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NCPMA ADVANCED MAPPING

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NORTH CAROLINA PROPERTY MAPPERS ASSOCIATION

ADVANCED MAPPING COURSE

SECTION 9

THE USE OF GIS IN PARCEL MAPPING


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9.1 Computer Technology

Computer technology is the most exciting development in cadastral mapping. Although

manual tools and systems served well for many years, computers are more valuable for

complex work. Advances in processing speed, memory, and display resolution for graphics

have made computers more useful than ever.

Some advantages of using computers in a modern mapping program are speed, precision,

flexibility, productivity, display capabilities, storage and retrieval efficiencies, and improved

efficiency in the corrections of errors and omissions. Possibly the two most notable specific

advantages are automated scale adjustments for updates, and elimination of shrinkage or

quality deterioration of base material (mylar and paper) over time.

The cadastral mapper can use computers in three general ways: (1) to meet the basic

functions of location, identification, and inventory; (2) as an analytical tool; and (3) as an

administrative tool to organize, display and use information.

For example: By electronic editing, the computer locates for correction such errors as lines

that do not join and polygons (parcels) that do not close. An advanced computerized

mapping system can be used to analyze property characteristics for identification of under

assessed parcels. It can sort by characteristic, for appraisal purposes. Neighborhoods can be

defined and analyzed. Sophisticated spatial research such as trend surface analysis is

possible.

Interaction with other databases, such as soils, zoning, flood plains, school districts, political

jurisdictions, and other physical, social, governmental, and economic spatially oriented

factors, can make possible the development of sophisticated systems that merge mapping and

computer assisted mass appraisal (CAMA).

Linking assessment records and parcel maps improves assessment activity records. For

example, a map can be created to show which parcels have not been physically inspected

within a specified period of time. The system can also produce a routing plan for field

inspection. Those improvements lacking information on characteristics such as square

footage can be pinpointed.

9.2 Computers

Computer technology was first implemented in many county assessors’ offices in the 1970’s.

Indeed for many counties their first exposure to computers was in the tax office. Today

computers are used in every office in some way or another. A computer is a set of hardware

and software components that work together as a system to run programs and store data.


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Working together they are capable of performing a series of logical or arithmetic operation at

high speed. They manipulate information by adding, comparing and storing numbers or

digits. This digital information is a combination of ones and zeros that form what is known

as machine language. Contrary to popular belief, computers can’t think, what they can do is

follow instructions very quickly.

Computer hardware is the computer’s physical equipment; any part of a computer you can

touch. The computer is made up of five hardware units:

The input unit, by which data and instruction are taken into the system, i.e.

keyboard, mouse/puck, scanner.

The data storage unit, where data and instructions are taken into the system;

disk drive, Network Area Storage (NAS), Storage Area Network (SAN), etc.

A control unit, the directory force that switches and integrates the various

units into a whole system (processor);

the output unit, where the processing results and other systems messages are

displayed for interpreting: monitor, printer and plotter.

Most communication with a computer is through a computer keyboard. When you hit the

letter R the signal determines the meaning of the key. For instance, when you press the letter

for capital R the computer uses a look-up table to supply the ASCII processing code the

binary number 01010010.

ASCII is a shared electronic language – the American Standard Code for Information

Interchange, therefore computers can pass information back and forth without translation.

Table 9-1 illustrates a few examples of binary code

TABLE 9-1 Binary Code

A 01000001

a 01100001

Space 00100000

00101110

2 00110010

The capacity to store information in a computer is measured in bytes. A byte is an ASCII

Character and consists of eight bits (binary digits). A bit and a byte are represented by the

following figure.


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Figure 9-1, the letter “A” in binary code.

BYTE CHARACTERS EQUAL TO

Kilobyte (KB) 1,024 One page of double spaced text

Megabyte (MB) 1,048,576 One book

Gigabyte (GB) 1,073,741,824 Shelf full of books in a library

Terabyte (TB) 1,099,511,627,776 Entire library of books

Petabyte (PB) 1,125,899,906,842,624 3 Years of NASA EOS data

Exabyte (EB) 1,152,921,504,606,846,976 5 EB- All words ever spoken by humans

Computers store information as data which is individual facts, unorganized but capable of

being organized. Therefore, Data Processing is a series of steps that converts raw data into

useful information.

