Nyerges GIS Database Primer
GISDBP_chapter_2_v20.doc
Chapter 2
Design and Use of Reference Systems in GIS Database Models
Abstract
GIS database analysts will face many choices to identify the most appropriate database model
design related to reference systems in a GIS project. The chapter presents material about
basemaps as reference information, focusing on geodetic, cadastral and land survey information
as related to coordinate systems. Different spatial reference systems use different coordinate
systems based on how large a geographic area is required in the basemap and how accurate the
coordinates must be for the application. Longitude and latitude coordinates that span the entire
earth surface are often not suitable for urban and regional database model applications.
Universal Transverse Mercator coordinate regions often cover larger areas than State Plane
coordinate systems, and thus the accuracy of state plane coordinates will be better. Coordinates
underpin land records, transportation records and water resource records in database models. We
draw from several published database models to describe database possibilities.
This chapter describes several approaches to geospatial reference systems suitable for use in
urban-regional applications of GIS. Remember that a data model language implemented in a
particular context results in a database model – the (schema) model of a particular database –
specified in terms of entity-object classes. The database models in this chapter focus on
basemaps, land records, transportation, and water resource concerns for three decision situations
– planning, improvement programming, and project implementation. Each one of those database
contexts requires a “basemap” data. Therefore, we start this discussion with the elements of a
basemap database model.
A basemap is a display that provides users with geographic orientation, and basemap data are the
elements that form the basemap database. It should be clear, that the term basemap is a relative
term, because not all people and/or applications make use of the same basemap features. That is,
orientation is a matter of context. Although base data change from urban-regional application to
application, the common character is still orientation, or what we might call spatial reference.
In a GIS context, coordinates are a foundation of databases, and surveys underlie coordinates.
The fundamentals of basemaps involve surveys and coordinate systems. Surveys are different
than coordinate systems. Surveys measure distances between objects located on a surface,
whereas coordinate systems lay out a continuous abstract numbering system (two-dimensional or
three-dimensional) for representing horizontal and vertical dimensions of space.
2.1 Geodetic Surveys
Geodesy is a sub-discipline of science/engineering in which professionals measure the shape of
the earth. Measuring the shape of the earth in two horizontal and one vertical dimension is a
fundamental basis of spatial reference, and is called a geodetic survey. A geodetic survey is a
triangulation network that covers large land masses to establish spatial reference (Figure 2.1).
Measurements on the earth’s surface are marked using geodetic control monuments. A
monument is a "spatial reference object" as something you can see, e.g., brass disc pounded into
5-1
Nyerges GIS Database Primer
GISDBP_chapter_2_v20.doc
the ground so others can find it. A network of control monuments is established to cover a large
surface, e.g., North America, Europe, South America, Asia. Three points (monuments) act as
vertices of a triangle to define a planar surface. A collection of triangles is used to cover a
surface as a reference datum (or reference surface).
Figure 2.1 Example of a geodetic triangulation network.
(US Coast and Geodetic Survey 1967)
Datums used for the US are the North American Horizontal Datum established in 1983 and a
North American Vertical Datum established in 1988 (Dana 1999, Doyle 2006). The North
American Horizontal Datum is based on the World Geodetic Survey (WGS) of 1984. The North
American Vertical Datum of 1988 is based on the World Geodetic Vertical Survey (WGVS) of
1988. WGS and WGVS were funded by the US Department of Defense as part of an initiative to
re-measure the shape of the earth on both a horizontal and vertical datum. The horizontal
positions were established in 1983 using the global positioning satellite system. The Global
Positioning System (GPS) is a constellation of satellites - 26 originally with a couple dropping
out of service over the years – that measure horizontal and vertical positions anywhere on the
surface of the earth. The datums are the physical evidence of the comprehensive surveys,
whereas GPS positioning is the measurement of single points.
