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Spatial Analysis (3D)

Spatial Analysis (3D). When last we visited…. Buffering – another tool. Buffering (building a neighborhood around a feature) is a common aid in GIS analysis. Using Buffers to Select. Select the features Save the features as a layer (Export). Putting it all together.

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Spatial Analysis (3D)

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  1. Spatial Analysis (3D) CS 128/ES 228 - Lecture 12b

  2. When last we visited… CS 128/ES 228 - Lecture 12b

  3. Buffering – another tool • Buffering (building a neighborhood around a feature) is a common aid in GIS analysis CS 128/ES 228 - Lecture 12b

  4. Using Buffers to Select • Select the features • Save the features as a layer • (Export) CS 128/ES 228 - Lecture 12b

  5. Putting it all together • Siting a nuclear waste dump • Build Layer A by selecting only those areas with “good” geology (good geology layer) • Build Layer B by taking a population density layer and reclassifying it in a boolean (2-valued) way to select only areas with a low population density (low population layer) • Build Layer C by selecting those areas in A that intersect with features in B (good geology AND low population layer) • Build Layer D by selecting “major” roads from a standard roads layer (major roads layer) CS 128/ES 228 - Lecture 12b

  6. Siting the Dump, Part Deux • Build Layer E by buffering Layer D at a suitable distance (major roads buffer layer) • Build Layer F by selecting those features from C that are not in any region of E (good geology, low population and not near major roads layer) • Build Layer G by selecting regions that are “conservation areas” (no development layer) • Build Layer H by selecting those features from F that are not in any region of G (suitable site layer) See also: Figure 6.9, p. 121 CS 128/ES 228 - Lecture 12b

  7. On to 3-D CS 128/ES 228 - Lecture 12b

  8. Some (More) GIS Queries • How steep is the road? • Which direction does the hill face? • What does the horizon look like? • What is that object over there? • Where will the waste flow? • What’s the fastest route home? CS 128/ES 228 - Lecture 12b

  9. Types of queries • Aspatial – make no reference to spatial data • 2-D Spatial – make reference to spatial data in the plane • 3-D Spatial – make reference to “elevational” data • Network – involve analyzing a network in the GIS (yes, it’s spatial) CS 128/ES 228 - Lecture 12b

  10. 1984 technology 1997 technology 3-D Computational Complexity CS 128/ES 228 - Lecture 12b

  11. Approximations • In the vector model, each object represents exactly one feature; it is “linked” to its complete set of attribute data • In the raster model, each cell represents exactly one piece of data; the data is specifically for that cell • THE DATA IS DISCRETE!!! CS 128/ES 228 - Lecture 12b

  12. Image from: http://www.ian-ko.com/resources/triangulated_irregular_network.htm Surface Approximations With a surface, only a few points have “true data” The “values” at other points are only an approximation The are determined (somehow) by the neighboring points The surface is CONTINUOUS CS 128/ES 228 - Lecture 12b

  13. Types of approximation • GLOBAL or LOCAL • Does the approximation function use all points or just “nearby” ones? • EXACT or APPROXIMATE • At the points where we do have data, is the approximation equal to that data? CS 128/ES 228 - Lecture 12b

  14. Types of approximation • GRADUAL or ABRUPT • Does the approximation function vary continuously or does it “step” at boundaries? • DETERMINISTIC or STOCHASTIC • Is there a randomness component to the approximation? CS 128/ES 228 - Lecture 12b

  15. Display “by point” • Notice the (very) large number of data points • This is not always feasible • “Draw” the dot Image from: http://www.csc.noaa.gov/products/nchaz/htm/lidtut.htm CS 128/ES 228 - Lecture 12b

  16. Display “by contour” • More feasible, but granularity is an issue • Consider the ocean… • “Connect” the dots Image from: http://www.csc.noaa.gov/products/nchaz/htm/lidtut.htm CS 128/ES 228 - Lecture 12b

  17. Display “by surface” • Involves interpolation of data • Better picture, but is it more accurate? • “Paint” the connected dots Image from: http://www.csc.noaa.gov/products/nchaz/htm/lidtut.htm CS 128/ES 228 - Lecture 12b

  18. Voronoi (Theissen) polygons as a painting tool • Points on the surface are approximated by giving them the value of the nearest data point • Exact, abrupt, deterministic CS 128/ES 228 - Lecture 12b

  19. 1- X w y W = *y + (1-)*x Smooth Shading • Standard (linear) interpolation leads to smooth shaded images • Local, exact, gradual, deterministic CS 128/ES 228 - Lecture 12b

  20. or Image from: http://www.ian-ko.com/resources/triangulated_irregular_network.htm TINs – Triangulated Irregular Networks • Connect “adjacent” data points via lines to form triangles, then interpolate • Local, exact, gradual, possibly stochastic CS 128/ES 228 - Lecture 12b

  21. Simple Queries? • The descriptions thus far represent “simple” queries, in the same sense that length, area, etc. did for 2-D. • A more complex query would involve comparing the various data points in some way CS 128/ES 228 - Lecture 12b

  22. slope aspect Slope and aspect • A natural question with elevational data is to ask how rapidly that data is changing, e.g. “What is the gradient?” • Another natural question is to ask what direction the slope is facing, i.e. “What is the normal?” CS 128/ES 228 - Lecture 12b

  23. What is slope? • The slope of a curve (or surface) is represented by a linear approximation to a data set. • Can be solved for using algebra and/or calculus Image from: http://oregonstate.edu/dept/math/CalculusQuestStudyGuides/vcalc/tangent/tangent.html CS 128/ES 228 - Lecture 12b

  24. Solving for slope • In a raster world, we use the equation for a plane: z = a*x + b*y + c and we solve for a “best fit” • In a vector world, it is usually computed as the TIN is formed (viz. the way area is pre-computed for polygons) CS 128/ES 228 - Lecture 12b

  25. Our friend calculus • Slope is essentially a first derivative • Second derivatives are also useful for… convexity computations CS 128/ES 228 - Lecture 12b

  26. Image from: http://www.friends-of-fpc.org/tutorials/graphics/dlx_ogl/teil12_6.gif What is aspect? • Aspect is what mathematicians would call a “normal” • Computed arithmetically from equation of plane Shows what direction the surface “faces” CS 128/ES 228 - Lecture 12b

  27. Matt Hartloff, ‘2000 • Delaunay “Sweep” algorithm uses Voronoi diagram as first step CS 128/ES 228 - Lecture 12b

  28. Jackson Hole, WY …then shades result based upon slopes and aspects CS 128/ES 228 - Lecture 12b

  29. Visibility • What can I see from where? • Tough to compute! CS 128/ES 228 - Lecture 12b

  30. When is an Elevation NOT an Elevation? • When it is rainfall, income, or any other scalar measurement • Bottom Line: It’s one more dimension (any dimension!) on top of the geographic data CS 128/ES 228 - Lecture 12b

  31. Network Analysis • Given a network • What is the shortest path from s to t? • What is the cheapest route from s to t? • How much “flow” can we get through the network? • What is the shortest route visiting all points? Image from: http://www.eli.sdsu.edu/courses/fall96/cs660/notes/NetworkFlow/NetworkFlow.html#RTFToC2 CS 128/ES 228 - Lecture 12b

  32. Network complexities All answers learned in CS 232! CS 128/ES 228 - Lecture 12b

  33. Conclusions • A GIS without spatial analysis is like a car without a gas pedal. It is okay to look at, but you can’t do anything with it. • A GIS without 3-D spatial analysis is like a car without a radio. It may still be useful, but you wish you had the “luxury”. CS 128/ES 228 - Lecture 12b

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