Lecture 22: Introduction to Terrain and Network Analysis ------Using GIS-- Introduction to GIS

Lecture 22: Introduction to Terrain and Network Analysis

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

Part 1. Terrain analysis


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

•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

Note the triangular facets defined by points


Three Dimensional data — TIN


Here is a simple TIN skeleton, without the facets painted


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

TINs


• 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


•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


•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


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

TINs


DEM to TIN


•Note how the result changes with the z value

TINs

Z= 50 Z= 200


•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


• 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



A TIN Shown from above


TINs Shown in 3D


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


Using Arc Scene

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


Using Arc Scene

Here we overlay roads and urban areas on a DEM


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


Steepest Path• Another feature in Arc Scene is generation of a steepest path on a terrain surface


The latest in 3d visualization

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

For both movies and incredible 3 terrain renderings.

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


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



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


Part 2. Network analysis


Network Analysis

• Networks defined as a set of interconnected line entities, generally arcs, whose attributes share some common theme primarily related to flow.

• Arcs in a network must share the attributes necessary for analyzing these flows (speed limits, frictions, etc.)


Adapted from UC Berkeley GIS Center

Network Arcs Can Have Direction

• Directed - flows allowed to move in a single direction (down stream, one way street)

• Undirected - flows allowed in both directions (two way streets)



A network is a system of linear features connected at intersections and interchanges.

These intersections and interchanges are called nodes

Arc

Node


Basic elements of a network


The linear feature connecting any given pair of nodes is called an arc.

Network Attributes of interestAttributes that can be attached to a network:

–origin, destination coordinates

–shape point coordinates

–street name/highway number

–aliases on street name

–directionality: one way traffic?

–classification: freeway, arterial, collector, residential

–speed limit, congestion (impedance) or travel time

–traffic volume

–length: driven length vs digitized length

–scenic value

–connectivity (turn tables)



Network ProblemsPath finding

paths

tours

Allocation

supply and demand

impedance

Spatial interaction

accessibility

interaction



Path findingFinding the shortest or least-cost manner in which to visit a series of locations in a network. The cost may be determined by distance or by travel-time or a combination of factors calculated as a cost value. Distance that factors in things that slow you down is known as functional distance

Often the parameter that is minimized in path finding is travel time. This factors in things like topography, traffic volume, average speed, stops etc.



Computing best routes at the continental scale (e.g. how do I get from LA to New York?) is easy, because small variations in distance measures are relatively unimportant.

At the intra-city level, the optimization criterion is travel time, which depends on legal restrictions (stop signs, traffic signals, one ways) and congestion (which varies by the minute), hence the margin of uncertainty in routing is far greater.


Path finding


This would not happen if Ponch and John had Path finding software


“Ponch, there’s a 4-12 going on at the video arcade, what’s the fastest way to get there?”

“Gee, I dunno John, let’s get off our bikes so we can look at our big, cumbersome map, and try to figure it out?”

Location - Allocation

Allocation assigns portions of a network to a location based on predetermined criteria. Essentially an area of the network is assigned to be serviced or served by a facility at a given location. It is based on Supply, Demand, and Impedance.



Allocation Assigns Portions of a Network to a Center


These street segments can be used to define a service area


Allocation Assigns Portions of a Network to a Center


Source: http://www.geo.umass.edu/courses/geo594a/warmup/warmup3/

Allocation analysis yields service areas for each of these facilities: that is, the areas they can reach within a specified driving time.

Allocation



Spatial interaction

Accessibility = How Connected is a Node

Accessibility is an aggregate measure of how reachable a location is from other locations.



Accessibility



Usefulness of Network Analysis

Used by retailers in market studies for siting new facilities

Used by utility company in managing their infrastructure: water, sewer, power

Used by consumers to get directions

Used by agencies to map out service areas: fire, police, public transportation facilities.


Documents

Lecture 22: Introduction to Terrain and Network Analysis ------Using GIS-- Introduction to GIS