Digital Terrain Modeling: Principles and Methodology - Chapter 14 doc

Applications of Digital Terrain ModelsDTMs have found wide applications since their origin in the late 1950s, in various disciplines such as mapping, remote sensing, civil engineering, m

Applications of Digital Terrain Models

DTMs have found wide applications since their origin in the late 1950s, in various disciplines such as mapping, remote sensing, civil engineering, mining engineering, geology, geomorphology, military engineering, land planning, and communications (Catlow 1986; Petrie and Kennie 1990; Li and Zhu 2000; Maune et al 2001) In this chapter, brief descriptions of various applications will be given

14.1 APPLICATIONS IN CIVIL ENGINEERING

The first application of DTM is in civil engineering, more precisely, highway engin-eering In 1957, Roberts (1957) proposed the use of DTMs for highway design One year later, Miller and Laflamme (1958) used the data to set up a cross-section (pro-file) model and coined the concept of DTMs for the first time Thereafter, Roberts and his colleagues at MIT developed the first terrain modeling system This system could not only interpolate in the sections (profiles), but also calculate the cut-and-fill between sections and provide useful data for engineering design By 1966, they had been able to provide programs for road design using DTMs, most of which were based on the cut-and-fill calculation Many techniques originally developed for road design have been applied to construction engineering, such as the design of reservoirs and dams DTMs have also been widely applied to other related engin-eering such as mining In this section, applications in road enginengin-eering and water conservancy will be described briefly

14.1.1 Highway and Railway Design

The development of a transportation network is complicated, aiming to provide

a network to satisfy the needs of society The design process can be split into steps such as site investigation, route planning and design, earthwork calculation, pavement

285

(a) (b)

Figure 14.1 Cases involved in the design of roads and railways: (a) excavation in the form of

cuts (cross section); (b) embankments in the form of fills (cross section); (c) digging

in the form of tunnel (profile); and (d) construction in the form of bridge (profile).

design, bridge and tunnel design, and so on DTMs help in route planning and design and earthwork calculation

Due to variations in the terrain, it is unlikely that a road or railway can be construc-ted without any earthwork In most cases, tunnels and bridges need to be construcconstruc-ted, hills and lowlands modified (Figure 14.1) Earthwork can take the form of excava-tion or the construcexcava-tion of embankments, to carry an elevated highway or railway Normally in a road or railway design, both cuts and fills will be necessary

Designers make every effort to select a route passing through areas with stable geological conditions, with gentle slopes and small curves to minimize earthwork Traditionally, such work was done on contour maps Nowadays, DTMs are widely used for drawing plans, profiles (along the designed central line), and cross sections, for computing the volume of earthwork, for generating perspective views, and even for producing 3-D animation As various routes are possible for a given project, the aim of the design is to obtain an optimal route

The basic requirements for landscape modification with a TIN are the ability to insert and delete points in a triangulation (seeChapter 5)and to assign to them a particular height Although it is desirable to have additional tools that are specific

to the particular application, terrain manipulation can be done with these alone — other techniques are described inChapter 15 Figure 14.2shows a triangulated surface where roadside elevation values have been added to the original terrain, and some

of the original data points have been modified Figure 14.3shows a 3-D view of this model, which may then be used to evaluate the feasibility of the proposed route Figure 14.4shows a highway on the TIN-based DTM with shading

In mining, DTMs have also been used to compute earth volume and to simulate mining progression

14.1.2 Water Conservancy

There are different types of water conservancy projects, such as reservoirs and canals

A canal project is similar to a road project, but there are differences The major difference is that water cannot naturally flow uphill Therefore, a canal is normally not allowed to have upward slopes

Figure 14.2 A triangulated terrain surface with road lines added.

Figure 14.3 A 3-D view of the triangulated terrain surface with road lines.

Figure 14.4 A highway designed on TIN-based DTM with shading.

