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Spatial Data Formats

Spatial Data Formats. Need data for a GIS?. Just photograph a topographic map Better yet, download one from the internet But are the roads, buildings, and other “objects” on this photo GIS layers?. Stages of development:.

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Spatial Data Formats

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  1. Spatial Data Formats CS 128/ES 228 - Lecture 4b

  2. Need data for a GIS? • Just photograph a topographic map • Better yet, download one from the internet • But are the roads, buildings, and other “objects” on this photo GIS layers? CS 128/ES 228 - Lecture 4b

  3. Stages of development: • Conceptual model: select the features of reality to be modeled and decide what entities will represent them • Spatial data model: select a format that will represent the model entities • Spatial data structure: decide how to code the entities in the model’s data files CS 128/ES 228 - Lecture 4b

  4. 2. Spatial data models • Raster • Vector • Object-oriented and… attribute data Spatial data formats: Fig. 3.1 in 3rd ed. CS 128/ES 228 - Lecture 4b

  5. Raster format • Features represented by cell contents • Spatial precision limited by cell size • Surfaces modeled as continuous values (almost) Fig. 3.9 in 3rd ed. CS 128/ES 228 - Lecture 4b

  6. Vector format • Discrete features explicitly represented • Spatial precision limited by number format • Surfaces shown by contours rather than continuous values CS 128/ES 228 - Lecture 4b Fig. 3.9 in 3rd ed.

  7. Object-oriented formats Leave details for CS majors Fig. 4.17 in 3rd ed. CS 128/ES 228 - Lecture 4b

  8. Thematic data (a.k.a. “attribute data”) • Quantitative or descriptive • May represent 1 or many themes • Tied to a spatial reference • Represented differently in raster vs. vector formats CS 128/ES 228 - Lecture 4b

  9. Scales of measurement Heywood et. al. 2006 – Table 2.1 CS 128/ES 228 - Lecture 4b

  10. Spatial modeling in raster format • Basic entity is the cell • Region represented by a tiling of cells • Cell size = resolution • Attribute data linked to individual cells CS 128/ES 228 - Lecture 4b

  11. Attribute data in raster format Attribute data are used to create symbology for each cell CS 128/ES 228 - Lecture 4b

  12. Additional attribute data • Some GISs provide a VAT linked to individual cells (e.g. ArcInfo GRID) • VAT data then accessible to database management system Unlimited additional fields  CS 128/ES 228 - Lecture 4b

  13. Attribute data for a vector layer • Each entity is linked to a row in an attribute table • Themes not (usually) displayed but available via Identify tool CS 128/ES 228 - Lecture 4b

  14. Vectors are good at modeling … • roadways or wiring consist of discrete components; types and order of the connections are key • Spaces between the network components generally not of interest … networks Bottom : http://www.dunereview.com/electricalupgrade-1.htm CS 128/ES 228 - Lecture 4a

  15. Rasters are good at modeling … • they model a continuous feature as a 2- or 3-D layer • every location has a value, even if only interpolated from discrete samples … surfaces CS 128/ES 228 - Lecture 4a Both: http://snobear.colorado.edu/Markw/Research/ESRI/ESRI.html

  16. Topographic maps use contours… …but the elevation between contour lines is undefined CS 128/ES 228 - Lecture 4a

  17. Digital terrain models Every cell has an elevation value Fig. 3.32 in 3rd ed. CS 128/ES 228 - Lecture 4a

  18. Precision agriculture Aerial photograph Soil pH Crop yield CS 128/ES 228 - Lecture 4a

  19. Oceanography Estimate of phytoplankton distribution in the surface ocean: global composite image of surface chlorophyll a concentration (mg m-3) estimated from SeaWiFS data (Source: NASA Goddard Space Flight Center, Maryland, USA and ORBIMAGE, Virginia, USA). CS 128/ES 228 - Lecture 4a

  20. Rasters are a type of Tesselation • A closed shape or polygon that repeats on all sides without any gaps or overlaps • Three regular polygons tesselate the plane: Equilateral triangle Square Hexagon CS 128/ES 228 - Lecture 4b

  21. Tilings In 1922 Escher visited the Alhambra palace and saw the wall tilings of the Moors. He was excited to find other artists who had been captivated by tilings, but also made this revealing comment: "What a pity their religion forbade them to make graven images." CS 128/ES 228 - Lecture 4b

  22. Escher’s “tesselations” CS 128/ES 228 - Lecture 4b

  23. Quilters also tesselate Designing Tesselations by Jinny Beyer CS 128/ES 228 - Lecture 4b

  24. Effects of resolution – raster Larger cells: • less precise spatial fix • line + boundary thickening • features too close overlap - less detail possible Fig. 3.10 in 3rd ed. CS 128/ES 228 - Lecture 4b

  25. many data sets available easy to overlay multiple themes able to represent multiple continuous surfaces different file formats readily inter-converted fast computer lookup and display Advantages of raster format CS 128/ES 228 - Lecture 4b

  26. poor representation of discrete objects exact boundary location difficult constant resolution throughout the region modeled generates very large data sets difficult to change projection or coordinate system Limitations of raster format CS 128/ES 228 - Lecture 4b

  27. Raster layers don’t share well CS 128/ES 228 - Lecture 4b

  28. Raster layers are normally projected • Note the datum and projection/ coordinate system • Special software needed to re-project CS 128/ES 228 - Lecture 4b

  29. Summary: Raster format • Huge amounts of spatial data are available in raster format • Rasters are the format of choice for continuous features • Rasters do a poor job of representing discrete features CS 128/ES 228 - Lecture 4b

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