9.3 Types of Computers

Fat/Thick Client Computer

Most PC’s in commercial use are fat client computers based on the IBM PC, (INTEL or

similar microprocessors), or the Apple Computer (Motorola or similar microprocessors).

These machines and their operating systems are intended for a single user. The architecture

of a fat client computer is designed so that the majority of the processing is done by the

computer and passes data for communication and storage to a server.

Thin/Slim Client Computer

Thin Client Computers are either hardware or software based computers that employ client-

server architecture. With this architecture, the processing is done by a central server and the

client computer’s function is to convey input and output from the user to the server. It is

01000001 ㄩ

Bit

Byte


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essentially a modern day “dumb terminal”. With the advancements in virtualization due to

the relative cost of processers, RAM, and hard drive space, this is starting to become more

common. Remote Desktop Protocol (RDP) is a scaled down version of this that is common

in most operating systems and can be used as a thin client.

9.4 Administration

Administering a computer system involves performing many different functions, which are

essential to the operation and use of the machines.

System backup is the regular copying of all data from the computer's disks to

tape or replicating the data to other drives or servers. With modern SAN

technology, data is often striped across multiple hard drives in a RAID and

then replicated to another set of hard drives or a second SAN. The advantage

of this is that you can perform a rollback (also known as a snapshot or restore

point) where you can change the data or software back to how it was at a

previous point in time using a software interface.

User support and problem resolution involve advising users on solving

software and hardware problems.

Installations of new hardware must be done by an administrator. Software

installation, upgrades, and patches are also installed by an administrator and

are often pushed out from the server using an MSI or other deployment

program.

System security involves understanding and reducing the vulnerabilities of

the computer system and network, controlling access to software and data

via passwords and protection schemes, preventing computer viruses, and

controlling physical access to the network via telecommunication and

modems. Problems increase with the openness of the network and the

number of users on the system.

Software programs are the coded instructions stored in the computer to tell it precisely and

completely what to do with the data provided. An operating system is software provided by

the computer's manufacturer, containing a series of steps to solve a problem or perform a

function. Application programs, on the other hand, are specific instructions for processing

specific types of data that manages the interaction between users and hardware. For

example, these programs tell the computer what to do with the resources to run programs or

send files to plotters and printers.

9.5 Computerized Mapping Systems

Computerized mapping systems types can be classified, in ascending order of sophistication,

as Computer-Assisted Drafting (CAD), Automated Mapping/Facilities Management

(AM/FM), or Geographic Information Systems (GIS).


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9.5.1 Computer-Assisted Drafting (CAD) is used to produce maps.

CAD is software that typically supports civil, mechanical, and other engineering design

activities. CAD software can display data, perform engineering calculations and complete

limited attribute processing. CAD data elements include all graphics needed to draw a map:

lines, line strings, text, and symbols. These may or may not be referenced to a coordinate

system, which can be used to represent a mapping grid, such as state plane coordinates. Data

in a CAD system is organized on layers that are conceptually like registered overlays. The

layers can be used to organize map features by theme, such as property lines versus streams,

or by type of data, such as linework versus text.

CAD is limited in that it cannot analyze or process base data. CAD does not store area

(polygons) but can measure the area of a plane figure. Also CAD can measure the length of

lines or distances between points. Basically, CAD systems tend to focus drawing functions.

The content of a map would be represented in the computer exactly as it was in the manual

system, but it is possible to combine several map sheets into one computer generated map or

to use only portions of individual maps.

9.5.2 Automated Mapping/Facility Management

AM/FM is a system based on CAD technology and used by utilities to manage mapping and

attribute data regarding their physical plant. For example, an electric utility would use

AM/FM to store the location and attributes of its power lines, poles, transformers, and so on.

AM/FM uses CAD graphic data elements to represent map features. Like CAD, these are

referenced to a map coordinate system and organized on layers by map theme. However,

AM/FM goes a step further by defining relationships among utility system components.

