SurfacesExample for Crime Data3 basic methods for representing a surface:Storing & Converting Surface DataDigital Elevation ModelGRID as a Storage MethodTriangulated Irregular NetworkTIN as a Storage MethodCreating a TINContour (isolines) LinesOther Concepts: Breaklines and Massed PointsOther concepts: surface polygonsHard and Soft Breaklines/PolygonsTerrainsViewing & Processing Surface DataInterpolationSlide 17Slide 18Slide 19Slide 20Slide 21Slide 22Draping an ImageBriggs GISC 6382 UT-Dallas 1SurfacesSurfaces involve a third 'z' dimension (height/elevation/magnitude, quantity) in addition to x,y planimetric location. Any type of continuous data can be represented as a surface, whether it be ground elevation, barometric pressure, rainfall, crop yield, noise levels, population density, sales intensity, land value, income, crime rates, etc.Briggs GISC 6382 UT-Dallas 2Example for Crime Data3D Model of Crime Density in Boston, MAhttp://gis.mit.edu/classes/11.521/lectures/Lecture_14/Lect14.htmBriggs GISC 6382 UT-Dallas 33 basic methods for representing a surface:•DEM (digital elevation model): set of regularly spaced sampled ground points in the x and y dimensions (although spacing not necessarily the same in each) accompanied by an elevation measure (z dimension). The DEM terminology was introduced by USGS.Two concepts used for determining elevation at points within the grid cells:–Lattice: each point represents a value on the surface only at the center of the grid cell–Surface grid considers each sample as a square/rectangular cell with a constant surface value.•TIN (Triangulated Irregular Network) a set of adjacent, non-overlapping triangles with x, y coordinates and z vertical elevations for their vertices, along with topological relationship between the triangles and their adjacent neighbors.•Contour lines: lines of equal elevation, drawn at a given interval (e.g. every 6 or 25 feet)The general term digital terrain model (DTM) may be used to refer to any of the above surface representations when in digital form. DEM sometimes used synonymously with DTM—don’t.Storing & Converting Surface Data•3-D surfaces are normally stored in one of two forms within ArcGIS –as a GRID, which is ArcInfo's general raster format–as a TIN which is a vector format for surfaces•However, when you download data from the Internet, surface data may be in other formats, such as –DEM format, as originally developed by USGS–SDTS (Spatial Data Transfer Standard) format, which is an FGDC (Federal Geographic Data Committee) standard–E00 which is ESRI’s text formatted for distributing coverages and GRIDS–Points and breaklines•Conversion to GRID or TIN is generally required for display or analysis within the ArcGIS system –Generally, ArcToolbox has capabilities for converting these formats to GRIDs or TINs•Contour lines can be stored as vector lines in a coverage, shapefile, or geodatabase, –can only be used for map display but not analysis, so this is not a recommended format for surface storage.Briggs GISC 6382 UT-Dallas 5Digital Elevation Model•a sampled array of elevations (z) that are at regularly spaced intervals in the x and y directions. •two approaches for determining the surface z value of a location between sample points. –In a lattice, each mesh point represents a value on the surface only at the center of the grid cell. The z-value is approximated by interpolation between adjacent sample points; it does not imply an area of constant value. –A surface grid considers each sample as a square cell with a constant surface value. Advantages•Simple conceptual model•Data cheap to obtain•Easy to relate to other raster data•Irregularly spaced set of points can be converted to regular spacing by interpolationDisadvantages•Does not conform to variability of the terrain•Linear features not well representedBriggs GISC 6382 UT-Dallas 6GRID as a Storage Method•GRIDs are ESRI’s raster data format–Use for storing DEMS or other data in raster format•GRID stores data as either:•Integer: in which case there is an associated value attribute table (VAT) which contains one record for each different value in the raster (thus there are normally substantially fewer records in the VAT table than there are cells in the raster); this record stores the value itself, a count of the number of cells with that value, and any additional attributes the user wishes to to attach. Thus, the values could be codes for soil type and the VAT could contain fertility measures, soil name, construction suitability codes, etc. If you select a record in the VAT, all cells with that value will highlight in the View or Scene.•Floating point: (number with a decimal point) in which case there is no VAT table, and simply one decimal value per cell•Integer GRIDS are generally substantially faster to process.Briggs GISC 6382 UT-Dallas 7Triangulated Irregular Network•Advantages–Can capture significant slope features (ridges, etc)–Efficient since require few triangles in flat areas–Easy for certain analyses: slope, aspect, volume•Disadvantages–Analysis involving comparison with other layers difficulta set of adjacent, non-overlapping triangles computed from irregularly spaced points, with x, y horizontal coordinates and z vertical elevations.Briggs GISC 6382 UT-Dallas 8•TINs–are the most useful method for representing a continuous surface in a vector GIS system.–data sets comprising any combination of contours, breaklines and point elevations (either DEM or massed points) can be combined as input to create a TIN•TINS are especially useful for analytical purposes -Good model for representing surfaces -slope and aspect easily derived-simplify the calculation of surface area and volumeTIN as a Storage MethodBriggs GISC 6382 UT-Dallas 9Creating a TINNo Breaklines Soft Breaklines Hard BreaklinesThe DataThe TriangulationThe Surface3D ViewBreak linesLinear features which define and control surface behavior in terms of smoothness and continuity. http://www.ian-ko.com/resources/triangulated_irregular_network.htmBriggs GISC 6382 UT-Dallas 10Contour (isolines) LinesAdvantages•Familiar to many people•Easy to obtain mental picture of surface–Close lines = steep slope–Uphill V = stream–Downhill V or bulge = ridge–Circle = hill top or basin Disadvantages•Poor for computer representation: no formal digital model•Must convert to raster or TIN for