, currently as a
Shapefile, and in the future as a geodatabase layer. It is recommended that the GIS
Division be responsible for the maintenance and management of this layer; Planning
Department personnel should be secondary resources (these personnel should be crosstrained on the various methodologies used for creation, maintenance, and management
of this layer). The County’s address point layer should be updated to include a new
point (record) each time that a new parcel polygon is added to the parcel basemap.
Address Points and Geocoding
An alternative means for generating address points is based on geocoding using the
street centerline layer. Geocoding (also known as address matching) with the street
centerline layer yields address points based on an interpolated value using address
ranges for each line segment. Although this method is somewhat effective in creation of
address points, it is not nearly as accurate as an address point layer as referenced
above. In geocoding, points are offset from the street centerline by a set distance
(typically 50 feet) in order to place them in relative proximity to each parcel’s centroid
(center point). While this methodology provides a quick means for generating address
points, an inherent lack of positional accuracy for each point is problematic (see Figure
3-9).
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Interpolated locations, such as those yielded by geocoding, have traditionally been
inaccurate and often times inconsistent—this depends both on the accuracy of the
underlying street centerlines used as well as the configuration parameters used for
geocoding. One of the key limitations of using an offset parameter in order to place
points closer to their “true” location is that addresses close to an intersection or end of a
line segment will not have the same offset as those near the center of a street segment
(see Figure 3-10).
Main Street
50 foot
offset
2nd Avenue
1 st Avenue
Main Street
Figure 3-9: An address interpolated to a street segment center will be placed at
the specified 50 foot offset.
Main Street
10 foot
offset
2nd Avenue
1st Avenue
Main Street
50 foot offset
Figure 3-10: Depending on address-matching logic used during geocoding, an
address interpolated near an intersection (or street segment end) will be
placed at a different offset. If the specified 50 foot offset were to be used in
this case, the address point would be placed near the end of the line segment
and offset by 50 feet, thereby placing the point close to the middle of the
intersecting street.
Other limitations of geocoding include the interpolation logic used for address matching.
Interpolated addresses are subject to address matching rules, which in some instances
may place a point within a block group centroid, or even within a Zip Code centroid, if
address information is ambiguous and/or incomplete. For example, if a record does not
have all relevant address information, such as an incomplete street number or a
misspelled street name, that record may be geocoded to a Zip Code centroid.
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