An important feature of AM/FM is that the attributes of the utility system are also stored in a

separate data table. These records are linked to the graphic data elements by a unique

identification number, such as a utility manhole number. These attribute data describe the

characteristics of the component.

9.5.3 GIS

A Geographic Information System (GIS) is best suited for the analysis of geographic data

and therefore most useful for county government mapping and analysis applications. GIS is

similar to CAD and AM/FM in that it references graphic data elements to an X - Y

coordinate system and it separates map features by layer (also referred to as a map theme or

coverage). The GIS may divide the entire area being mapped into separate files, much like

map sheets, but it handles all of the data in these files as though they were one seamless map

file. In addition to graphic (sometimes referred to as spatial) data, a GIS also stores attribute

data. These are associated with the spatial data and provide further descriptive information

about them, similar to an AM/FM system. For instance, in a GIS a parcel would be defined

as an area, and its descriptive data might include the parcel identification number, owner's


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name, acreage, value, and so on. This attribute data is placed in a database separate from the

graphics data. GIS also differs from CAD and AM/FM in that the spatial relationships

among all data elements are defined. This convention, known as data topology, goes

beyond merely describing the location and geometry of map features. Topology also

describes how linear map features are connected, how areas are bounded, and which areas are

contiguous. To define map topology, a GIS uses a special database structure. As in a CAD

system, all map features are related to a geographic coordinate system. But unlike a CAD

system, which defines map features as lines or symbols, a GIS defines map features as points,

nodes, lines, areas, and islands

9.6 Topological Data Structure

Three basic elements or data structures are required to make a GIS “intelligent”.

1. Geographic Location: A given set of coordinate points can be used to precisely locate a

feature on a map. These are the “X”,“Y” and even “Z” coordinates discussed in Section

One.

2. Relative Position. This refers to the connectivity and adjacency of features. Polygons

(i.e., parcels) that share common boundaries can be identified as well as line segments

(i.e., highway R/W’s) that are part of a common network. The relative position

information, based on the appropriate structuring of a database, is referred to as

“topology”.

3. Descriptive Data. This is alpha/numeric data that is associated with the topology of the

graphics. Essentially, there is a lot of information in the world, which is tied to

geography, but is not graphic in nature. Such information is often referred to as “attribute

data” and might include elements such as land and building values, land use, names of

property owners, etc.

Spatial Data includes points, nodes, lines, areas/polygons:

1. Points. Points are used to represent the location of objects defined by a single set of X

and Y coordinates. In some cases, these are feature points that can be identified on the

surface of the earth (e.g. spot elevation, fire hydrants). In other cases, they are attribute

points for areas such as property centroid. The locations of attribute points may be

digitized or may be calculated by computer software.

2. Nodes. Nodes represent the ends of intersections of lines (linear objects). Nodes exist at

the first (beginning point) and last (ending point) set of X and Y coordinates of a line.

Each node may also reflect an intersection with one or more additional lines. A node

plays an important role in topological definition and, as such, should reference each line

that insects it.

3. Lines. Lines are strings of coordinates that run between nodes. Each line has a minimum

of two X and Y State Plane Coordinate Pairs (straight line). Lines also include references

to attribute points, the beginning and ending node numbers, and identifiers of areas (when

applicable) on the right and left of the line.


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4. Areas. Areas are polygons which are defined by a series of lines. These lines must, in

combination, totally encircle or close the area they represent. Areas also include

references to attribute points and other associated data. The boundary of an area is

defined by a listing of the lines that comprise the area’s border.

9.6Topological Concepts

Connectivity Lines connect to each other at nodes

Area Definition Lines that connect to surround an area define a polygon

Contiguity Lines have direction and left and right sides


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Figure 9-3


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Figure 9-4


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Figure 9-5


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A GIS can automatically maintain a connection between coordinate records and attributes

records. See figure 9-6 below.