5-2
Nyerges GIS Database Primer
GISDBP_chapter_2_v20.doc
Triangles in a survey datum bend along the edges of the control network as needed in piece-wise
planar surfaces. Piece-wise planar surfaces are used for representing the curved surface of
continents as, for example, the control network triangulation map of Alaska. With the horizontal
datum we associate coordinate points (longitude and latitude). Location of a monument can be
represented as a coordinate point expressed in terms of longitude and latitude.
2.2 Geographic Coordinates
Three types of coordinate systems are commonly used as the coordinate reference frameworks in
urban-regional applications: Longitude-Latitude coordinate reference (also called geographic
coordinate system), Universal Transverse Mercator (UTM) coordinate reference, and State Plane
coordinate reference. Coordinate reference systems provide the orientation across a single
continuous coverage of space.
Longitude-Latitude (elevation is important, but not relevant right now) cover a large area, e.g.
the entire earth as needed (Figure 2.2). Although longitude-latitude coordinates are available
across the entire earth, the mathematics (trigonometry) needed to process this coordinate system
is more complex than on a flat, two-dimensional (planar) surface. It is easier to work with twodimensional planar surfaces because the mathematics is based on Euclidian geometry rather than
trigonometry for computations.
Figure 2.2 Graticule showing longitude-latitude coordinate system.
5-3
Nyerges GIS Database Primer
GISDBP_chapter_2_v20.doc
Conversion between a curved two-dimensional reference system expressed as longitude-latitude
to a flat planar reference system is performed using a map projection, or more accurately, a
coordinate transformation. Thus, we convert a curved two-dimensional spatial reference
(Longitude, Latitude) to planar two-dimensional spatial reference (x, y). Two of the most popular
coordinate transformations in the US for urban-regional GIS applications are UTM coordinate
reference and State Plane coordinate reference.
2.3 Universal Transverse Mercator Coordinates
UTM coordinate system consists of 60 zones of 6 degrees longitude around the earth for the
northern hemisphere and 60 zones for the southern hemisphere. Zones 10-19 cover the USA (see
Figure 2.3). The UTM coordinate system is based on the Transverse Mercator map
transformation for all zones. A UTM coordinate system is more useful for regional work than for
urban-area work because the geographic coverage is large in comparison to urban areas. The
drawback is that this larger area introduces more distortion (error) across that space.
Figure 2.3 Universal Transverse Mercator coordinate system.
2.4 State Plane Coordinate Systems
The State Plane Coordinate System contains multiple zones for most states, although for a few
states a single zone is used (Figure 2.4). The map projections used are Transverse Mercator,
5-4
Nyerges GIS Database Primer
GISDBP_chapter_2_v20.doc
Lambert Conformal Conic, and Space Oblique Mercator. The main idea is to use a zone
configuration that minimizes distortion to within +/- 1 foot accuracy. Transverse Mercator is
used when a state is more north-south in extent, except for very large states such as California.
For example, Illinois, which is a north-south state, uses the Transverse Mercator projection to
establish an east and west zone to reduce distortion (See Figure 2.5). Lambert Conformal Conic
is used when a state is more of east-west extent. For example, Washington State uses north and
south zones to cover the state (Figure 2.6). All zones use a false origin that places the land area
in the upper right quadrant (that quadrant with positive x and positive y numbers) of a Cartesian
coordinate system.
Figure 2.4 State plane coordinate reference zones across the contiguous US.
5-5
Nyerges GIS Database Primer
GISDBP_chapter_2_v20.doc
Figure 2.5 State plane coordinates based on Transverse Mercator for Illinois, showing false
origin and coordinate axes for the West Zone.
Figure 2.6 State plane coordinates based on Lambert Conformal Conic for Washington South
Zone.
5-6
Nyerges GIS Database Primer
GISDBP_chapter_2_v20.doc
Coordinate data and the projections upon which they are based are described in a GIS using
metadata. The Greenvalley lowland coordinate system is created from a Lambert Equal-Area
projection (Figure 2.7a) and the metadata for this projection can be stored in the GIS (Figure
2.7b).
Figure 2.7a Defining a coordinate system using a Lambert Azimuthal Equal-Area map projection
in ArcInfo.