In a reservoir project, the water volume needs to be estimated, the location of the dam determined, various critical water and outlet water levels designed, and drainage planned Reservoir volume and area are two major features DTMs can be used to replace traditional contour maps to assist in selecting a site for the dam and estimating water volume The procedure for the computation of water volume is as follows:

1 Let points(xA,yA) and (xB,yB) be the two points on the dam axis, then the equation

for this axis is

wherek = (yB− yA)/(xB− xA) and b = yA+ kxA

2 Compute the intersection points(x i,y i ) of the dam with the contour lines at different

levels, which are derived from the DTM of the reservoir area

3 Compute the area of the irregular polygon formed by each contour (e.g., thekth

contour) and the designed dam:

A k= 1

2

(x i+1 + x i )(y i+1 − y i ), k = 1, 2, , m (14.2)

4 Compute the volume between the adjacent two areas (e.g., thekth and the (k+1)th):

V =1

whereH is the height difference between the two adjacent areas.

5 Compute the volume contained by the reservoir:

In the end, the curves showing the relationships between water level and reservoir volume and between water level and water area can also be produced with ease

14.2 APPLICATIONS IN REMOTE SENSING AND MAPPING

DTMs have many applications in remote sensing and mapping, such as topographic mapping (contours), thematic mapping, orthoimage generation and image analysis, map revision, and so on In this section, only the applications in orthoimage genera-tion and remote sensing image analysis are discussed

14.2.1 Orthoimage Generation

To make images useful as backdrops for other thematic information and base maps,

it is desirable that the images have characteristics similar to those of maps This means that the same scaling, orientation, and projection into a geo-referencing system (e.g., a national geodetic system) should be adopted To accomplish this,

a number of requirements must be fulfilled:

1 All image points should be registered in a geo-referencing system such as a national geodetic (or grid) system

2 Each point (pixel) of the resulting image should have the same scale if the ground area is small, or else scale variations should follow a map projection

3 The relative relationships between features should also be retained

Remote sensing images, either satellite or aerial images, do not have such good characteristics due to the distortions caused by the imperfections of camera or scanner systems, the instability of platforms (tilts and flying height variations), atmospheric refraction, the earth’s curvature, and terrain height variations The two most serious factors are the instability of the platform and terrain height variations Therefore, geometric rectification is required

To rectify the images, the relationship between the image and ground points needs

to be established For aerial photography, relationships were discussed inChapter 3 and expressed by Equation (3.3) For scan image, Equation (3.3) can still be used but only for each scan line Therefore, it is not practical to use Equation (3.3) to model scan images, because there are normally thousands of scan lines in a frame, and this results in too many unknowns In practice, a polynomial (normally second or third order), as listed inTable 4.1,is used to approximate geometric transformation models A few control points from both the image and the ground are measured as reference points to solve the coefficients of this model Then, all points on the image can be transformed to the ground This is geometric rectification in which distortions are corrected, minimized, or redistributed However, the distortion caused by terrain variation is still there To remove this distortion, as shown in Figure 14.5, a DTM is required

In Figure 14.5(a), the distortion caused by relief is shown The ground point A has a heightz over a reference datum This causes a displacement “aa” on the image.

In the case of rectification, if the height of each pixel on the image, for example,

“a” in Figure 14.5, is found from the DTM, then a correction can be applied The determination of the ground height is done by an iterative process (Albertz et al 1999),

as shown in Figure 14.5(b)

A ′

A

O

n

a ′

B

N

a

z

H H–z f

X2X4X5X3 X1

z1

z2 z4z

5z3

Iteration of the ground coordinates

O O

a

Figure 14.5 Image distortion due to relief and its correction: (a) image distortion caused by

relief and (b) intersection of ground surface with light ray.

B

A

B

Sun

Figure 14.6 Effect of topographic variation on image brightness.