Figure 9-6

Parcel Centroid # x, y pairs

Centroid # Owner Deed Bk/Pg Date Address

1 Robert Martin 577 / 97 2-3-1977 5 Steele St

2 Meghan Donohue 989 / 701 11-10-1989 7 Steele St

3 Marie Monteith 1191 / 14 1-2-1998 4 Steele St

4 Scott Tabb 1491 / 767 7-21-2000 6 Steele St

5 Randy Holman 857 / 401 8-9-1984 9 Steele St

6 Joe Hunt 601 / 318 11-17-1979 8 Steele St

9.7 Organizing Map Information

To this point it has been determined that geographic data is stored as a series of x, y

coordinate pairs representing points, lines and polygons. Also, the relationship between

these features is made possible through topology. With all this information, it is necessary to

understand how the data can be organized and used.

Layers - Map features are logically organized into sets of layers (also called themes or

feature classes) of information. A layer consists of topologically linked geographic features

and their associated descriptive data stored as an automated map.

5 536,376 , 1,473,219

100.00’

5

115.27’

1

8

2

.

3

7’

1

7

4

.

0

8’


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Figure 9-7


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9.8 Representing Descriptive Data in the Computer

9.8.1 Attributes – Are non-graphic descriptive information about features, characteristics or

elements of a database. Attributes are stored as sets of numbers and characters.

Parcel Number Unique number for each parcel

Owner’s Name Name of owner

Address Property address

Property Type 1=Residential

2=Commercial

3=Farmland

4=Industrial

5=Government

Status V=Vacant

I=Improved

Another way to illustrate the same thing is:

Map Number Parcel

Number

Owner Property

Address

Property

Type

Status

25 10 Jones Oak Street Residential Vacant

The descriptive information for a feature, such as a parcel, can be stored in a tabular data file.

In this case a record stores all the information about one occurrence of a feature (a point, line

or polygon) and an item stores one type of information (attribute information) for all features

in the database. These data files are known as feature attribute tables.

A row within a Feature Attribute Table is referred to as a Record and a column within a

Feature Attribute table is referred to as an item. This is illustrated in Figure 9-8.

Figure 9-8

Centroid # Owner Deed Bk/Pg Date Address

1 Robert Martin 577 / 97 2-3-1977 5 Steele St

2 Meghan Donohue 989 / 701 11-10-1989 7 Steele St

3 Marie Monteith 1191 / 14 1-2-1998 4 Steele St

4 Scott Tabb 1491 / 767 7-21-2000 6 Steele St

5 Randy Holman 857 / 401 8-9-1984 9 Steele St

6 Joe Hunt 601 / 318 11-17-1979 8 Steele St


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9.8.2 Connecting Features and Attributes - The power of GIS lies in the link between the

graphic (spatial) data and the tabular (descriptive) data. There are three (3) noteworthy

characteristics of this connection:

A one-to-one relationship between features on the map and records in the feature

attribute table.

The link between the feature and the record is maintained through the unique

identifier assigned to each feature.

The unique identifier is physically stored in two places: in the files containing the x, y

coordinate pairs and with the corresponding record in the feature attribute table.

The coordinate records and the attribute records share a common element: the feature ID

(also known as the object ID). The feature ID associates the attributes with the geographic

feature.

Once this connection is established, it is possible to query the map to display attribute

information, or create a map based on the attributes stored in the feature attribute table.

9.8.3 Attribute Relate and Join Relational Operators -A relate uses a common item to

establish temporary connections between corresponding records in two tables. In a relate,

each record in one table is connected to a record in another table that shares the same value

for a common item. A relate has the effect of making a feature attribute table “wider” by

temporarily adding feature attributes which aren’t actually stored in the feature attribute

table.. Figure 9-9 shows two tables that can be related based on the parcel number. A join is

similar but the connection is permanent by merging the two tables into one larger table. This

is illustrated in Figure 9-10.

Figure 9-9

Parcel Number Owner

9675-01-4312 Holman, R

9675-01-7519 Wefer, L

9675-01-9648 Wheelock, D

Parcel Number Neighborhood Code Area

9675-01-4312 8PK 110 A .92

9675-01-7519 RAQ 210 B .78

9675-01-9648 R30 105 A 1.04


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Figure 9-10

Parcel Number Owner Neighborhood Code Area

9675-01-4312 Holman, R 8PK 110 A .92

9675-01-7519 Wefer, L RAQ 210 B .78

9675-01-9648 Wheelock, D R30 105 A 1.04

Linking both spatial and non-spatial within a GIS allows the user to make inquiries and

perform analysis similar to the following examples:

This GIS will allow you to “search by owner” and list those owner’s who match the search

criteria. See figure 9-11.

