Lecture03 - Maps as Numbers Getting Started with GIS...

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Unformatted text preview: Maps as Numbers Getting Started with GIS Chapter 3 Maps as Numbers Maps GIS requires that both data and maps be GIS represented as numbers The GIS places data into the computer’s The computer’ memory in a physical data structure (i.e. files and directories) Files can be written in binary or as ASCII text Files Binary is faster to read and smaller, ASCII Binary can be read by humans and edited but uses more space Chapter 3: Maps as Numbers 3.1 Representing Maps as Numbers 3.1 3.2 Structuring Attributes 3.2 3.3 Structuring Maps 3.3 3.4 Why Topology Matters 3.4 3.5 Formats for GIS Data 3.5 3.6 Exchanging Data 3.6 Do we know the difference? A. 1010 1001 1000 1000 0010 A. B. 23e2 1712 a2b5 fff0 B. C. 1323 1652 1710 3214 C. D. abcdefghijkl D. E. αβχδεφργηιϕκλ E. αβχδεφργηιϕκλ 1 ASCII Codes Binary vs. HEX vs. ASCII Features vs. Fields The Data Model A logical data model is how data logical are organized for use by the GIS GISs have traditionally used either GISs raster raster vector vector 2 Rasters and vectors can be flat files … if they are simple Beethoven is vector … Mozart is raster! Spot the data structure Spot the data structure A. RASTER A. B. VECTOR B. A. RASTER A. B. VECTOR B. 3 Spot the data structure A. RASTER A. B. VECTOR B. Attribute data Features and Maps A GIS map is a scaled-down digital GIS scaledrepresentation of point, line, area, and volume features While most GIS systems can handle While raster and vector, only one is used for the internal organization of spatial data Only one can be used in combined Only operations across layers A geographical flat file Attribute data are stored logically in flat Attribute files A flat file is a matrix of numbers and flat values stored in rows and columns, like a spreadsheet Both logical and physical data models Both have evolved over time DBMSs use many different methods to DBMSs store and manage flat files in physical files 4 A raster data model uses a grid. Generic structure for a grid One grid cell is one unit or holds one attribute One Every cell has a value, even if it is “missing” Every missing” A cell can hold a number or an index value cell standing for an attribute A cell has a resolution, given as the cell size cell in ground units Often create a “mask” to cover part of Often mask” rectangle not in AOI The mixed pixel problem The Grids and missing data 5 Rasters are faster... Fat lines Points and lines in raster format have to move to a Points cell center Lines can become fat Lines Areas may need separately coded edges Areas Each cell can be owned by only one feature Each As data, all cells must be able to hold the maximum As cell value Rasters are easy to understand, easy to read and Rasters write, and easy to draw on the screen RASTER The quad-tree structure A grid or raster maps directly onto a programming grid computer memory structure called an array Grids are poor at representing points, lines and Grids areas, but good at surfaces Grids are good only at very localized topology, and Grids weak otherwise Grids are a natural for scanned or remotely sensed Grids data Grids suffer from the mixed pixel problem Grids Grids must often include redundant or missing data Grids Grid compression techniques used in GIS are runGrid runlength encoding, R-trees and quad trees R- 6 Range (R-) Trees Vectors Wisconsin Top: transportation and urban places from VMAP0 Bottom: Census tracts The Vector Model A vector data model uses points stored by vector their real (earth) coordinates Lines and areas are built from sequences Lines of points in order Lines have a direction to the ordering of Lines the points. Polygons can be built from points or lines Polygons Vectors can store information about Vectors topology VECTOR At first, GISs used vector data and cartographic spaghetti At structures Vector data evolved the arc/node model in the 1960s Vector In the arc/node model, an area consist of lines and a line In consists of points Points, lines, and areas can each be stored in their own Points, files, with links between them The topological vector model uses the line (arc) as a basic The unit. Areas (polygons) are built up from arcs The endpoint of a line (arc) is called a node. Arc junctions The are only at nodes Stored with the arc is the topology (i.e. the connecting Stored arcs and left and right polygons) 7 One arc End node Intermediate nodes Vectors just seemed more correcter TIN must be used to represent volumes TIN Vector can represent point, line, and area Vector features very accurately Vectors are far more efficient than grids Vectors