5-7
Nyerges GIS Database Primer
GISDBP_chapter_2_v20.doc
Figure 2.7b Metadata description in ArcCatalog for projection of Figure 2.7a.
2.5 Cadastral Control Surveys
Although coordinate systems are the fundamental basis of establishing coordinate data values for
points in spatial databases, many people (hence GIS applications) obtain reference to the world
through a land feature. Land feature boundaries are described using land surveys. There are two
types of land surveys – cadastral control land surveys and local property surveys. Cadastral land
surveys are used for describing large tracts of private/public land. They provide descriptions of
such areas so that local property surveyors can use that cadastral control to reference smaller
properties. Local property surveys are used for measuring boundaries of a private property,
commonly smaller areas. Land surveys establish the data that are used in populating database
models. We address cadastral surveys in this section and local property surveys in section 2.2.
Cadastral surveys are of two types in the US. (Figure 2.8). One is metes and bounds and the
second is US Public Land Survey System (USPLSS).
5-8
Nyerges GIS Database Primer
GISDBP_chapter_2_v20.doc
Figure 2.8 Cadastral surveys in the continental US.
(Bureau of Land Management 1973)
Metes and bounds is the land description system of the 13 original colonies and the Spanish
colonies. It is a non-systematic (feature-based) approach to land boundary description. That is,
the monuments for metes and bounds were pre-existing large features, such as large trees, large
rocks, river bends, etc. The surveys were developed independently of each other based on local
features.
USPLSS contains many surveys across the country (Figure 2.9). It is a systematic approach to
land boundary description. The systematic approach is due to the use of a mathematical
measurement of regular sized areas needed to measure very large amounts of land. The USPLSS
was started by George Washington, a topographer (land surveyor/mapper) by profession. After
American Revolution, US government lacked cash resources to pay soldiers for their effort, but
was rich in land resources. With considerable land to the west of the original 13 colonies,
property was sold in order to pay soldiers and fill the US Treasury. The survey process, called
the Northwest Ordinance Survey of 1785, began in the Ohio Valley. Five surveys were started,
likely because it was a challenge to get the process right with the survey technology of the day.
5-9
Nyerges GIS Database Primer
GISDBP_chapter_2_v20.doc
Figure 2.9 Surveys of the USPLSS (Bureau of Land Management 1963).
The surveys became more systematic as they moved from east to west, resulting in larger tracts
in the central and western US. For example, the Mississippi survey covers the central states, and
the Willamette Survey covers the Pacific Northwest states of Oregon and Washington.
Each survey starts from initial point (Figure 2.10) which lies at the intersection of the principal
meridian and base line. A meridian is a line of longitude and a base line is a line of latitude. In
the Willamette Survey the initial point was physically marked by the Willamette Stone. The
Willamette Stone was a small stone obelisk originally located in the western hills of Portland,
Oregon. Thus, it marked the intersection and origin of the Willamette Meridian and Willamette
Baseline, which defined the grid system of sections and townships from which all real property
in the states of Oregon and Washington has been measured. Initial points demarcate the origin
for measuring townships. In Figure 2.10, townships are measured six miles per side. Each block
within the figure represents one township. At every fourth township north and south, a readjustment accounts for the convergence of longitudinal lines toward the north pole. Townships
are enumerated in north-south and east-west directions. Townships north of the base line are
labeled “north”; and townships south of the base line are labeled “south”. Townships east of the
principal meridian are labeled “east” and those west of the principal meridian are labeled “west”.
Townships are enumerated north/south using a “T” for township and enumerated east/west using
an “R” for range in grid reference of Figure 2.10.
5-10
Nyerges GIS Database Primer
GISDBP_chapter_2_v20.doc
Figure 2.10 Grid layout of a USPLSS survey (Bureau of Land Management 1973).