9.8.4 Spatial Join and Relate

A spatial join is a join that is not based on an attribute but based on the geographic location

of a feature. An example of this would be to spatially join the parcels to the flood plain. The

result would incorporate the appropriate flood information into the attributes of the parcel so

that it can be queried. A spatial relate is similar in that it is based in the geographic location.

Since it is a relate, it does not merge the attributes but establishes a connection between them

based on the location. A common version of this is the Select By Location function using the

intersect option. You can query the parcel layer (with the original attributes intact) to see

which ones occupy the same location as the flood zone.

9.9 Data Conversion Technology

In order to use a CAD System or GIS for mapping, it is necessary to have a method to

convert manual data to digital data. Actually, since the organization deals with different

types of data it is beneficial to have more than one way to convert manual data into digital

form. There are three recognized methods of converting data;

Digitizing

Coordinate Geometry

Scanning


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9.9.1 Digitizing - A common form of converting manual data to digital data is by digitizing.

It is defined as the process of using a digitizing tablet and cursor to encode the locations of

geographic features. This is accomplished by pushing a digitizer button and recording an x,

y coordinate.

Digitizing has the following advantages:

The ability to correct errors or distortions in the original maps at the time of data

capture.

Highly reliable human recognition of map objects.

The ability to interpret ambiguous or incomplete information and select the relevant

required information at the time of data capture.

Digitizing has the following disadvantages:

The process is labor intensive and time consuming.

9.9.2 Coordinate Geometry - Coordinate Geometry or COGO is a program used to generate

digital map features from geometric descriptions. Mathematical algorithms compute

coordinates from geometric descriptions such as bearing and distances; the coordinates are

stored and used to generate map displays.

Typically bearings and distances for each segment of the parcel description are entered into a

computer as prompted by the COGO application. The program then “plots” the parcel

precisely in accordance with the deed description. See Figure 9-18 below.

Figure 9-18

4 3

2

1

N 87º 30’E 141.17’

N 32º

15’W

101.19’ S 34º

47’ E

89.01’

N 89º W 73.41’


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Coordinate Geometry has the following advantages:

The exact shape and size of a geometric description can be duplicated within the

computer.

Parcel descriptions can be created separately and located on the digital database.

Coordinate Geometry has the following disadvantages:

The process is very labor intensive and very time consuming.

9.9.3 Scanning - Scanning converts paper documents (maps) to digital or computer readable

raster images. This produces a raster image on the map. For some GIS applications, the

pixels must be converted from the raster image into a vector data structure that defines linear

features by their endpoints and vertices.

Scanning has the following advantages:

Scanning can be used to capture total document images, annotation and text and

points, lines and shapes.

The scanning method has the ability to capture large amounts of data in a short

period of time.

Scanning has the following disadvantages:

Raster images have limited use because they have no “intelligence.”

It is difficult overlay to raster images on one another.

9.9.4 Heads-Up Digitizing - Heads-up digitizing has become a common form of digitizing.

It typically involves scanning a map as a raster image, then drawing vectors over it using the

mouse. Other forms include drawing a feature with a mouse by using other features on the

map as a visual aid.

9.10 Graphic Data Formats

Computer graphic images are found in two formats, raster and vector.

Raster - A format for storing, processing and displaying graphic data in which graphic data

are stored as values for uniform grid cells or pixels. Similar to the way a fax machine

transmits a paper document over telephone lines, various tones and colors of a map are


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converted into numeric values and placed on a finely divided grid. Each square of the grid is

called a pixel. Common examples of raster data are .jpg / .bmp / .tif / .sid / .png

Figure 9-19 illustrates a raster structure where each grid cell or pixel is referenced by a row

and column number and contains a number representing the type or value of the attribute

being mapped. In raster structures, a point is represented by a single cell, a line by a number

of neighboring cells strung out in a given direction, and a polygon by an aggregation of

neighboring cells.

