{"id":499,"date":"2014-01-24T21:12:05","date_gmt":"2014-01-24T21:12:05","guid":{"rendered":"http:\/\/opentextbc.ca\/natureofgeographicinformation\/?post_type=chapter&p=499"},"modified":"2015-05-27T15:45:06","modified_gmt":"2015-05-27T15:45:06","slug":"1-overview-6","status":"publish","type":"chapter","link":"https:\/\/opentextbc.ca\/natureofgeographicinformation\/chapter\/1-overview-6\/","title":{"raw":"National Spatial Data Infrastructure II","rendered":"National Spatial Data Infrastructure II"},"content":{"raw":"

7.1. Overview<\/h2>\r\nChapters 6 and 7 consider the origins and characteristics of the framework data themes <\/strong>that make up the United States' proposed National Spatial Data Infrastructure (NSDI). Chapter 6 discussed the geodetic control and orthoimagery themes. This chapter describes the origins, characteristics and current status of the elevation, transportation, hydrography, governmental units and cadastral themes.<\/strong>\r\n

Objectives<\/h3>\r\nStudents who successfully complete Chapter 7 should be able to:\r\n
    \r\n\t
  1. Given a regular or irregular array of spot elevations, construct a triangulated irregular network, interpolate contour intervals and draw contour lines;<\/li>\r\n\t
  2. Compare vector and raster representations of terrain elevation;<\/li>\r\n\t
  3. Acquire and view digital elevation data from the National Elevation Dataset;<\/li>\r\n\t
  4. Calculate an interpolated spot elevation based on neighboring elevations;<\/li>\r\n\t
  5. Contrast the characteristics of three global elevation data products;<\/li>\r\n\t
  6. Describe the characteristics and current status of the NSDI hydrography, transportation, and governmental units themes as implemented in USGS' National Map; and<\/li>\r\n\t
  7. Interpret the size and relative location of a land parcel designated in terms of the U.S. Public Land Survey System.<\/li>\r\n<\/ol>\r\n

    Comments and Questions<\/h3>\r\nRegistered students are welcome to post comments, questions, and replies to questions about the text. Particularly welcome are anecdotes that relate the chapter text to your personal or professional experience. In addition, there are discussion forums available in the ANGEL course management system for comments and questions about topics that you may not wish to share with the whole world.\r\n\r\nTo post a comment, scroll down to the text box under \"Post new comment\" and begin typing in the text box, or you can choose to reply to an existing thread. When you are finished typing, click on either the \"Preview\" or \"Save\" button (Save will actually submit your comment). Once your comment is posted, you will be able to edit or delete it as needed. In addition, you will be able to reply to other posts at any time.\r\n\r\nNote: the first few words of each comment become its \"title\" in the thread.\r\n

    7.2. Checklist<\/h2>\r\nThe following checklist is for Penn State students who are registered for classes in which this text, and associated quizzes and projects in the ANGEL course management system, have been assigned. You may find it useful to print this page out first so that you can follow along with the directions.\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n
    Chapter 7 Checklist (for registered students only)<\/caption>\r\n
    Step<\/th>\r\nActivity<\/th>\r\nAccess\/Directions<\/th>\r\n<\/tr>\r\n<\/thead>\r\n
    1<\/th>\r\nRead<\/strong>\u00a0Chapter 7<\/td>\r\nThis is the second page of the Chapter. Click on the links at the bottom of the page to continue or to return to the previous page, or to go to the top of the chapter. You can also navigate the text via the links in the GEOG 482 menu on the left.<\/td>\r\n<\/tr>\r\n
    2<\/th>\r\nSubmit\u00a03 practice quizzes<\/strong>including:\r\n
      \r\n\t
    • Contouring<\/li>\r\n\t
    • DLGs and DEMs<\/li>\r\n\t
    • Interpolation<\/li>\r\n<\/ul>\r\nPractice quizzes are not graded and may be submitted more than once.<\/td>\r\n
    		Go to ANGEL > [your course section] > Lessons tab > Chapter 7 folder > [quiz]<\/td>\r\n<\/tr>\r\n
    3<\/th>\r\nPerform\u00a0\u201cTry this\u201d activities<\/strong>including:\r\n
      \r\n\t
    • Draw a contour map<\/li>\r\n\t
    • Explore Digital Line Graph hypsography<\/li>\r\n\t
    • Explore a Digital Elevation Model<\/li>\r\n\t
    • Download and view an extract from the National Elevation Dataset<\/li>\r\n<\/ul>\r\n\u201cTry this\u201d activities are not graded.<\/td>\r\n
    		Instructions are provided for each activity.<\/td>\r\n<\/tr>\r\n
    4<\/th>\r\nSubmit theChapter 7 Graded Quiz<\/strong><\/td>\r\nANGEL > [your course section] > Lessons tab > Chapter 7 folder > Chapter 7 Graded Quiz. See the Calendar tab in ANGEL for due dates.<\/td>\r\n<\/tr>\r\n
    5<\/th>\r\n\u00a0Readcomments and questions<\/strong>posted by fellow students. Add comments and questions of your own, if any.<\/td>\r\n\u00a0Comments and questions may be posted on any page of the text, or in a Chapter-specific discussion forum in ANGEL.<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n \r\n