Vectors work well with pen and light-plotting Vectors lightdevices and tablet digitizers Vectors are not good at continuous Vectors coverages or plotters that fill areas Start node TOPOLOGY Basic arc topology Topological data structures dominate GIS software Topological Topology allows automated error detection and Topology elimination Rarely are maps topologically clean when digitized or Rarely imported A GIS has to be able to build topology from GIS unconnected arcs Nodes that are close together are snapped Nodes Slivers due to double digitizing and overlay are Slivers eliminated 8 Arc/node map data structure with files Topological errors The bounding rectangle Topology Matters The tolerances controlling snapping, The elimination, and merging must be considered carefully, because they can move features Complete topology makes map overlay Complete feasible Topology allows many GIS operations to be Topology done without accessing the point files 9 Vector overlay Vectors and 3D Volumes (surfaces) are structured with Volumes the TIN model, including edge or triangle topology TINs use an optimal Delaunay TINs triangulation of a set of irregularly distributed points TINs are popular in CAD and surveying TINs packages New points New labels Slivers Constructing the TIN TIN: Triangulated Irregular Network Start at random Refine by flipping yes no Way to handle field Way data with the vector data structure Common in some Common GISs and most AM/FM packages More efficient than a More grid Surface is interpolated Surface through the triangles 10 FORMATS Most GIS systems can import different Most data formats, or use utility programs to convert them Data formats can be industry standard, Data commonly accepted or standard Vector Data Formats Vector formats are either page definition Vector languages or preserve ground coordinates. Page languages are HPGL, PostScript, and Page Autocad DXF GeoPDF gaining acceptance GeoPDF Script languages like GML, SVG, KML Script True vector GIS data formats are DLG and True TIGER, which has topology KML sample <Placemark> <name>Untitled Path</name> <LineString> Placemark> <LineString> <tessellate>1</tessellate> <altitudeMode>relativeToGround</altitudeMode> altitudeMode> relativeToGround</altitudeMode> <coordinates> -134.148103,37.752967 -128.917074,38.803008 -125.166954,39.583592 -122.137625,39.656880 -120.421783,40.036311 -118.298157,40.235316 -114.348386,40.631532 -112.670431,40.761033 -111.916045,40.681939 -110.177711,40.653055 -109.544331,40.619327 -107.155697,40.642007 -105.410526,40.421505 -103.192299,40.430138 -102.853712,40.427904 -98.168302,40.363524 -97.093391,40.308754 -94.831304,40.479175 -93.760070,40.395392 -84.913828,39.466651 -84.414888,39.387332 -81.380660,39.188551 -80.276261,38.977744 -77.811560,38.872542 -75.062267,38.521146 -72.006956,38.101733 -66.67819,37.664687 </coordinates> </LineString> </Placemark> </LineString> </Placemark> 11 The TIGER data structure Raster Data Formats Another view A DEM Most raster formats are digital image Most formats Most GISs accept TIF, GIF, JPEG or Most encapsulated PostScript, which are not georeferenced DEMs are true raster data formats DEMs GeoTIF now common, tif plus wrl GeoTIF 12 Multi-resolution NED: Puget Sound DEMs and UTM (7.5 minute 30m) 1-arc-second EXCHANGE 1/3-arc-second 1/9-arc-second Vector to raster exchange errors Most GISs use many formats and one data structure Most If a GIS supports many data structures, changing If structures becomes the user’s responsibility user’ Changing vector to raster is easy; raster to vector is Changing hard Data also are often exchanged or transferred Data between different GIS packages and computer systems The history of GIS data exchange is chaotic and has The been wasteful 13 GIS Data Exchange Data exchange by translation (export and import) can lead to Data significant errors in attributes and in geometry In the United States, the SDTS was evolved to facilitate data In transfer SDTS became a federal standard (FIPS 173) in 1992 SDTS SDTS contains a terminology, a set of references, a list of SDTS features, a transfer mechanism, and an accuracy standard FGDC has published metadata standards FGDC Both DLG and TIGER data are available in SDTS format Both Other standards efforts are DIGEST, DX-90, the Tri-Service Other DXTriSpatial Data Standards, and many other international standards OpenGIS Consortium has pioneered open standards and OpenGIS interoperability Format conversion still and issue, but much better! Format Efficient data exchange is important for the future of GIS Efficient Transfer Standards Interoperability: OpenGIS 14 ...
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