Figure 9-19 Raster Structure

Vector - A format for processing and displaying graphic data. Vector data are represented

by strings of coordinates representing the true position of features that are shown as points,

lines and areas. Vector based systems are analogous to “connect the dot” pictures.

Information is entered into the system by defining points and lines. Attributes are then

assigned to the points, the lines connecting the points, and the polygons which are defined by

the attributes which lie on either side of the line. It is this spatial relationship between the

points, lines and polygons which give the GIS its topology, or intelligence.

Vectors can be easily updated and locations of lines and points can be changed and

new connections can be added.

GIS software can add “intelligence” to the points and lines. It is possible to

describe the area enclosed by lines as polygons and then add attributes to the

polygons.

Vector data requires less computer memory for storage.


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9.11 Coordinate System Selection

Coordinate systems are the mathematical systems that tie control networks together - the

common thread. Coordinate systems are also used to tie the control to the base map and the

cadastral layer.

The selection of a coordinate system for a GIS should follow the principles of GIS described

in that chapter. That is, the GIS is most useful when it is registered on a consistent and

continuous coordinate system. This means that the coordinate system should accomplish

three goals:

1. The coordinate system ties individual geographic control points to the control

network. If the geographic network serves its purpose of providing a common

reference for all maps and features in the jurisdiction, then the coordinate system

must be related to the geographic control.

2. The coordinate system must be mathematically and rigorously developed. This

means that the coordinate system will allow GIS maps and geographic data to be

precisely related to each other. It will also allow the GIS to incorporate data from

many sources into one system. Data from one surrounding jurisdiction can be

matched to data from others.

3. For a coordinate system to provide benefits, it must extend over the entire GIS

project. For example, if a county has two different coordinate systems, combining all

the data into one cohesive system would be difficult.

In any jurisdiction there are many options for coordinate systems that will meet these criteria.

Experts knowledgeable in coordinate systems may differ on specific solutions for any

particular area. These differences concern how well the coordinate systems relate to

statewide systems, how well they tie the control points together, and what the mathematical

basis of the coordinate system should be. Here in North Carolina, the most common is the

State Plane coordinate system using the North American Datum of 1983 (NAD83) in feet.

However, there are many areas that use the datum of 1927 (NAD27) and some agencies use

meters.

Sometimes as a GIS user, you will get GIS data from various sources that do not match the

coordinate system you are using. It may be in a different projection or datum. In order to

use the data, a transformation may be necessary. A transformation is the process by which

data is converted from one datum or projection to another. It also is commonly referred to as

re-projecting the data.


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9.12 Data Management

9.12.1 Storage of Digital Data

Instead of having map cabinets for maps and filing cabinets for assessment cards, the

computer stores the digital data. The digital data remain in the computer or on a computer

disk until retrieval is requested. Today, one of the most common storage methods is to store

the data in a Relational Database Management System (RDBMS) usch as Microsoft SQL

Server or Oracle. Smaller GIS shops will often employ scaled down versions of these such

as Microsoft SQL Server Express, Microsoft Access (ESRI personal or file geodatabase), or

a proprietary format. It is becoming more common for the data to be housed on a separate

server from the software such as a SAN or NAS. This aides in efficiency, back-up,

replication, and flexibility.

9.12.2 Database management

The GIS database is usually complex. Moreover, data changes and new data come from

many sources. Likewise, there are usually several departments in the organization

that rely on GIS data to get their work done. Therefore, overall accountability for the GIS

database is usually assigned to one or, at most, a few individuals. This generally includes

responsibility for the following issues regarding both graphic (spatial) and non-graphic

(attribute) data:

Metadata ("data about the data").

Data sources.

Data organization.

Data distribution.

Data format standards.

Plotting standards (i.e., graphic conventions).

Data security.

Horizontal and vertical mapping accuracy.

Map projections.

Data maintenance (even though one or more GIS users may make the edits).


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9.12.3 GIS Data Management

Managing and maintaining the GIS database in the Assessor’s department is relatively

straightforward since there is one “owner” of the data. It controls both access to the

maintenance of the database. But most counties eventually move toward an “enterprise

GIS,” making GIS data available to users in non-technical departments as well. The greatest

return on an investment in GIS occurs when as many departments in the County have access

to GIS data as possible, as opposed to just one or two departments, even though they may be

“heavy” users.

