Thao Nguyen

Tuesday, October 11, 2005

Ontology and epistemology of space

M. Raubal (2000). Ontology and epistemology for agent-based wayfinding simulation. in: S. Winter (Ed.), Geographical Domain and Geographical Information Systems - EuroConference on Ontology and Epistemology for Spatial Data Standards, 22-27 September 2000, La Londe-les-Maures, France. GeoInfo Series 19, pp. 85-87, Institute for Geoinformation, TU Vienna, Austria.

Wayfinding and orientation are important parts of people's daily lives. We have to find our ways through cities, through buildings, or along streets and highways. Many times people find it difficult to perform such tasks because they are not provided with adequate "knowledge in the world." Environments lack sufficient wayfinding information or their architectures are badly designed, therefore they are too complex to facilitate wayfinding. Agent-based simulation of human wayfinding before actual construction of a built environment helps to determine where people face wayfinding difficulties, why they face them, and how wayfinding information and design have to be changed to avoid such difficulties.

Ontology and epistemology of space are basic concerns during the development of an agent-based wayfinding model. By defining the ontology for a specific wayfinding domain or environment, we describe what is in this domain in a general way. Paying attention to epistemology allows us to focus on the wayfinding agent's knowledge and beliefs. In this work we look at ontology and epistemology from the viewpoint of ecological science, a multidisciplinary advance to the study of living systems, their environments, and the reciprocity between the two. In particular, we consider the sub field of ecological psychology, which proposes to study the information transactions between living systems and their environments, especially in regard to the perceived significance of environmental situations for the planning and execution of purposeful behaviors.

Our main focus lies on wayfinding in buildings and we use wayfinding in airports as a case study. The ontology of this wayfinding environment is based on the ideas of J. J. Gibson, a proponent of ecological psychology, who investigated how people visually perceive their environment. Accordingly, we subdivide the wayfinding environment into a medium, substances, and surfaces. We move in a medium (of light, sound, odor, etc.) in which there are points of observation and lines of locomotion. The substances differ in chemical and physical composition, and are structured in a hierarchy of nested units. The medium is separated from the substances of the environment by surfaces. In our case, substances are cognizing agents, such as a passenger or an employee of the airport, and non-cognizing objects, such as a door, a sign, or a check-in counter.

The epistemological question of what the wayfinding agent can know about the environment and how it can accumulate such knowledge is modeled through affordances. Gibson described the process of perception as the extraction of invariants from the stimulus flux. Surfaces absorb or reflect light and Gibson's radical hypothesis was that the composition and layout of surfaces constitute what they afford. Affordances are therefore specific combinations of the properties of substances and surfaces taken with reference to an observer. There are many affordances in the environment but the wayfinding agent perceives only the affordances relevant for the specific wayfinding task, such as a door affords opening and moving through, or a hallway affords moving along. In an airport where all the necessary wayfinding information is on yellow signs, the wayfinding agent will only utilize the affordance of perceiving a yellow sign and will ignore signs in other colors.

Agents have to be coupled with the environment in which they perceive and act. The nature of this connection is the following: the environment provides percepts (i.e., affordances) to the agent, the agent decides upon and performs actions in (and therefore on) the environment, which in turn provides new percepts (i.e., affordances), etc. The complexity of this process is influenced by the properties of the environment.

Research in spatial ontology and epistemology is an important basis for the setup of an agent-based model for wayfinding. Perception and cognition of the agent can only be modeled in a useful way if the ontological and epistemological foundations are well established. In this work we try to connect ontology of space, epistemology of space, and spatial cognition, in order to come up with a practical agent-based simulation tool for wayfinding in buildings. Such a tool will help to design buildings that facilitate wayfinding

The Ontology of Space

The Ontology of Space
by Adam Reed

The concept of space is fundamental to physics, and therefore to all physical science. But does this concept correspond to a real existent? The naﶥ answer would be "no:" to the non-scientist, space denotes emptiness, the absence of existents. And of course absence is not an existent. Fortunately, this naﶥ view is wrong. To demonstrate exactly how it is wrong, one must track the concept of space back to the very identity of existence qua existence. And existence is identity.

What, then, is Identity?

The identity of an existent consists of the measurable values ("measurements") of its attributes. Thus for space to exist, its attributes would have to have measurements. But in what sense can space have measurements? What is a measurement?or more precisely, a measurable value of an attribute?anyway? For identity to refer to existence, the components of identity must be existentially extrinsic to consciousness. So the relevant referent is not the result of the conscious action of measurement. It is rather the existential fact that the action of measurement measures. Indeed, the primacy of existence implies that while one's ability to measure that existential value?and arrive at a result of measurement?is contextual and depends on the state of one's knowledge, the existential value remains the same?even while one's ability to measure it, and the nature of the result of the measurement, undergoes transformations commensurate with the knowledge in which one's measurement procedures are grounded.

