1. Surface Analysis Tools
TransCAD
Surface Analysis Tools
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2. Digital Elevation Models (DEM)
A Digital Elevation Mode (DEM) is a specialized database that represents the relief of a surface between points of known elevation. Similar to other geographic data sets, DEMs are referenced to a known coordinate system. The coordinate values are stored as degrees of longitude and latitude.
Creation of a DEM begins with the capture of an irregular array of elevations. These points, commonly referred to as the “original observations,” are used as input to an interpolation routine that will approximate the nature of the surface between sample points.
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3. Digital Elevation Models (DEM)
Original elevation observations may be derived from any one or a combination of sources such as ground surveys, photogrammetric data capture, and cartographic data sources. The process of interpolation typically results in the creation of one of two commonly used DEM data structures: a rectangular grid of elevations, or a triangulated irregular network (TIN).
Rectangular grids provide a simple data structure in which data values are interpolated to create an evenly spaced grid across a geographic region. Rectangular grids can be stored in a grid geographic file and displayed as a grid layer.
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4. Grid Layers
A grid layer stores a set of values for a regular array of cells; the cells are usually square. Grid layers are another type of may layer that TransCAD can use, in addition to point layers, for surface analysis and 3D maps. Grid layers allow you to store any regular array of values. A common type of grid layer is a digital elevation model with regularly-spaced heights for a surface.
A grid layer is stored in a grid geographic file. A grid geographic file stores just a value for each cell in the grid. The coordinates of the corners of the grid cells do not need to be stored, because they can be computed from the origin of the grid and the height and width of each cell, which are also stored in the grid geographic file.
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5. Preparing for Surface Analysis
To analyze a surface based on a point layer, TransCAD must create the triangulated irregular network (TIN). TransCAD can support multiple levels of interpolation during the creation of a TIN to smooth contour lines during contour generation.
Interpolation creates new “height” values between sample points. Level 0 does no contour smoothing, while levels 1, 2, and 3 provide increasing levels of interpolation and smoothing.
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6. To Prepare for Surface Analysis Using a Point Layer
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7. To Calculate a Spot Elevation
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8. To Create a Profile Drawing
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9. To Do Visibility Analysis from a TIN
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10. To Generate Contours from a TIN
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12. To Show 3D Terrain
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14. To Create a Rectangular Grid DEM from a TIN
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15. Creating an Area Geographic File from a TIN
A triangulated irregular network (TIN) is a transient data structure that is created “in the background” to support surface analysis. When you close the Surface Analysis toolbox the TIN is deleted. TransCAD allows you to save the TIN as triangular facets in an area geographic file and to display the TIN as a map layer.
In addition, TransCAD generates a data table that stores the gradient and aspect of each facet in the TIN. Aspect is defined according to the diagram to the right, and describes the compass direction that a effective themes that illustrate the characteristics of a surface.
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16. To Create an Area Geographic File from a TIN
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17. To Create a 3D Map
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