Usually, each of these departments is responsible for a particular type (i.e., map theme or

coverage) of GIS data. Moreover, they usually have independent GIS operations. To

implement an enterprise GIS program, therefore, the organization must manage and maintain

GIS data “owned” by several departments.

9.12.3.1 Centralized GIS Database

The obvious approach to enterprise GIS is to store all data on a single server and make it

available to the entire county over its LAN. While several different departments are

responsible for maintaining their respective GIS themes, they can still access and edit this

data over the LAN.

Department A

GIS Data Server

Internet Users

Other

Applications

Department B Department C


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The chief advantages of this approach are the following:

Only one database needs to be to managed (backed up, archived, etc.).

Only one server and server software license must be purchased.

All data is under the physical control of only one department.

The chief disadvantages are the following:

Departments that “own” certain GIS data themes do not have physical control of

them.

Data “traffic” on the organization’s LAN backbone will increase significantly.

The heaviest GIS users are departments that “own” GIS data. Since all editors

must access their data over the LAN backbone, its data transmission volumes will

increase accordingly. While vector GIS data files are typically only somewhat larger

than normal “office’ data files (e-mail, word processing, etc.), raster GIS data files are

typically very must larger. Thus the transmission of GIS data, especially raster data,

may severely tax the available bandwidth of the LAN backbone.


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9.12.3.2 Distributed GIS Databases

Another approach is to physically distribute the GIS data on multiple servers, one for each

department that “owns” (i.e., maintains) the respective data themes. While each department

has access to “its’ data over a departmental LAN, other departments also have access to it

over the LAN backbone. Figure 9-24 illustrates the concept of distributed GIS databases.

Department A

GIS Data Server

Internet Users

Distributed GIS

Department B

GIS Data Server

Figure 9-24 Distributed GIS Databases


Departments that “own” certain GIS data themes have physical control of them.

The heaviest GIS data users access their department’s data over its own LAN, so

GIS data transmission volumes on the organization’s LAN backbone are significantly

lower.


Each department must manage its own database, which requires duplicate

personnel skills and training.

Multiple servers and server software, licenses must be purchased.

GIS data is under the physical control of several departments, so effectiveness

may vary.


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9.12.3.3 Master and Working GIS Databases

There is yet another approach which combines the best features of the first two, but is

usually the most costly of the three. Under this strategy, departments work with their

respective GIS data files located on their own “working” data server, while copies are stored

on a “master” data server that is accessed by the entire organization. “Snapshot” copies of

the “working” data are uploaded to the “master” data server on a periodic basis. Figure 8-20

illustrates the use of master and working GIS databases.

Department A

GIS Data Server

Internet Users

Master and Working GIS Databases

Department B

GIS Data Server

Master GIS Data

Server

Other Applications


Departments that “own” certain GIS data themes have physical control of them,

while the master database, upon which the entire organization relies, is under the

physical control of only one department. Once again, this often the

organization’s IT department.

The heaviest GIS data users access their department’s data over its own LAN, so

GIS data transmission volumes on the organization’s LAN backbone are lower.

When changes are made to the working databases, the database will not be

current until it is refreshed.


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This approach requires the greatest investment in training and personnel.

This approach requires the greatest investment in data servers and server

software.

9.12.3.4 Replicated Databases

Replicated databases are similar to Master and Working in that there is more than one server.

There are a couple of variations of replication.

One is an editing server and a viewing server where one or more servers are used to house

data for editing by various editors. The data that has been edited is then synchronized with

the viewing server. This improves security and reduces the load, especially in an

environment where there are a lot of users or applications that view the data.

Web

ADFEditing

Department A

GIS Editing Server

Internet

Users

Replicated GIS Database

Viewing

Department CEditing

Department B

GIS Viewing Server

Other

Applications


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Another common way of replicating is to have multiple editing servers. Each department or

entity has a server in which they maintain their data. Changes to their data are sent via a

synchronization process to the other department’s server so that they can see the changes.

Likewise the other department’s server will send that department’s changes to your server.