Each township is (theoretically) composed of 36 sections, six sections in a row and six sections
in a column, numbered in serpentine manner (Figure 2.11). Each of the sections measures 1 mile
on a side by 1 mile on a side, hence the area is 1 square mile (Figure 2.12). Each section is
composed of section subdivisions: one-half (1/2) sections, quarter (1/4) sections, and quarterquarter (1/4 1/4) sections (1/4 of 1/4 of a square mile is 40 acres). Section corners are marked
with physical monuments such as those in Figure 2.13.
Figure 2.11 Section numbering scheme within a township (Bureau of Land Management 1973).
5-11
Nyerges GIS Database Primer
GISDBP_chapter_2_v20.doc
Figure 2.12 Section subdivisions (Bureau of Land Management 1973).
Figure 2.13 Physical monuments for section and quarter section corners.
(Bureau of Land Management 1974)
5-12
Nyerges GIS Database Primer
GISDBP_chapter_2_v20.doc
A township section can contain metes & bounds surveys (Figure 2.14). Those metes and bounds
local surveys would have been developed before the USPLSS was put in place in that particular
area (Figure 2.14).
Figure 2.14 Local property surveys (Bureau of Land Management 1973).
2.6 GIS Database Model Frameworks
The survey systems described above form the foundation of a basemap database model, and
would result in a GIS database layout like that for the City of Seattle (Figure 2.15). However,
there is no single definitive interpretation of what is included in a basemap database model, as it
depends on the application context. For example, a basemap for regional databases composed by
federal agencies to describe large tracks of land contains seven different types of basemaps. (See
Table 2.1) Federal basemaps include government areas basemap, natural boundaries,
administrative areas, cartographic basemap, engineering basemap, cadastral, and survey control.
There are several features within each of those basemap categories (the indented feature names
in Table 2.1), that reveal the considerable variation among information used to establish
basemaps. An urban basemap database model example is provided in Figure 2.16. This
database model originated from the common categories of more than 70 municipalities in
Ontario Province, Canada. The commonality might have reduced the number of categories as
compared with Figure 2.16, and the capital improvement features included are seldom associated
with basemap features. Nonetheless, the point here is that database models are different across
GIS applications.
5-13
Nyerges GIS Database Primer
GISDBP_chapter_2_v20.doc
Figure 2.15 City of Seattle quarter section library.
Table 2.1 Federal Agency Basemap Regional Database Model Feature Categories
Animal_Habitat
Biodiversity
Wetlands
Government areas:
Interests_Land
Land_Ownership
Parks
Place_Name
Streets
Address_range
Zip-Postal_Code
Census
Demographics
Political_Boundary
Legal_Area
Administrative Areas
Administrative Areas
Service_Territory
School_District
Neighborhood
Public_Transport_Route
Political_Boundary
Activities
Inspection
Maintenance
Natural Boundaries
Geology
Hydrology
Soils
Climate
Vegetation
Cartographic Basemap
Landuse
Park
School
Golfcourse
Landmark
Industrial_Complex
Church
Airport
Capital_Improvement_Project
Permit
Schedule
Trail
Walking
Bicycle
Contour_Line
Elevation_Point
Block_Outline
Engineering Basemap
5-14
Nyerges GIS Database Primer
GISDBP_chapter_2_v20.doc
Right_of_way
Floodplain
Water
Sewer
Stormwater
Transportation
Railroad
Street_Centerline
Intersection
Bridge_or_Tunnel
Electric
Gas
Telecommunications
Cable_TV
Steam
Street_furniture
Cadastral
TaxParcel
Land_Ownership
Owner_Parcel
Separate_Right
Encumbrance
Regulated_Use
Use_Restriction
Boundary
Address
Figure 2.17 Basemap portion of an urban database model.
(Excerpted from ESRI 2006)
5-15
Nyerges GIS Database Primer
GISDBP_chapter_2_v20.doc
Let us summarize coordinate control in terms of macro-scale, meso-scale, and micro-scale spatial
referencing. In macro-scale spatial referencing, geodetic surveys describe very large areas of
earth, hence coordinate systems describe the spatial positioning for those areas. Geodetic
monuments orient cadastral surveys, i.e., latitude/longitude point is used as “origin” for UTM
and State Plane coordinate systems. In meso-scale spatial referencing, cadastral control surveys,
with township and section referencing are used to control local property surveys – spatial
positioning for local property. State plane coordinates are used to describe spatial position for the
section corners, 1/4 section corners etc. In micro-scale spatial referencing, local property
surveys (described in more detail in land records section below) make use of a local coordinate
system when an electronic distance measuring instrument, like a GPS unit, is used to collect
coordination information.
