UNIT 35 - RASTER STORAGE

UNIT 35 - RASTER STORAGE

Compiled with assistance from Donna Peuquet, Pennsylvania State University

  • A. INTRODUCTION
  • B. STORAGE OPTIONS FOR RASTER DATA
  • C. RUN ENCODING
  • D. SCAN ORDER
  • E. DECODING SCAN ORDERS
  • REFERENCES
  • DISCUSSION AND EXAM QUESTIONS
  • NOTES

    The latter portion of this unit and the following two require familiarity with numbering systems in base 2 and 4 as well as techniques for conversion between these and decimal. You may wish to provide your students with some background material on these topics before tackling these units.

    UNIT 35 - RASTER STORAGE

    Compiled with assistance from Donna Peuquet, Pennsylvania State University

    A. INTRODUCTION

    Why use raster?

    Objectives

    B. STORAGE OPTIONS FOR RASTER DATA

    What if there is more than one layer?

    What do raster systems store in each pixel?

    Raster/Vector combinations

    C. RUN ENCODING

    Problems

    D. SCAN ORDER

    1. Row order

    2. Row prime order (Boustrophedon)

    3. Morton order

    4. Peano scan (also Pi-Order or Hilbert)

    Comparing scan orders

    E. DECODING SCAN ORDERS

    Method

    Generalization

    REFERENCES

    Abel, D.J., 1986. "Bit interleaved keys as the basis for spatial access in a front-end spatial database management system," Proceedings, Tesseral Workshop #2, Reading, England.

    Franklin, W., 1979. "Evaluation of algorithms to display vector plots on raster devices," Computer Graphics and Image Processing 11:377-397.

    Goodchild, M.F., and A.W. Grandfield, 1983. "Optimizing raster storage: an examination of four alternatives," Proceedings, AutoCarto 6, Ottawa, 1:400-7.

    Peuquet, D., 1981. "An examination of techniques for reformatting digital cartographic data, Part II, The vector-to-raster Process," Cartographica 18(3):21-33.

    DISCUSSION AND EXAM QUESTIONS

    1. What systems are used for topographic map indexing in the US and other countries? Discuss the use of digit interleaving in this context, using different national examples.

    2. The term metadata is used to refer to information carried with a map layer, such as its accuracy, numbers of rows and columns, type of data stored for each pixel, etc. Discuss the importance of metadata in limiting the operations which a user is allowed to perform on a map layer.

    3. Raster and vector have developed as two partially independent traditions in GIS. Summarize the dimensions of the raster-vector debate, particularly in the importance of spatial objects in the two systems.

    4. All of the scan orders discussed in this unit visit each pixel exactly once. Discuss the potential advantages, if any, of scan orders which visit certain pixels more than once. Give examples.

    5. Any raster GIS places restrictions on what can be stored in each pixel of a map and what operations can be carried out. Discuss this point as it applies to IDRISI, and any other raster GIS to which you may have access. Will it let you store an alphabetic value such as A in a pixel and then allow you to carry out arithmetic operations on this layer?

    6. Find out what raster storage option (row by row, run encoded, pixel by pixel, layer by layer, etc.) is used by IDRISI and any other raster GIS (GRASS, MAP, etc.) to which you have access.


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    Last Updated: August 30, 1997.