©2005 austin troy lecture 15: introduction to terrain analysis ------using gis-- introduction to...

©2005 Austin Troy

Lecture 15: Introduction to Terrain Analysis

------Using GIS--Introduction to GIS

By Austin Troy

University of Vermont

©2005 Austin Troy

Part 1. Terrain analysis


©2005 Austin Troy

Functions are scattered between:•Spatial Analyst (in Arc Map•3D Analyst (in Arc Map)•Arc Scene


Terrain in Arc GIS

SF Bay Area extruded my median income by zip code

©2005 Austin Troy

•Slope is calculated as the steepest path downhill for a neighborhood•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= tan-1(rise/run) or tan(Φ)= rise/run


Calculating Slope in 2 Dimensions

Φ

©2005 Austin Troy

•Raster slope is calculated by steepest path in neighborhood•In this case we have several slope calculations that we must summarize

• dZ/dx can be calculated in several different ways; simplest is


Calculating Raster Slope

55Z1

51Z2

48Z3

54Z4

43Z0

36Z5

53Z6

45Z7

38Z8

22

arctan

dy

dZ

dx

dZS

c

ZZ

dx

dZ

245

c

ZZ

dy

dZ

272

©2005 Austin Troy

•Aspect is the direction (in azimuth angle) of the steepest path of a neighborhood around a cell •It’s critical for calculating flow paths, viewsheds, etc.

•Note: Spatial Analyst asks for z value, which has nothing to do with these z values—it’s for unit conversion


Calculating Aspect with Rasters

dxdZ

dydZ

Aspect arctan

Where dZ/dy and dZ/dx can be defined in either of the ways just mentioned

©2005 Austin Troy

•Can be used for many hydrologic functions such as flow direction analysis, watershed generation and drainage network analysis (set of cells through which surface water flows), which in turn can be used to generate likely locations for streams•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•Pits/sinks prevent analysis of flow because there is no direction of steepest descent.


Uses of Slope and Aspect

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•This is a method for comparing two elevation surfaces to look for areas of change (deposition/ erosion).


Spatial Analyst:Cut and Fill

Source: ESRI help

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•SA can be used to generate contours from a raster layer

•Can choose contour interval, base contour and z factor (for unit shift)


Spatial Analyst: Countours

Source: ESRI help

©2005 Austin Troy

•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.


Three Dimensional data — TIN

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Note the triangular facets defined by points


Three Dimensional data — TIN

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Here is a simple TIN skeleton, without the facets painted

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TINs can be created from raster Digital Elevation Models or from mass points with elevation values.

TINs

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• TIN triangles vary in size with the complexity of the terrain; more complex terrain needs more smaller triangles, and hence more points •When creating a TIN form a DEM, Arc GIS uses the maximum z tolerance algorithm to determine how many and which sample points to use for defining the triangles that will represent the surface

TINs

©2005 Austin Troy


•Maximim z tolerance: Compares estimated height (in temporary TIN) to the grid height (in DEM). The differences are called offsets. The user specifies a maximum z-tolerance value which the offsets cannot be greater than. Hence, a high z value allows for considerable error, meaning fewer points will be used, while a low z value allows very little error meaning more will be used.

TINs

©2005 Austin Troy


•Points are selected through an iterative process. First it constructs a candidate TIN, and for each triangle in the TIN it calculates the elevation difference of the grid cells bounded by it. If the cell with the largest difference to triangle is greater than the z tolerance, that point is flagged for addition to the TIN model. Hence, a new point is added. It goes through every triangle in this way and then recalculates a new triangulation using the new points in addition. This continues until all points are within the specified z tolerance.

TINs

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• The Z tolerance specifies how close to the raster accuracy it will be.

TINs

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DEM to TIN

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•Note how the result changes with the z value

TINs

Z= 50 Z= 200

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•Breaklines: another feature that TINs have besides triangular facets; these represent discontinuities or changes on the surface of the land, like streams and roads.

TINs

Note: these lines are part of the TIN, not another layer

©2005 Austin Troy


• So now we know how points are chosen, but how does it choose to join points together into triangles?•The most common method of joining points to form triangles is Delaunay triangulation; in this method, all points are connected to their nearest neighbors to form triangles and triangles are as equi-angular, or compact as possible•This is done by radiating circum-circles; circles are scribed through three points and the computer tests to see if any points are inside, if no points inside, it makes a triangle

How are TINs generated

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A TIN Shown from above

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TINs Shown in 3D

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3D TINs vs. 3D gridsTIN’s facet resolution can be controlled in Arc View—the smaller the facets, the better the resolution

©2005 Austin Troy

Component of Arc GIS that allows for 3D visualization of vector features, raster surfaces and Triangulated Irregular Networks in 3 dimensions. Can show terrain as well other information extruded in the third dimension.


What is Arc Scene?

SF Bay Area extruded my median income by zip code

©2005 Austin Troy


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

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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

©2005 Austin Troy


Using Arc Scene•Can also drape photos on surfaces

+

=

©2005 Austin Troy


Using Arc Scene•Can also drape photos on surfaces

=

©2005 Austin Troy


Overlaying vector on TIN in 3D

Vector building footprints over a TIN, using elevation to define the bases of the buildings and number of stories to extrude them

Making complex scenes in ArcScene

• Can also use 3D symbols graduated in size, including many building and tree models from symbol palette

©2005 Austin Troy

Introduction to GIS

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Using Arc SceneWe can also create animations of fly throughs in ArcScene

Click here to see an animation created in Arc Scene.

http://uvm.edu/~nr143/animation.avi

©2005 Austin Troy


The latest in 3d visualizationhttp://www.visualizationsoftware.com/3dem/gallery.html

For both movies and incredible 3 terrain renderings.

http://www.visualizationsoftware.com/3dem/gallery.html

©2005 Austin Troy


The latest in 3d visualization3DN World Software: http://www.3dnworld.com/Visual Nature Studio and World Construction Set

By Kingsley Allan

Illinois State Water Survey [email protected]

http://www.3dnworld.com/

http://ilrdss.sws.uiuc.edu/gis_anim.asp

mailto:[email protected]

©2005 Austin Troy



By Erik Crews USDA Forest Service, National Forests in North Carolina [email protected]

http://www.cs.unca.edu/nfsnc/

©2005 Austin Troy



By Erik Crews USDA Forest Service, National Forests in North Carolina [email protected]

Visualizing proposed road

©2005 Austin Troy



By Jason Gregory Greystone Environmental

[email protected]

Visualizing effects of elk overpopulation

©2005 austin troy lecture 15: introduction to terrain analysis ------using gis-- introduction to...

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