Susan Cutter et al. Aug, 2002, The Professional Geographer, 54(3): David Tenenbaum GEOG 070 UNC-CH Spring 2005

Transcription

1 What is Geography? Geography is not just about city and country names Geography is not just about population and growth Geography is not just about rivers and mountains Geography is a broad field that studies all sorts of phenomena on the Earth s surface, including natural and human components, and these are termed human and physical geography. Geography is present in your everyday life: The air you breathe, the water you drink, the place where you live, the people you meet

2 Ten Big Questions that Geographers Answer 1. What makes places different from one another, and why is this important? 2. Is there a deeply human need to organize space by creating arbitrary borders, boundaries, and districts? 3. How to delineate space? 4. Why do people, resources, and ideas move? 5. How has the Earth been transformed by human action? 6. What role will virtual systems play in learning about the world? 7. How do we measure the immeasurable? 8. What role has geographical skill played in the evolution of human civilization and what role can it play in predicting the future? 9. How and why do sustainability and vulnerability change from place to place and over time? 10. What is the nature of spatial thinking, reasoning, and abilities? Susan Cutter et al. Aug, 2002, The Professional Geographer, 54(3):

3 Geographers Perspectives on the World Location matters Real-world relationships Horizontal connections between places Importance of scale (both in time and space)

4 Introduction to Maps Definition: A graphic depiction on a flat medium of all or part of a geographic realm in which real world features have been replaced with symbols in their correct spatial location at a reduced scale. To map is to transform information from one form to another --- Mathematics Earth surface map Paper --- Geography

5 Map Elements Most common elements include: map/spatial data title legend scale north arrow inset(s) textual information borders & neatlines coordinate grid

6 Data Symbolization There are a number of characteristics of symbols that we can use of to make visual distinctions in thematic information (Jacques Bertin s Visual Variables): Size Shape Color Hue (color) Color Value (intensity) Texture Orientation Arrangement

7 Representing the Real World In a GIS, the representation of real world phenomena makes use of digital data formats Earth surface Digital data representation

8 Binary Notation Everything is represented as 0s and 1s in a computer. These two-state forms correspond to yes/no, on/off, open/closed Binary Decimal 1 digit 0, 1 1 bit 0,1,2, 9 2 digits 00, 01 2 bits 00, 01, 10, 11 97, 99 3 digits 000, bits 000, 001, 010, , 003, 100, , , 999 One to one correspondence Decimal Binary ?

9 Binary Notation Decimal: 72,479 = 70,000 = ,000 = = = = Binary: Note: In binary = = 20

10 Bits and Bytes 8 bits = 1 byte 1 bit = 1 binary digit 1 byte = 8 bits bytes = 1 Kb 1024 Kb = 1 Mb 1024 Mb = 1 Gb 1024 Gb = 1 Tb 1024 Tb = 1 Pb

11 ASCII Encoding If computers store everything using 0s and 1s, then how are characters represented? The ASCII (American Standard Code for Information Interchange) code assigns the numbers 0 through 127 to 128 characters, including upper and lower case alphabets plus various special characters, such as white space etc. e.g. decimal 85 is assigned to represent upper case U. In binary, = 85. Thus the computer represents U using Files which contain information encoded in ASCII are easily transferred and processed by different computers and programs. These are called ASCII or text files.

12 Two Fundamental Sorts of Representations Regardless of what phenomena of interest we chose to include in our geographic representation, we first must choose between a model that either represents geography as discrete objects OR represents geography as fields Most GIS approaches focus on the discrete object view, and we ll look at it in greater detail, although we will look at field representations as well

13 Entities Data Objects In the discrete object view, we associate each and every entity we wish to represent with its own data object a digital building block used in a data structure to represent an entity (x,y) (x,y) (x,y) (x,y) (x,y) (x,y) (x,y) (x,y) point line polygon (area) (x,y) (x,y)

14 The Field View In contrast to the discrete object view, which models the world in terms of entities, the field view approaches the world as consisting of continuous phenomena: The field view represents the real world as a finite number of variables, each one defined at every possible position (p.69 of the text) This view is inherently problematic for digital representation, which necessarily has to discretize the world into some set of minimum units

15 Spatial Data Models Raster uses individual cells in a matrix, or grid, format to represent real world entities Vector uses coordinates to store the shape of spatial data objects

16 Vector Data Model - Objects Features in the real world are represented by objects that are chosen to approximate their shape Geographic Primitives Points 0 dimensional Lines 1 dimensional Polygons 2 dimensional