    7.3. Theme: Elevation<\/h2>\r\nThe NSDI\u00a0Framework Introduction and Guide<\/em>\u00a0(FGDC, 1997, p. 19) points out that \u201celevation data are used in many different applications.\u201d Civilian applications include flood plain delineation, road planning and construction, drainage, runoff, and soil loss calculations, and cell tower placement, among many others. Elevation data are also used to depict the terrain surface by a variety of means, from contours to relief shading and three-dimensional perspective views.\r\n\r\nThe NSDI Framework calls for an \u201celevation matrix\u201d for land surfaces. That is, the terrain is to be represented as a grid of elevation values. The spacing (or resolution) of the elevation grid may vary between areas of high and low relief (i.e., hilly and flat). Specifically, the Framework Introduction states that\r\n\r\nElevation values will be collected at a post-spacing of 2 arc-seconds (approximately 47.4 meters at 40\u00b0 latitude) or finer. In areas of low relief, a spacing of 1\/2 arc-second (approximately 11.8 meters at 40\u00b0 latitude) or finer will be sought (FGDC, 1997, p. 18).\r\n\r\nThe elevation theme also includes bathymetry\u2013depths below water surfaces\u2013for coastal zones and inland water bodies. Specifically,\r\n\r\nFor depths, the framework consists of soundings and a gridded bottom model. Water depth is determined relative to a specific vertical reference surface, usually derived from tidal observations. In the future, this vertical reference may be based on a global model of the geoid or the ellipsoid, which is the reference for expressing height measurements in the Global Positioning System (Ibid).\r\n\r\nUSGS has lead responsibility for the elevation theme. Elevation is also a key component of USGS\u2019 National Map. The next several pages consider how heights and depths are created, how they are represented in digital geographic data, and how they may be depicted cartographically.\r\n

    7.4. Vector and Raster Approaches<\/h2>\r\n \r\n\r\nThe terms\u00a0raster<\/strong>\u00a0and\u00a0vector<\/strong>\u00a0were introduced back in Chapter 1 to denote two fundamentally different strategies for representing geographic phenomena. Both strategies involve simplifying the infinite complexity of the Earth\u2019s surface. As it relates to elevation data, the raster approach involves\u00a0measuring elevation at a sample of locations<\/em>. The vector approach, on the other hand, involves\u00a0measuring the locations of a sample of elevations<\/em>. I hope that this distinction will be clear to you by the end of this chapter.\r\n\r\n\"Diagram\r\n\r\nVector and raster representations of the same terrain surface.\r\n\r\nThe illustration above compares how elevation data are represented in vector and raster data. On the left are elevation\u00a0contours<\/strong>, a vector representation that is familiar with anyone who has used a USGS topographic map. The technical term for an elevation contour isisarithm<\/em>, from the Greek words for \u201csame\u201d and \u201cnumber.\u201d The termsisoline<\/em>,\u00a0isogram<\/em>, and\u00a0isopleth<\/em>\u00a0all mean more or less the same thing. (See any cartography text for the distinctions.)\r\n\r\nAs you will see later in this chapter, when you explore Digital Line Graph hypsography data using Global Mapper or dlgv 32 Pro, elevations in vector data are encoded as attributes of line features. The distribution of elevation points across the quadrangle is therefore irregular. Raster elevation data, by contrast, consist of grids of points at which elevation is encoded at regular intervals. Raster elevation data are what\u2019s called for by the NSDI Framework and the USGS National Map. Digital contours can now be rendered easily from raster data. However, much of the raster elevation data used in the National Map was produced from digital vector contours and hydrography (streams and shorelines). For this reason we\u2019ll consider the vector approach to terrain representation first.\r\n

    7.5. Contours<\/h2>\r\n\"Perspective\r\n\r\n \r\n\r\nContour lines trace the elevation of the terrain surface at regularly-spaced intervals (Raisz, 1948. \u00a9 McGraw-Hill, Inc. Used by permission).\r\n\r\nDrawing contour lines is a way to represent a terrain surface with a sample of elevations. Instead of measuring and depicting elevation at every point, you measure only along lines at which a series of imaginary horizontal planes slice through the terrain surface. The more imaginary planes, the more contours, and the more detail is captured.\r\n\r\n\"Plan\r\n\r\nContour lines representing the same terrain as in the first figure, but in plan view. (Raisz, 1948. \u00a9 McGraw-Hill, Inc. Used by permission).\r\n\r\nUntil photogrammetric methods came of age in the 1950s, topographers in the field sketched contours on the USGS 15-minute topographic quadrangle series. Since then, contours shown on most of the 7.5-minute quads were compiled from stereoscopic images of the terrain, as described in Chapter 6. Today computer programs draw contours automatically from the spot elevations that photogrammetrists compile stereoscopically.\r\n\r\nAlthough it is uncommon to draw terrain elevation contours by hand these days, it is still worthwhile to know how. In the next few pages you\u2019ll have a chance to practice the technique, which is analogous to the way computers do it.\r\n

