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## Terrain Analysis (Surface Analysis)

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**------Using GIS--**Introduction to GIS TOC What is surface? Surface model DEM (lattice of points, grid) Contour TIN Surface Analysis in ArcGIS: Spatial Analyst, 3D Analyst , ArcScene Terrain Analysis (Surface Analysis)**Terrain Analysis**• Use of three-dimensional data to perform landscape or terrain evaluation. • Examples: • Use of digital elevation models to generate contour maps and perform slope analysis • Civil engineering calculations to determine cut-and-fill volumes and generate vertical profile drawings**What is a Surface?**• A surface is a continuous feature, without discrete intervals. • There is no way to store all the information about a surface. • We must approximate a surface with samples.**Surface Data**• Surface models are based on point data which associates a z value item with an x,y location. • The z value usually measures surface elevation and provides the surface model with its third dimension. • These z values can also be used to represent a wide variety of measurements. Common examples are:depth, temperature, rainfall, pollutants, barometric pressure and demographic distribution. • Surfaces which are generated to represent these data points become the model upon which to perform surface analysis.**Surface AnalysisCan be performed to answer any of the**following questions. • What is the elevation of a selected point on the surface? • Can specified points see each other? • What are the real distances traveled along the surface? • How much area does a region of the surface really cover? • How steep are different areas of the surface? • Which direction does a slope face?**Elevation Represented as Points(Lattice , grid; or DEM)**Lattice size and origin Points X/Y = 21 21 Origin (x,y) = 540000.00 3911940.000 Lattice distance between points: Distance in X = 30.000 Distance in Y = 30.000 Surface value in range: Min z=235.000 Max z=631.00 Lattice boundary: Xmin=540000.000 Xmax= 540600.000 Ymin= 3911940.000 Ymax = 3912540.000**Triangulation of a Terrain SurfaceSource: Heil, R.J. and**S.M. Brych, “An approach for consistent topographic representation of varying terrain”, Proceedings of the Digital Terrain Models (DTM) Symposium, Falls Church, VA: ASP and ACSM, 1978, p. 408, Figure 4b. drainage divide between mainstream and tributary stream tributary ridge line confluence of main mainstream and tributary stream B. An example of triangulation representing terrain.**------Using GIS--**Introduction to GIS Terrain Analysis in Arc GIS Functions are scattered between: Spatial Analyst (in Arc Map) 3D Analyst (in Arc Map) ArcScene**------Using GIS--**Introduction to GIS Calculating Slope in 2 Dimensions Slope is calculated as the steepest path downhill In the case of raster slopes, neighborhood is 8 cells surrounding any given cell Slope as %= rise/run*100; problem: approaches inf. Slope as degrees Φ Rise Φ run**------Using GIS--**Introduction to GIS Calculating Raster Slope Raster slope is calculated by steepest path in neighborhood dZ/dx can be calculated in several different ways; simplest is**------Using GIS--**Introduction to GIS Calculating Aspect with Raster Where dZ/dy and dZ/dx can be defined in either of the ways just mentioned Aspect is the direction (in azimuth angle) of the steepest path**------Using GIS--**Introduction to GIS Uses of Slope and Aspect Can be used for many hydrologic functions such as flow direction analysis, watershed generation and drainage network analysis One frequent problem with elevation data are pits or sinks which are cells that are lower than surrounding cells, usually because of random error in DEM**------Using GIS--**Introduction to GIS Spatial Analyst: Cut and Fill This is a method for comparing two elevation surfaces to look for areas of change (deposition/ erosion).**------Using GIS--**Introduction to GIS Spatial Analyst: Contours SA can be used to generate contours from a raster layer Can choose contour interval, base contour and z factor (for unit shift)**------Using GIS--**Introduction to GIS TIN TIN stands for Triangulated Irregular Network A TIN is a data structure that defines geographic space as a set of contiguous, non-overlapping triangles, which vary in size and angular proportion. Like grids, TINs are used to represent elevation surfaces, and can be created directly from files of elevation sample points, but with TINs these sample points are irregularly distributed.**------Using GIS--**Introduction to GIS Three Dimensional data — TIN Note the triangular facets defined by points**------Using GIS--**Introduction to GIS Here is a simple TIN skeleton, without the facets painted**------Using GIS--**Introduction to GIS TINs TINs can be created from raster Digital Elevation Models or from mass points with elevation values.**------Using GIS--**Introduction to GIS TINs • TIN triangles vary in size with the complexity of the terrain; more complex terrain needs more smaller triangles, and hence more points**------Using GIS--**Introduction to GIS DEM to TIN**------Using GIS--**Introduction to GIS TINs Shown in 3D**------Using GIS--**Introduction to GIS 3D TINs vs. 3D grids TIN’s facet resolution can be controlled in ArcGIS—the smaller the facets, the better the resolution**------Using GIS--**Introduction to GIS What is Arc Scene? Component of Arc GIS that allows for 3D visualization of vector features and raster surfaces. Can show terrain as well other information extruded in the third dimension.**------Using GIS--**Using Arc Scene • To extrude, go to properties of DEM or TIN and “obtain heights” from itself under “base heights” • Then can select various options • for rendering under the rendering tab**------Using GIS--**Introduction to GIS Using Arc Scene • Then you can “drape” vector thematic layers on top by defining their base height as being that of the DEM and then extruding by their height**------Using GIS--**Introduction to GIS Using Arc Scene • Can also drape photos on surfaces + =**------Using GIS--**Introduction to GIS Using Arc Scene • Can also drape photos on surfaces**------Using GIS--**Introduction to GIS Using Arc Scene Here we overlay roads and urban areas on a DEM**------Using GIS--**Introduction to GIS Overlaying vector on TIN Vector building footprints over a TIN, using elevation to define the bases of the buildings and number of stories to extrude them**------Using GIS--**Introduction to GIS Using Arc Scene We can also create animations of fly throughs in ArcScene**------Using GIS--**Introduction to GIS Steepest Path • Another feature in Arc Scene is generation of a steepest path on a terrain surface**------Using GIS--**Introduction to GIS The latest in 3d visualization**Terrain model of Mount Everest and its surrounding area**based on photogrammetric survey data. 3D vizualization of Mount Everest Source: Martin Sauerbier/Institute of Geodesy and Photogrammery**Figure 10.7 Multiple representations of Tryfan, north Wales**(Continued)**Figure 10.7 Multiple representations of Tryfan, north Wales**(Continued)**------Using GIS--**Introduction to GIS The latest in 3d visualization**------Using GIS--**Introduction to GIS The latest in 3d visualization**3D view of the retail trade areas for two stores operating**withinthe same market**Layering a Pollution Surface (Nitrogen Dioxide NOx) onto**Virtual London**Figure 8.12 Different Renditions of Virtual London. (a) The**GIS Model; (b) Within a multi-User Virtual World / Exhibition Space; (c) as a hardcopy model from a CAD/CAM printer**Example 3D object modelling in MapInfo**Source: Encom Technology Pty Ltd**Example 3D object modelling in MapInfo**Source: Encom Technology Pty Ltd