Table 14.1 Removal of Topographic Effect with Band Ratio

Image Brightness on Image Brightness on Slope Facing Away from Sun Slope Facing Sun Unit Red Band NIR Band Red/NIR Red Band NIR Band Red/NIR

14.2.2 Remote Sensing Image Analysis

In Chapter 12, shading was used to produce a vivid representation of terrain surface It was argued that the surface reflectance is different if the slope and aspect are different This means that the brightness of image pixels is also affected by the slope and aspect of the terrain surface Figure 14.6 shows such effects

However, the image from all the bands will suffer from the same effect for the same area Therefore, in remote sensing, band ration is widely used to remove such effects In Figure 14.6, there are two types of land cover, A and B The sunlight is from the lower-right corner Therefore, the surface on the right side will appear to

be brighter on the image Table 14.1 gives a possible example The ratios of the red and the near infrared (NIR) bands are the same although the absolute differences are very different With a DTM, the slope and aspect map of the terrain surface can easily

be produced, as discussed inChapter 13;thus, the topographic effect can then be removed In the end, a more reliable analysis could be made from the image after the removal of topographic effects

14.3 APPLICATIONS IN MILITARY ENGINEERING 14.3.1 Flight Simulation

Pilot training is difficult, costly, and sometimes dangerous It is natural to think about simulation so that the pilot can sit down in front of a special device to learn how to control an airplane In addition to pilot training, flight simulation can also be used for mission planning and rehearsal

Figure 14.7 Virtual battlefield environment simulation (You 1991).

In simulation, the DTM plays an important role 3-D rendering techniques are employed to simulate the terrain, often with LOD techniques Textures and other attributes can also be mapped onto the DTM surface to generate realistic scenery

To simulate scene changes while flying, the “fly-through” technique is used DTMs can also be used to guide cruise missiles This is done by matching the DTM surface stored in the computer with the real world sensed by the detectors on board the cruise missile

14.3.2 Virtual Battlefield

The virtual battlefield is a simulation of a potential battlefield generated in computers, which allows people to be involved Battlefield simulation provides a dynamic and stereo environment, which can be used to recapitulate the battle, evaluate the results, and gain experience DTM is used to simulate the battle environment Figure 14.7 is

an example of the virtual battlefield environment

A number of parameters can be derived from the DTM for the battlefield simu-lation, such as intervisibility, shields of the landform, exposed distance of a moving unit, the closest shielding distance to the target, and accessibility of the battle field

14.4 APPLICATIONS IN RESOURCES AND ENVIRONMENT 14.4.1 Wind Field Models for Environmental Study

In climatology, environment, and forestry, to predict the spread of forest fires and pollution, it is necessary to be able to predict the wind direction To do so, enough information about the wind model of the areas of interest needs to be obtained

A large mountain range exerts dynamic and thermodynamic effects on the atmosphere The dynamic effect means that with rotation and gravity, mountains force the air to flow like waves at various scales, which results in changes in the wave fronts The thermodynamic effect means that temperature differences between day

and night in different parts of the mountains result in local influence of hill and valley winds The direction and velocity of winds also vary with altitude, slope, aspect, and terrain roughness, which results in a complex and unstable wind field in mountain areas To model this, all the essential topographic parameters can be extracted from DTMs, such as

1 the lowest, highest, and average elevations of each grid cell

2 the lowest, highest, and average elevations of certain square areas delineated with specific conditions

3 the average slope of each cell

4 the percentage of cells containing such terrain features as ridges, valleys, and flat lands

5 the standard deviation of elevations and slopes

14.4.2 Sunlight Model for Climatology

The mountain climate is deeply influenced by terrain variations These effects result in different climates on the two sides of the mountain range Also, unique local climates may be formed on any part of a mountainous region because of dif-ferent combinations of altitudes, slopes, and aspects, as well as the shading effect of mountain ridges When analyzing the mountain climate, one has to consider not only the relatively invariable factors like geographic latitude and average altitude of the region, but also the micro topographic parameters such as the local height differences, aspect, and shading areas

