UNIT 26 - GENERAL COORDINATE SYSTEMS

UNIT 26 - GENERAL COORDINATE SYSTEMS


For Information that Supplements the Contents of this Unit:

Coordinate Systems (Dana/Geographer's Craft) -- Global systems: latitude, longitude, height, UTM; Local systems: public land rectangular surveys, metes and bounds; Miscellaneous systems: postal codes, navigational systems.
Geodetic Datums (Dana/Geographer's Craft) -- Introduction to geodetic datums; Figure of the earth: geometric models, reference ellipsoids, global coordinate systems, earth surfaces; Geodetic datums.
Global Positioning Systems (Dana/Geographer's Craft) -- US Department of Defense Satellite Navigation System; US GPS positioning services; GPS satellite signals; GPS data; position and time from GPS; GPS error sources; etc.
Latitude and Longitude (illustrated)
Modified UTM Grid Projections -- Article on applications for field and computer generated coordinate systems (McDowell).


  • A. INTRODUCTION
  • B. PLANE COORDINATE SYSTEMS - CARTESIAN COORDINATES
  • C. STORING COORDINATES
  • D. PLANE COORDINATE SYSTEMS - POLAR COORDINATES
  • E. GLOBAL COORDINATES - LATITUDE AND LONGITUDE
  • F. DETERMINING POSITION
  • REFERENCES
  • DISCUSSION AND EXAM QUESTIONS
  • NOTES

    This unit begins the module on coordinate systems. An understanding of this important topic is critical when students begin to build databases from many different sources. If you find it difficult to draw a good diagram illustrating latitude and longitude, any introductory Geography textbook will have at least one figure showing this.

    UNIT 26 - GENERAL COORDINATE SYSTEMS

    A. INTRODUCTION

    B. PLANE COORDINATE SYSTEMS - CARTESIAN COORDINATES

    Determining Coordinates

    Measuring Distance

    C. STORING COORDINATES

    Integers vs real numbers

    Computer Precision

    Precision of Cartesian Coordinates

    Propagation of coordinate errors

    D. PLANE COORDINATE SYSTEMS - POLAR COORDINATES

    E. GLOBAL COORDINATES - LATITUDE AND LONGITUDE

    Determining Coordinates

    Important Terms

    Measuring distance

    Question of Precision

    F. DETERMINING POSITION

    REFERENCES

    Goodchild, M.F., 1984. "Geocoding and Geosampling," in G.L. Gaile and C.J. Willmott, eds., Spatial Statistics and Models, Reidel Publishing Company, Dordrecht, Holland, pp. 33-53.

    Robinson, A.H., R.D. Sale, J.L. Morrison and P.C. Muechrcke, 1984. Elements of Cartography, 5th edition, Wiley, New York. See pages 63-71.

    Strahler, A.N. and A.H. Strahler, 1987. Modern Physical Geography, 3rd edition, Wiley, New York. See pages 3-8 for a description of latitude and longitude and various appendices for information on coordinate systems.

    The magazine GPS World is a good source of frequent update on the state of GPS technology and its applications.

    DISCUSSION AND EXAM QUESTIONS

    1. Determine the location of your current residence using a standard topographic map, in:

    a. Cartesian coordinates, using the lower left corner of the map as origin and the bottom and left edges as axes

    b. polar coordinates, using the lower left corner as origin and the left edge as the reference direction

    c. latitude and longitude.

    Estimate the accuracy of your results in terms of distance on the ground. How is this affected by the scale of the map? For each of the three sets of coordinates, estimate how many (a) decimal and (b) binary digits would be needed to store your answer. Assume that the range of Cartesian and polar coordinates is defined by the map sheet, but the range of latitude and longitude is defined by the globe.

    2. You are designing a database for Los Angeles County to store the boundaries of each land parcel. The required accuracy is 1 m. Assuming that Los Angeles County has dimensions on the order of 100 km by 100 km, estimate how many bits you will need to store each coordinate pair.

    3. Discuss the relative advantages of integer and real (floating point) representations of coordinates in spatial databases. When would you choose one and when the other, and why?

    4. Describe Cartesian and polar coordinate systems. What advantages do polar coordinates have, and in what types of applications might you expect them to be used?


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