Chapter 2 Design and Use of Reference Systems in GIS Database Models

Transcription

1 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 2-1

2 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 The North American Vertical Datum of 1988 is based on the World Geodetic Vertical Survey (WGVS) of 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. 2-2

3 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. 2-3

4 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 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. 2.4 State Plane Coordinate Systems Figure 2.3 Universal Transverse Mercator coordinate system. 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, 2-4

5 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. 2-5

6 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. 2-6

7 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 Conformal Conic map projection in ArcInfo. 2-7

8 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). 2-8

9 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. 2-9

10 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

11 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 Figure 2.11 Section numbering scheme within a township (Bureau of Land Management 1973). 2-11

12 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) 2-12

13 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 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. 2-13

14 Figure 2.15 City of Seattle quarter section library. Table 2.1 Federal Agency Basemap Regional Database Model Feature Categories Government areas: Animal_Habitat Interests_Land Biodiversity Land_Ownership Wetlands Parks Place_Name Administrative Areas Streets Administrative Areas Address_range Service_Territory Zip-Postal_Code School_District Census Neighborhood Demographics Public_Transport_Route Political_Boundary Political_Boundary Legal_Area Activities Inspection Natural Boundaries Maintenance Geology Hydrology Capital_Improvement_Project Soils Permit Climate Schedule Vegetation Cartographic Basemap Trail Landuse Walking Park Bicycle School Contour_Line Golfcourse Elevation_Point Landmark Block_Outline Industrial_Complex Church Airport Engineering Basemap 2-14

15 Water Sewer Stormwater Transportation Railroad Street_Centerline Intersection Bridge_or_Tunnel Electric Gas Telecommunications Cable_TV Steam Street_furniture Right_of_way Floodplain 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) 2-15

16 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 2-16

17 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 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, 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. 2-17

18 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 Geodetic Datums: Geographer s Craft. last accessed November 16, Doyle, D Development of the National Spatial Reference System, National Geodetic Survey, Coast and Geodetic Survey, last accessed November 16, 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, last accessed November 15, 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 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? 2-18

19 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. 2-19

20 township an area of the US Public Land Survey System constructed of (commonly) 36 sections. Measurements are commonly six miles on a side. 2-20