To illustrate the relation of an extrinsic measurable value to increasingly sophisticated methods of objective measurement, I will use the example of measuring color.

The simplest method of measurement, and the first one learned by children, is nominal measurement. The child first learns by ostensive definition the names of some exemplary colors?red, blue, yellow, green, orange, purple and so on. The sensation of color from the newly perceived object is then compared to the known exemplars, and the name of the closest match is attached to that color as a result. For example, if the closest match was previously identified by the name "blue," the result of nominal measurement will be "blue." This result depends on the context of what colors the child already knows. Once the child learns the color "turquoise," the same extrinsic value may yield a new name.

Once the child begins to learn about the qualities of light, it becomes possible to measure a color on a linear instead of a nominal scale. Instead of a best match to one of previously learned named colors, the given color is compared to the whole rainbow laid out by wavelength of light, and the wavelength at which the contrast is at a minimum is noted. Now the measurement of color is a wavelength, a number on a continuous scale?a scalar.

Studying light further, one will notice that colors of the same scalar wavelength may still look different from each other. That is because the retina of the human eye has three kinds of color-sensitive receptors, and light in which the same wavelength seems dominant may in fact contain different proportions of light that stimulates red, green and blue cells. One can measure the amount of red, green, and blue in the given color, and the result of this measurement is a vector?an ordered array?of three numbers.

Suppose, finally, one is measuring the same color in order to duplicate it in a camouflage paint for ships of war. Potential enemies may use detectors at any possible wavelengths, not just the wavelengths seen by the retinal cells of the human eye. In that case, one must measure and duplicate not just the proportion of red, green and blue, but the distribution of light intensities in the whole spectrum of wavelengths. The result of this last measurement is a continuous distribution of light intensity as a function of wavelength. And all these measurements?nominal, scalar, vector, and continuous function?are measurements of the same existential value of the color attribute of some existent. We see that in its existential sense as a component of identity, the concept of measurement is quite general; it may refer to something a great deal more complicated than the simple scalar numbers one ordinarily thinks of as "measurements."

What, then, are the measurable attributes of space? That would be an appropriate question if "space" were a unit of some concept. But space is not a unit?it is the collective name of all the locations in the universe, or perhaps of all the locations not otherwise occupied by specific material existents?particles and so on. So the question becomes: is a point, a specific location in some inertial frame of reference, an existent?

If one took the standard materialist tack, and confined the notion of an existent to entities of matter and energy only, then the answer would have been "no." However, once existence is understood as coextensive with identity, then the only requirement of an existent is that it have attributes with measurements. In the transcript of the discussion session on the topic of axiomatic concepts (p. 241 in the 1990 Meridian paperback edition of Introduction to Objectivist Epistemology), in answer to "Prof. B" during discussion of axiomatic concepts, Ayn Rand says that an existent is "anything which you can isolate, whether it is an entity, a relationship, an action, or an attribute." Therefore, if a point has an identity?attributes with measurements?then a point, in the sense of a location in space, is also a kind of existent.

Such attributes are not difficult to find. For example, the gravitational field of every mass extends throughout space. The electric field of a charge also extends through all points, and so on. Therefore the strength and direction of physical fields (such as the electric and magnetic fields) are attributes that have specific values at each specific location in every inertial frame of reference. Therefore every point in space is an existent in its own right. "Space" refers to a collection of real existents?and therefore space exists.

Note that space exists without necessarily having to be filled with a substance, such as the "ether" commonly postulated in late-nineteenth century physics. Whether a substantial "ether" exists is a question for physics, not philosophy. As long as there is no physical evidence of a substantial "ether," the sensible presumption remains that there is no such thing

Friday, October 07, 2005

Vietnam Demographics

Demographic Indicators: 2005 and 2025

2005 2025
Births per 1,000 population.................... 17 13
Deaths per 1,000 population.................... 6 6
Rate of natural increase (percent)............. 1.1 0.7
Annual rate of growth (percent)................ 1.0 0.6
Life expectancy at birth (years)............... 70.6 75.6
Infant deaths per 1,000 live births............ 26 14
Total fertility rate (per woman)............... 1.9 1.8

Midyear Population Estimates and Average Annual Period Growth Rates:
1950 to 2050
(Population in thousands, rate in percent)

Year Population Year Population Period Rate

1950 25,348 2005 83,536 1950-1960 2.2
1960 31,656 2006 84,403 1960-1970 3.0
1970 42,577 2007 85,262 1970-1980 2.3
1980 53,715 2008 86,117 1980-1990 2.3
1990 67,283 2009 86,968 1990-2000 1.6