17 Vector Data Model - Topology Topology defines spatial relationships. The arcnode data structure supports the following topological concepts: Area definition: Arcs connect to surround an area, defining a polygon Containment: Nodes (or arcs) can be found within a polygon Connectivity: Arcs connect to each other at shared nodes Contiguity: Arcs have a defined direction, and left and right sides

18 Raster Data Model The raster data model represents the Earth s surface as an array of two-dimensional grid cells, with each cell having an associated value: Cell (x,y) Cell value rows Cell size = resolution columns

19 Cell Size & Resolution The size of the cells in the raster data model determines the resolution at which features can be represented The selected resolution can have an effect on how features are represented: 10 m Resolution 1 m Resolution 5 m Resolution

20 Rules for Assigning Cell Values Cell values are assigned to cells accorded to some set of rules, and selecting those rules differently can also effect the representation of features:

21 Georeferencing GOAL: To assign a location to all the features represented in our geographic information data In order to do so, we need to make use of the following elements: ellipsoid/geoid To determine a position datum on the Earth, you ll need to understand how projection these elements relate to coordinate system each other in order to scale specify a position The next few lectures will introduce you to these elements

22 Our knowledge of the shape of the Earth has evolved in 3 steps: 1. Flat surface: There are some people today that still believe that the earth is flat (International Flat Earth Society) 2. Sphere: Shape of the Earth In 600 BC, Pythagoras wrote that he believed that we must live on a body of a perfect shape a perfect sphere

23 2. Sphere cont.: Shape of the Earth In ~200 BC, Greek philosopher Aristotle argued that the Earth must be a sphere, giving the following evidence: Ships always disappear from view hull first, mast last, rather than becoming ever smaller dots on the horizon of a flat earth Also, further evidence was provided by making an astronomical observation, that the Earth s shadow on the moon is always circular

24 3. Ellipsoid: Shape of the Earth In 1670, based on his theory of gravity, Isaac Newton proposed that the Earth must bulge slightly at the equator due to the centrifugal force generated by the Earth s rotation, therefore we might expect a slight flattening of the shape of the Earth at the poles. Newton suggests that the Earth is elongated slightly along the equator

25 3. Ellipsoid cont.: Shape of the Earth Newton s theory was confirmed by a series of measurements taken from by expeditions sent to Ecuador and Finland to measure the ground distance of one degree of latitude (measured using astronomical angles). It was found that the distance near the pole was slightly greater due to the flattening of the Earth One degree of latitude on the ground is different in length in these two locations due to the flattening of the Earth near the poles

26 What is a Projection? Map projection - The systematic transformation of points on the Earth s surface to corresponding points on a planar surface The easiest way to imagine this is to think of a light bulb inside of a semi-transparent globe, shining features from the Earth s surface onto the planar surface

27 Projections Distort Because we are going from the 3D Earth 2D planar surface, projections always introduce some type of distortion When we select a map projection, we choose a particular projection to minimize the distortions that are important to a particular application

28 Three Families of Projections There are three major families of projections, each tends to introduce certain kinds of distortions, or conversely each has certain properties that it used to preserve (i.e. spatial characteristics that it does not distort): Three families: 1. Cylindrical projections 2. Conical projections 3. Planar projections 3 2 1

29 Preservation of Properties Every map projection introduces some sort of distortion because there is always distortion when reducing our 3- dimensional reality to a 2-dimensional representation Q: How should we choose which projections to use? A: We should choose a map projection that preserves the properties appropriate for the application, choosing from the following properties: 1. Shape 2. Area 3. Distance 4. Direction Note: It may be more useful to classify map projections by the properties they preserve, rather than by the shape of their developable surfaces

30 Tissot s Indicatrix Tissot s Indicatrix is a graphical tool which we can use to assess the properties preserved by a projection Tissot s Indicatrix allows us to take a feature that is a perfect circle before projection, and then see how it looks once projected (usually the distortion causes it to be elliptical in shape) We can calculate s = "area scale" = the product of semimajor and semi-minor axes of the ellipse

31 Coordinate Systems We are going to discuss two kinds of coordinate systems: The geographic coordinate system expresses positions in terms of latitude and longitude, and like all spherical coordinate systems, the definitions of these quantities are in terms of angles Planar coordinate systems place a Cartesian grid on a 2-dimensional surface (like our projected maps and geodatabases) and express a position in terms of (x,y) coordinate pairs

32 The Geographic Coordinate System Every coordinate system needs to have an origin where the coordinate values are zero, and the geographic coordinate system s origin is specified using: The Equator is the origin for latitude positive in N. hemisphere negative in S. hemisphere The Prime Meridian is the origin for longitude positive in E. hemisphere negative in W. hemisphere