Popular GPS units lack enough precision to be used for local property surveys – engineering
level measurements. However, surveying-grade GPS units are capable of improving their
positional accuracy by triangulating positioning with the position of a satellite and the fixed
ground station. Currently, these units achieve a sub-centimeter positional accuracy.
2.7 Comparing and Contrasting Basemap Database Model Needs
Database model considerations are different than data model considerations. Data models
support the implementation of database models. Consequently, any software language used for
encoding a database model must be complete enough to express the information needs inherent
in the database model. Therefore, we first consider the database model need as a need for
information content. Then we consider the needs of the data model as a way of supporting the
creation of that information content.
The database model considerations are the data category names together with the attribute
content, whereas the data model need focuses on topologic capability. That is, the spatial
topologic data objects needed to store the line topology of a geodetic network as part of
basemap, is a different kind of need than simply to store a data category of “survey lines” or
“monuments”. Thus, a database designer would have to make a decision to store survey lines as
topological data or as shapes (a non-topological type of data organization) only. An example
would be whether monuments are to be stored as nodes supporting the topological relationship of
connectivity or as geometric points.
Comparing basemap database model needs is a matter of understanding the context for
information use. To do that we must consider basemap as a term relative to other data which
leads us to the question of “basemap for whom?” In this particular case, we will assume the
information user to be the “GIS analysts”, as they are the ones most likely to make decisions
about what basemaps to use. In the following sections, we broaden that idea of “information
user” and carry it all the way to “policy/decision maker”.
2.8 Summary
The fundamental content of a basemap is the coordinate control – that is the control points to
establish the coordinate reference system in relation to the “real world” positions. When making
5-16
Nyerges GIS Database Primer
GISDBP_chapter_2_v20.doc
choices about database models, a GIS analyst can consider the issues from the perspective of
planning-oriented, programming-oriented, and project-oriented information contexts. Planning
database models focus on large geographic domains, with many locations and coarse resolution
for control monuments. Programming database models focus on several places in the geographic
domain, region-wide coordinates, and coarse resolution for control monuments. Impacts have
not been a big part of this process. Project database models focus on few locations, but more
local geographic domain and fine monument resolution.
When comparing/contrasting data model needs, we ask whether topological construct categories
are needed for basemaps. In regards to needs for a planning basemap data model, a shapefile
non-topological structure is probably fine, as it is a “broad-based” orientation. However, this
depends on the other value-added data categories that will make use of the “base data” in
reference. In regards to needs for a programming basemap data model, a shapefile nontopological structure is likely appropriate as there is not typically any need for adjacency
relationships. However, this depends on the other value added data categories that will make use
of the “base data” in reference. In regards to needs for a project basemap data model, shapefile
non-topological structure is likely fine, as basemap data is still fundamentally only “referential”
in character. However, this depends on the other value added data categories that will make use
of the “base data” in reference. The message here is that basemap data model often depends on
the context of the data layer that is serving as the database model for the basemap. As such, a
geometry-only content is fine if no application requires topological connectivity. In the next
section on land records we return to this issue, put it in context, and expand the concept of a
basemap a bit more.
Basemaps are seldom ever solely used as a document to address decision situations – hence
basemap for what other data? Of course, parts of organizations are responsible for developing
the “basemap data” – but these responsibilities are always carried in cooperation with users of
other “value added” data categories such as land records. The next chapter about land records
will address this issue of comparing database models and data models from an information
“value added” context.
2.9 References
Brown, P. 1993 In Multipurpose Land Information Systems: The Guidebook, v.1, edited by
Brown, P.M. and Moyer, D.D., ch. 5. Federal Geodetic Control Committee, Washington, DC,
1989.