2000 79,060 2010 87,814 2000-2010 1.1
2001 79,999 2020 96,341 2010-2020 0.9
2002 80,908 2030 102,802 2020-2030 0.6
2003 81,791 2040 106,363 2030-2040 0.3
2004 82,663 2050 107,773 2040-2050 0.1

Midyear Population, by Age and Sex: 2005 and 2025
(Population in thousands)

------------2005----------- ------------2025-----------

TOTAL 83,536 41,362 42,174 99,978 49,756 50,222
0-4 6,965 3,612 3,353 6,747 3,471 3,276
5-9 7,577 3,936 3,641 7,084 3,649 3,436
10-14 8,736 4,518 4,218 7,002 3,614 3,388
15-19 9,067 4,668 4,399 6,801 3,516 3,285
20-24 8,659 4,411 4,248 6,781 3,508 3,272
25-29 7,423 3,769 3,655 7,359 3,805 3,554
30-34 6,737 3,385 3,353 8,432 4,335 4,097
35-39 6,051 2,998 3,053 8,695 4,443 4,252
40-44 5,726 2,785 2,940 8,254 4,165 4,089
45-49 4,690 2,204 2,487 6,997 3,503 3,494
50-54 3,281 1,514 1,767 6,246 3,073 3,173
55-59 2,169 973 1,195 5,470 2,635 2,835
60-64 1,628 700 928 4,966 2,320 2,647
65-69 1,559 657 902 3,822 1,689 2,132
70-74 1,334 545 789 2,408 1,014 1,394
75-79 984 381 603 1,341 518 823
80-84 570 192 378 760 259 500
85-89 261 81 179 491 153 339
90-94 95 27 68 233 64 169
95-99 21 6 15 75 19 56
100+ 3 1 2 15 3 12

Source: U.S. Census Bureau, International Data Base, April 2005 version.

Thursday, October 06, 2005

Torsten Hägerstrand: Time Geography

Background As late as the 1960s, there were no accepted models linking the spatial and temporal capacities and restraints on individual behavior. Torsten Hägerstrand, professor in the Department of Social and Economic Geography at Sweden's Lund University, had studied human migration in the 1960s. In August 1969, he presented a paper to the European Congress of the Regional Science Association in Copenhagen, Denmark. Although the paper was ostensibly an argument for regional scientists to address the individual human element in their aggregate models, at the heart of it was the need to examine the spatial and temporal coordinates of human activity. The spatial-temporal model that he unveiled was destined to change the course of history in the social sciences.

Innovation Historically, social scientists studying the effects of space on human behavior tended to treat time as an external factor, something that is relevant to understanding a given phenomenon, but not essential. Activity choices were seen being made in the context of distance alone, such as with the gravity model, and often these decisions were seen in an aggregate sense, with individual decisions viewed as minor variations of those of larger zonal-based groups.

Torsten Hägerstrand's paper, What about People in Regional Science? published in 1970, challenged such long-held beliefs. Having spent many years researching human migration patterns, he was convinced that the study of human beings as groups and aggregate populations masked the true nature of human patterns of movement. "It was primitive economics to assume that banks should worry about the identity of coins," he noted. "Is it advanced or primitive social science to disregard the identity of people over time in the same fashion?" While he felt that social scientists should "leave it to the historian[s] to concern [themselves] with biographies of sample individuals," he believed that an understanding of disaggregate spatial behavior was paramount.

Along with using the individual human as the unit of study, Hägerstrand also emphasized the importance of time in human activity. "Time has a critical importance when it comes to fitting people and things together for functioning in socio-economic systems," he noted. Hence, a given location may be near an individual, but if a person cannot allocate enough time to travel to it, spatial proximity alone will not be enough to allow the person to visit it.

Hägerstrand came up with the concept of a space-time path to illustrate how a person navigates his or her way through the spatial-temporal environment. The physical area around a given individual is reduced to a two-dimensional plane, on which his or her location and destination are represented as zero-dimensional points. Time is represented by the vertical axis, creating a three-dimensional "aquarium" representing a specific portion of space-time. The path of a stationary individual will appear as a vertical line between the starting and ending times, and a specific location (or "station") will trace a vertical "tube" in the same manner. If an individual moves between two stations over a period of time at a constant speed, it will draw a sloped line in the three-dimensional space-time between the two tubes. The faster an individual travels, the sooner he or she will reach the destination, and the more sloped the line will be.