DTMs play an important role in sunlight modeling The incidence direction of sunlight is defined by the functions of date, time, and the latitude and longitude of the area The sunlight received by each cell is also dependent on the slope, aspect, and altitude of the cell To produce a precise sunlight model, the coordinates of grid cells are transformed into latitude and longitude, the angle between the incidence direction of the sunlight and the outward normal of the grid cell is then calculated, the shade status is judged by a hidden surface algorithm of the 3-D perspective, and then the instantaneous sun radiation can be accurately calculated The sun radiation

of a grid cell in 1 day can be obtained by summarizing all instantaneous sun radiation Similarly, the sun radiation on one grid cell during a month, a season, or a year can

be calculated Of course, the sun radiation at one time period on one slope surface can be obtained by accumulating all the radiation values of those grid cells on the surface

14.4.3 Flood Simulation

The flat areas of river basins are often flooded after heavy rain Therefore, it is necessary to study flood risks To do so, potential flood levels and velocity are the two major parameters to be considered The DTM has been used to simulate floods

In such a simulation, with a given rainfall, the amount of water from different catch-ments can be estimated, as described inChapter 13.After considering the capacity

of the river, the amount of water to be accumulated can be computed Then, the area

(a) (b)

Figure 14.8 Flood areas simulated with satellite images superimposed The color plate can

be viewed at http://www.crcpress.com/e_products/downloads/download.asp?cat_

no = TF1732.

to be flooded can also be estimated Figure 14.8 shows an example of the flood areas simulated using a DTM, with satellite images superimposed

14.4.4 Agriculture Management

Recently a popular term — precision farming — has come into use It means that that farmers can control the quantity of water, fertilizer, and pesticides placed on different areas of the farm land, based on the attributes of the land, such as soil type and condition, slope, the condition of the crops, and so on Slope (as well as aspect) information can be derived from a DTM

Slope is a type of spatial information important to soil erosion In some developing countries, areas with steep slopes are still farmed, resulting in serious soil erosion This was also the case, for example, in China However, in the late 1990s, the Chinese government ordered that no lands with a slope over a certain value should be farmed

In this way, the situation of soil erosion has been improved

14.5 MARINE NAVIGATION

The topographic surface is usually observed above sea level, but clearly the sub-sea terrain surface is important in various applications Terrain model construction is often more difficult, as the overall surface form is often not available at the time of sampling Observations of the sea floor are often made along ships’ tracks, giving a highly anisotropic distribution that requires special interpolation techniques to reconstruct plausible surfaces (Gold and Condal 1995)

However, the most important marine application of DTMs is undoubtedly in ship navigation This is most commonly based on the use of electronic or paper mar-ine charts, but is now being considered for 3-D representations (Gold et al 2004) Figure 14.9shows a navigator’s view of the East Lamma Channel, Hong Kong, together with the superimposed chart symbols Figure 14.10 shows the use of dynamic 3-D safety contours, highlighting the safe channel for a particular ship’s draught.Figure 14.11 shows how the dynamic intersection of the tidal sea surface with the terrain is used for

a collision-avoidance system, with the kinetic Voronoi diagram (derived from TIN) as the basis — collisions are only possible between the generators of adjacent cells

Figure 14.9 A navigator’s view of the East Lamma Channel.

Figure 14.10 Channel safety contours based on sub-sea topography.

Figure 14.11 The intersection of the terrain and sea surface: collision detection using kinetic

Voronoi diagrams.

and night in different parts of the mountains result in local influence of hill and valley winds The direction and velocity of winds also vary with altitude, slope, aspect, and terrain. .. valleys, and flat lands

5 the standard deviation of elevations and slopes

14. 4.2 Sunlight Model for Climatology

The mountain climate is deeply influenced by terrain. .. Band Red/NIR Red Band NIR Band Red/NIR

14. 2.2 Remote Sensing Image Analysis

In Chapter 12, shading was used to produce a vivid representation of terrain surface