Bureau of Land Management (BLM), 1963: Principal meridians and base lines governing the
United States public land surveys [map], 1:8,000,000. U.S. Department of the Interior, U.S.
Government Printing Office, Washington, DC
Bureau of Land Management, 1973: Manual of Instructions for the Survey of the Public Lands of
the United States, U.S. Department of the Interior, U.S. Government Printing Office,
Washington, DC, 333 pp.
5-17
Nyerges GIS Database Primer
GISDBP_chapter_2_v20.doc
Bureau of Land Management, 1974: Restoration of Lost or Obliterated Corners & Subdivision of
Sections, U.S. Department of the Interior, U.S. Government Printing Office, Washington, DC, 40
pp.
Dana, P. 1999. Geodetic Datums: Geographer’s Craft.
http://www.colorado.edu/geography/gcraft/notes/datum/datum_f.html, last accessed November
16, 2006.
Doyle, D. 2006. Development of the National Spatial Reference System, National Geodetic
Survey, Coast and Geodetic Survey, http://geodesy.noaa.gov/PUBS_LIB/develop_NSRS.html,
last accessed November 16, 2006.
Epstein, E. and P. Brown (1993) Land interests in D. Moyer, Multipurpose Land Information
Systems: the Guidebook, Chapter 4, National Oceanic and Atmospheric Administration,
Bethesda MD.
ESRI (Environmental Systems Research Institute) 2006 Urban Basemap Database Model,
http://support.esri.com/index.cfm?fa=downloads.dataModels.filteredGateway&dmid=33,
last accessed November 15, 2006.
National Research Council, 1982: Modernization of the Public Land Survey System, National
Academy Press, Washington, DC, 74 pp.
US Coast and geodetic Survey 1967.Status of horizontal control, United States [map], July 1,
1967, Washington, DC : the Survey.
2.10 Review Questions
1. What is a datum?
2. What is a survey and how does it differ from a coordinate system?
3. A spatial reference system would include street address systems and coordinate systems. From
your personal experience about street address systems, how do they differ from coordinate
systems?
4. What is the difference between a geodetic survey and longitude-latitude coordinate system?
5. Describe the UTM coordinate system
6. Describe the State Plane Coordinate System
7. What is the difference between the State Plane Coordinate System and US Public Land Survey
System?
5-18
Nyerges GIS Database Primer
GISDBP_chapter_2_v20.doc
8. Why would we choose to use either UTM or State Plane as a coordinate system for a
database?
9. Why does the US have a US Public Land Survey System, and what is the value of the land
partitioning approach used in the system?
10. What role does the spatial reference system play among the basic components of a basemap
database model?
2.11 Glossary
basemap – the foundation for urban and regional database models that rely on proper choice of a
coordinate system to match the geographic extent and accuracy needs of the GIS project.
cadastral – land record information constituted of fiscal or juridical categories
class – a generic term for a data category composed by bundling observations of like kinds; for
example a feature class in ArcGIS
coordinate systems – a way of providing a numeric description of a space (commonly in 2 or 3
dimensions)
database model – A schema and data dictionary associated with the outcomes of a particular
database design process.
geodetic survey – a large area (continent in scope) survey to measure coordinates for longitude,
latitude.
land survey – field work that develops a description of the extent of areas; a major example is
the US Public Land Survey System
metes and bounds – a form of land survey that makes use of physical land marks for survey
corners.
property survey – land survey at a micro scale.
planimetric – land record information developed from ground features than can be seen in aerial
photographs.
section – in the US PLSS a 1 one mile by 1 mile surveyed portion of land; 36 sections constitute
a township.
survey – collecting observations about the world, see land survey, property survey, geodetic
survey.
5-19
Nyerges GIS Database Primer
GISDBP_chapter_2_v20.doc
township – an area of the US Public Land Survey System constructed of (commonly) 36
sections. Measurements are commonly six miles on a side.
5-20