Hägerstrand used the space-time path to demonstrate how human spatial activity is often governed by limitations, and not by independent decisions by spatially or temporally autonomous individuals. He identified three categories of limitations, or "constraints": capability, coupling, and authority. Capability constraints refer to the limitations on human movement due to physical or biological factors. Thus, for example, a person cannot be in two places at one time. A person also cannot travel instantaneously from one location to another, which means that a certain tradeoff must be made between space and time. Those with access to cars and bullet trains have a spatial-temporal advantage over those who are limited to their feet or bicycles for transportation. A coupling constraint refers to the need to be in one particular place for a given length of time, often in interaction with other people. This coincidence of space-time paths is described (in an electrician's jargon) as "bundled" paths in a station's tube. In other words, your space-time path must temporarily link up with those of certain other people to accomplish a particular task. This could mean anything from visiting the supermarket to going to work for the day. Lastly, an authority constraint is an area (or "domain") that is controlled by certain people or institutions that set limits on its access to particular individuals or groups. For example, a person's space-time path is normally not permitted to enter a sensitive military base or private club.

A space-time path represents the path taken by an individual, but any one path is only one of many that can actually be taken by a person in a given amount of time. A space-time "prism" is the set of all points that can be reached by an individual given a maximum possible speed from a starting point in space-time and an ending point in space-time. For example, if a man has to leave home at 11:00 a.m. and return home by 1:00 p.m., and he can travel at a maximum of 50 miles per hour, a point 50 miles away would be unreachable by 11:30 a.m. (hence outside of his prism). He could arrive at that point at noon, but would have no time to stay, since he was exactly on the outermost extent of the prism, and would have to immediately turn back. However, if he traveled at 100 miles per hour, the prism's boundaries would widen, and the point would easily be reachable by 11:30 a.m. Instead of having to immediately turn back, he could stay at that station for a full hour before leaving, amounting to a significant savings of time. In essence, the physical life-paths that we can take are controlled by the constraints in each of our space-time prisms, known more precisely as "potential path spaces," or PPSs.

Hägerstrand's concept of space-time was powerful because it was simple. Although its inspiration was derived from the study of human migration patterns, it quickly took hold across the social science spectrum during the 1970s. Space-time geography revolutionized the study of transportation accessibility largely because of its ability to represent individual behavior in a reasonably accurate manner. In 1976, Bo Lenntorp, one of Hägerstrand's associates at Lund University, studied how increased bus services in the city of Karlstad could increase the number of areas within the city that would be accessible to a person given a particular individual "activity program." Three years later, Lawrence Burns further elaborated on the accessibility aspect of the space-time model by demonstrating the impact of altering factors such as differing modes of travel, increased transportation options, even the hour of the work commute. For example, Burns showed how space-time prisms are in effect narrower during rush hour, and how flextime schedules could enable commuters to maximize their prisms by traveling at optimal times of the day. He also foresaw the advent of modern communications technologies having significant impacts on time savings.

Throughout the 1980s and 1990s, Hägerstrand's model continued to influence fields ranging from city planning to social equity. In 1991, Harvey Miller from the University of Utah demonstrated how space-time prisms could be applied to modern transportation GIS systems. Miller pointed out that the two-dimensional footprint of the PPS, known as the "potential path area", or PPA, was conceptually similar to potential paths taken along a network system of arcs and nodes to determine accessible areas for a given location. Several years later, Mei-Po Kwan of Ohio State University demonstrated that space-time models could show disparities in gender accessibility -- even from the same household -- that were invisible in traditional spatial gravity models.

Over thirty years after it was first introduced, Hägerstrand's space-time model continues to provide new ways of understanding human activity in space, and promises novel solutions for solving difficult issues of transportation and access in modern society.

Innovation diffusion as a spatial process. Translated by A. Pred. (Chicago: University of Chicago Press, 1967).

"On the definition of migration." (Lund: Lunds Universitets Kulturgeografiska Institution, Rapporter och Notiser, 9, 1973).

"The impact of transport on the quality of life." (Lund: Lunds Universitets Kulturgeografiska Institution, Rapporter och Notiser, 13, 1974).

"The domain of human geography." Directions in geography, ed. R. J. Chorley, 67-87. (London: Methuen, 1973).

"Space, time and human conditions." Dynamic allocation of urban space, ed. A. Karlqvist et. al. (Lexington: Saxon House Lexington Book, 1975).

Related Works
Pred, A. "The choreography of existence: Comments on Hagerstrand's time-geography and its usefulness." Economic Geography, 53:207-21 (1977).

Pred, A., ed. Space and time in geography: Essays dedicated to Torsten Hagestrand. (Lund: Gleerup, 1981).

Kellerman, A. Time, space and society: Geographical-societal perspectives. (Boston: Kluwer Academic Publishers, 1989).

Miller, Harvey J. "Modelling accessibility using space-time prism concepts within geographical information systems." International Journal of Geographical Information Systems, 5:3, 287-302 (1991).

Kwan, Mei-Po. "Space-time and integral measures of individual accessibility: A comparative analysis using a point-based network." Geographical Analysis, 30:3, 191-216 (1998).

Wednesday, April 27, 2005

Message of The Day

Final Project due on MONDAY, Hurry up!