This maintains “ownership” of data and editing rights by various departments, but also

ensures that everybody has up to date data.

Department A

GIS Data Server

Internet Users

Replicated GIS Database

Department B

GIS Data Server Other Applications

9.13 Data Maintenance

There is yet another important issue in enterprise GIS, regardless of which of the above

strategies is chosen. This is the management of the data editing process. If the organization

has only one GIS data editor, this is not a problem. This editor logs into a file, makes

changes, then logs out. However, if several editors must access the same GIS data server,

significant problems can result.

The problem occurs when two users must work on the same file. Suppose both editors are

permitted to simply “check out” the file to their workstation, make changes, and then save

the file again. The changes made by the first editor who saves a file will be lost when the

other editor saves his or her copy of the file. The general rule is, “The last one who saves

wins.” This problem is more significant for the large county that has several dozen GIS data

editors in one department constantly making changes, as opposed to the small county with

one or two editors in a department. While the fact that there are numerous GIS files reduces

the frequency of such conflicts, the problem must nonetheless be planned for and dealt with.

There are three approaches to solving this problem.


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9.13.1 File Level Locking Under this strategy, the GIS permits only one user to edit, or to

copy a file for editing, at a time. When another user tries to access the same file for editing,

the system replies that the file is not available for editing or copying at that time and that he

should try again later.

File (Layer) Level Lock

This approach may work very well for the small county mentioned above, but will probably

not be satisfactory for the large county. It may prefer the second strategy, known as “feature-

level” locking. This is illustrated in Figure 9-29.

9.13.2 Feature-level Locking Under feature-level locking, the GIS permits two or more

editors to access the same file, but prevents them from simultaneously editing the same

feature in that file. This effectively

solves the large county’s problem and enables a number of GIS data editors to work at will

with very little chance of conflict

Feature Level Lock

Parcel Being

Edited


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9.13.3 Versioned Data Versioned data is where an alternative state of the GIS database is

created for editing. When a database is versioned, there are no locks created but instead the

changes are saved to delta tables known as “A & D Tables”. These tables store the features

to be added and deleted from the database. The master/default version of the database (prior

to the alternate version being created) is not changed at this time. There is then a process for

synchronizing the changes where the changes from the A & D tables are incorporated into

the master version of the database. This process is often called Reconcile (synchronizing the

changes) and Post (posting the changes to the default state of the database.

If the same feature is edited by more than one user, there are a couple of different ways to

resolve it. The first is when user A saves their edits, then user B saves their edits, user B will

get a warning message that the data has changed since they started editing. User B then must

review the data to ensure that it is correct before attempting to save it again. This happens

when the editors are using the same version of the database. The second is when the users

are using different versions of the database. During the Reconcile process, if there are

conflicting edits to a feature, the software will allow the user to resolve those conflicts.

Another useful feature of versioned data is the ability to capture historical data so that a user

can see what a feature was at a previous point in time. This is accomplished by time-

stamping and moving the edits from the A & D tables to H tables during the Post process.


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9.14 Review Questions

1. What is the difference between Raster and Vector Data?

2. What is a Byte?

3. What is larger, a Megabyte or a Kilobyte?

4. List and briefly describe the three types of topological data structures. What is an

example of each?

5. What is Topology?

6. If you receive data that is in another coordinate system, what must you do in order to

be able to use the data within your GIS?


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9.14.1 Review Question Answers

1. What is the difference between Raster and Vector Data?

Raster data

2. What is a Byte?

A sequence of ones and zeros in binary code

3. What is larger, a Megabyte or a Kilobyte?

A Megabyte

4. List and briefly describe the three types of topological data structures. What is an

example of each?

A Point is a one dimensional cartographic object that represents a single location. An

example would be a fire hydrant. A Line is a two dimensional cartographic object

that represents the connection between two points. An example would be a seteet

segment. A Polygon is a three dimensional cartographic object that represents an

area. An example would be a parcel polygon

5. What is Topology?

Topology is the spatial relationships of data elements. It describes how features are

connected, bounded and if they are contiguous.

6. If you receive data that is in another coordinate system, what must you do in order to

be able to use the data within your GIS?

Perform a data transformation

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