    7.6. Contouring By Hand<\/h2>\r\nThis page will walk you through a methodical approach to rendering contour lines from an array of spot elevations (Rabenhorst and McDermott, 1989). To get the most from this demonstration, I suggest that you print the\u00a0illustration in the attached image file<\/a>. Find a pencil (preferably one with an eraser!) and straightedge, and duplicate the steps illustrated below. A \u201cTry This!\u201d activity will follow this step-by-step introduction, providing you a chance to go solo.\r\n\r\n \r\n\r\n\"Step\r\n\r\nBeginning a triangulated irregular network.\r\n\r\nStarting at the highest elevation, draw straight lines to the nearest neighboring spot elevations. Once you have connected to all of the points that neighbor the highest point, begin again at the second highest elevation. (You will have to make some subjective decisions as to which points are \u201cneighbors\u201d and which are not.) Taking care not to draw triangles across the stream, continue until the surface is completely triangulated.\r\n\r\n\"Step\r\n\r\nComplete TIN. Note that the triangle sides must not cross hydrologic features (i.e., the stream) on a terrain surface.\r\n\r\nThe result is a\u00a0triangulated irregular network (TIN)<\/strong>. A TIN is a vector representation of a continuous surface that consists entirely of triangular facets. The vertices of the triangles are spot elevations that may have been measured in the field by leveling, or in a photogrammetrist\u2019s workshop with a stereoplotter, or by other means. (Spot elevations produced photogrammetrically are called\u00a0mass points<\/strong>.) A useful characteristic of TINs is that each triangular facet has a single slope degree and direction. With a little imagination and practice, you can visualize the underlying surface from the TIN even without drawing contours.\r\n\r\nWonder why I suggest that you not let triangle sides that make up the TIN cross the stream? Well, if you did, the stream would appear to run along the side of a hill, instead of down a valley as it should. In practice, spot elevations would always be measured at several points along the stream, and along ridges as well. Photogrammetrists refer to spot elevations collected along linear features as\u00a0breaklines<\/strong>\u00a0(Maune, 2007). I omitted breaklines from this example just to make a point.\r\n\r\nYou may notice that there is more than one correct way to draw the TIN. As you will see, deciding which spot elevations are \u201cnear neighbors\u201d and which are not is subjective in some cases. Related to this element of subjectivity is the fact that the fidelity of a contour map depends in large part on the distribution of spot elevations on which it is based. In general, the density of spot elevations should be greater where terrain elevations vary greatly, and sparser where the terrain varies subtly. Similarly, the smaller the contour interval you intend to use, the more spot elevations you need.\r\n\r\n(There are algorithms for triangulating irregular arrays that produce unique solutions. One approach is called\u00a0Delaunay Triangulation<\/strong>which, in one of its constrained forms, is useful for representing terrain surfaces. The distinguishing geometric characteristic of a Delaunay triangulation is that a circle surrounding each triangle side does not contain any other vertex.)\r\n\r\n\"Step\r\n\r\n \r\n\r\nTick marks drawn where elevation contours cross the edges of each TIN facet.\r\n\r\n \r\n\r\nNow draw ticks to mark the points at which elevation contours intersect each triangle side. For instance, see the triangle side that connects the spot elevations 2360 and 2480 in the lower left corner of the illustration above? One tick mark is drawn on the triangle where a contour representing elevation 2400 intersects. Now find the two spot elevations, 2480 and 2750, in the same lower left corner. Note that three tick marks are placed where contours representing elevations 2500, 2600, and 2700 intersect.\r\n\r\nThis step should remind you of the equal interval classification scheme you read about in Chapter 3. The right choice of contour interval depends on the goal of the mapping project. In general, contour intervals increase in proportion to the variability of the terrain surface. It should be noted that the assumption that elevations increase or decrease at a constant rate is not always correct, of course. We will consider that issue in more detail later.\r\n\r\n\"Step\r\n\r\nThreading elevation contours through a TIN.\r\n\r\nFinally, draw your contour lines. Working downslope from the highest elevation, thread contours through ticks of equal value. Move to the next highest elevation when the surface seems ambiguous.\r\n\r\nKeep in mind the following characteristics of contour lines (Rabenhorst and McDermott, 1989):\r\n