33 Planar Coordinate Systems Once we start working with projected spatial information, using latitude and longitude becomes less convenient We can instead use a planar coordinate system that has x and y axes, an arbitrary origin (a Cartesian plane), and some convenient units (e.g. ft. or m.) When applied in a geographic context: Eastings are x values Northings are y values

34 Planar Coordinate Systems Some common examples of planar coordinate systems that are in use: The Universal Transverse Mercator Coordinate System is used throughout the world In the United States, the State Plane Coordinate System is used, which provides coordinate grids for each state, using multiple zones and grids in many of the larger states in order to minimize projection distortions

35 Universal Transverse Mercator

36 Universal Transverse Mercator We can describe the location of each zone in terms of its central meridian, which falls on on a meridian with an integer value of longitude

37 Universal Transverse Mercator The central meridian, which runs down the middle of the zone, is used to define the position of the origin Distance units in UTM are defined to be in meters, and distance from the origin is measured as an Easting (in the x-direction) and a Northing (in the y-direction) The x-origin is west of the zone (a false easting), and is placed such that the central meridian has an Easting of 500,000 meters

38 State Plane Coordinate System The State Plane Coordinate System (SPCS) is only defined and used in the United States Like UTM, it is divided into zones, but here zones are fully contained within states Some larger states contain multiple zones Original units are feet, many states are now switching to meters

39 Representing Scale on Maps Definition: The scale of a map is ratio between distances on the map and the corresponding distances in the real world. Scale representation on the map: Representative fraction (RF): 1:100,000, 1 to 100,000, or 1/100,000 Verbal: 1 inch is equal to 50 miles 10 miles Graphic: Scale bar Purpose (or a kind of question that scale can answer): Scale information allows us to answer questions like: 1 inch on a 1:24000 map represents what distance on the surface of the Earth? (2000 feet)

40 Map Scale and Spatial Resolution Spatial resolution is a concept that is related to map scale: It is determined by the smallest physical mark that can drawn (or discerned) on a map accurately This limits the smallest geographic feature or distance that can be represented accurately on the map This is directly related to the scale of the map - Why? Consider the relationship between the size of the smallest mark we can make (or see/accurately measure) and the smallest feature in the real world that can be represented on the map

41 The Two Types of Data in GIS Spatial data: Describing where things are AND Attribute data: Describing what things are Example: A point specified by UTM coordinates Easting = 50,000 m Northing = 5,000,000 m Zone =17 This specifies the location of a point of the ground The nature of the real-world feature located at this point would be recorded in the attribute data Traditionally, geographic data and attributes were recorded on paper too (maps), and these had the same problems as a phone book

42 Advantages of Databases over Files DBs avoid redundancy and duplication DBs reduce data maintenance costs Applications are separated from the data Applications persist over time Support multiple concurrent applications DBs facilitate better data sharing Security and standards can be defined and enforced using DBs

43 Disadvantages of Databases over Files Expense of databases Complexity of databases Performance of databases especially with complex data types (including spatial data) Integration with other systems can be difficult, especially if those systems don t use the same data model

44 The Role of DBMS in GIS System Geographic Information System Database Management System Task Data load Editing Visualization Mapping Analysis Storage Indexing Security Query Data

45 Relational Data Model The relational model organizes data in a series of twodimensional tables, each of which contains records for one kind of entity records PID # Fields Name Major Phone # John Geog David Comm This model is a revolution in database management It replaced almost all other approaches in database management because it allows more flexible relations between kinds of entities

46 Relation Rules (Codd, 1970) Only one value in each cell (intersection of row and column) All values in a column are about the same subject Each row is unique No significance in column sequence No significance in row sequence

47 Global Positioning System (GPS) A space-based 3-dimensional measurement and positioning system that operates using radio signals from satellites orbiting the earth Created and maintained by the US Dept. of Defense and the US Air Force The system as a whole consists of three segments: satellites (space segment) receivers (user segment) ground stations (control segment) Note: Russia and a European consortium are implementing similar systems.

48 GPS Space Segment (Satellites) 24 NAVSTAR satellites in the GPS constellation orbit the Earth every 12 hours ~11,000 miles altitude (a very high orbit) positioned in 6 orbital planes (4 per plane) orbital period & planes are designed to keep 4-6 satellites above the horizon at any time everywhere on the planet controlled and monitored by five ground stations around the globe

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50 GPS User Segment (Receivers) Ground-based devices that can read and interpret the radio signals from several of the NAVSTAR satellites at once Use timing of radio signals to calculate the receiver s position on the Earth's surface Calculations result in varying degrees of accuracy that depend on: quality of the receiver user operation of the receiver local & atmospheric conditions current status of system