This chapter has attempted to provide a ’gentle’ introduction to GIS. It has dis- cussed the nature of GIS tools and GIS as a field of scientific research. Much of the technical detail has been intentionally left out in favour of a broader discus- sion of the key issues relating to both of these topics. The chapter has looked at the purposes of GIS and identified understanding objects and events in geo- graphic space as the common thread amongst GIS applications, and that spatial data and spatial data processing are key factors in this understanding. A simple example of a study of the EL Ni ˜no effect provided an illustration, without the technical details.
It was noted that the use of GIS commonly takes place in several phases: data capture and preparation, storage and maintenance, manipulation and analysis, and data presentation. Before we get to discussing these phases, the following two chapters provide more discussion on important background concepts and issues. In Chapter 2, we will focus the discussion on different kinds of geo- graphic phenomena and their representation in a GIS, and discuss appropriate instances of when to use which. Chapter3is devoted to a discussion of data pro- cessing systems for spatial data, namely, GIS, databases and spatial databases. Following these last two chapters, the remaining structure of the book follows the phases identified above. In Chapter5we look at the phase of data entry and preparation: how to ensure that the (spatial) data is correctly entered into the GIS, such that it can be used in subsequent analysis. Analysis of geoinformation is the focus of Chapter 6. It discusses the most important forms of spatial data analysis in some detail, and looks at issues related to spatial modelling.
The phase of datavisualizationis the topic of Chapter7. This chapter deals with fundamental cartographic principles: what to put on a map, where to put it, and what techniques to use for specific types of data. Sooner or later, almost all GIS users will be involved the presentation of geoinformation (usually of maps), so it is important to understand the underlying principles.
Questions
1. Take another look at the list of professions provided on page 26. Give two more examples of professions that people are trained in at ITC, and describe a possible relevant problem in their ‘geographic space’.
2. In Section 1.1.1, some examples are given of changes to the Earth’s geog- raphy. They were categorized in three types: natural changes, man-made changes and a combination of the two. Provide additional examples of each category.
3. What kind of professionals, do you think, were involved in the Tropical Atmosphere Ocean project of Figure 1.1? Hypothesize about how they obtained the data to prepare the illustrations of that figure. How do you think they came up with the nice colour maps?
4. Use arguments obtained from Figure 1.1 to explain why 1997 was an El Ni ˜no year, and why 1998 was not. Also explain why 1998 was in fact a La Ni ˜na year, and not an ordinary year.
5. On page37, we made the observation that we would assume the data that we talk about to have been put into a digital format, so that computers can operate on them. But often, useful data has not been converted in this way. From your own experience, provide examples of data sources in non- digital format.
6. Assume the El Ni ˜no project is operating with just four buoys, and not 70, and their location is as illustrated in Figure1.4. We have already computed
120°E 140°E 160°E 180° 160°W 140°W 120°W 100°W 80°W 0° 30°S 20°N 10°N 10°S 20°S 30°N B0341 B0871 B8391 B9033 ••
Figure 1.4: Just four mea- suring buoys
the average SSTs for the month December 1997, which are provided in the table below. Answer the following questions:
• What is the expected average SST of the illustrated location that is precisely in the middle of the four buoys?
• What can be said about the expected SST of the illustrated location that is closer to buoy B0341? Make an educated guess at the tempera- ture that could have been observed there.
Buoy Position SST
B0341 (160◦W, 6◦N) 30.18◦C
B0871 (180◦W, 6◦N) 28.34◦C B8391 (180◦ W, 6◦ S) 25.28◦C B9033 (160◦ W, 6◦ S) 28.12◦C
7. In Table1.2, we illustrated some stored measurement data. The table uses one row of data for a single day that some buoy reports its measurements. How many rows do you think the table will store after a full year of project execution?
The table doesnotstore the geographic location of the buoy involved. Why do you think it doesn’t do that? How do you think these locations are stored?
Geographic information and Spatial
data types
2.1
Models and representations of the real world
As discussed in the previous chapter, we use GISs to help analyse and under- stand more about processes and phenomena in the real world. Section 1.2.1 re- ferred to the process ofmodelling, or building a representation which has certain characteristics in common with the real world. In practical terms, this refers to the process of representing key aspects of the real world digitally (inside a com- puter). These representations are made up of spatial data, stored in memory in the form of bits and bytes, on media such as the hard drive of a computer. This digital representation can then be subjected to various analytical functions (computations) in the GIS, and the output can be visualized in various ways.
Modelling is the process of producing an abstraction of the ‘real world’ so that some part of it can be more easily handled.
Depending on the application domain of the model, it may be necessary to ma- nipulate the data with specific techniques. To investigate the geology of an area, we may be interested in obtaining a geological classification. This may result in additional computer representations, again stored in bits and bytes. To examine how the data is stored inside the GIS, one could look into the actual data files, but this information is largely meaningless to a normal user.
As highlighted in in Figure 2.1, the process of translating the relevant aspects of the real world into a computer representation of it is a domain of expertise by itself. It might be achieved through direct observations using sensors, and digitizing (converting) the sensor output for computer usage. This is the domain ofremote sensing, the topic of Principles of Remote Sensing[53]. We may also do
this by indirect means: for instance, by making use of the output of a previous project, such as a paper map, and re-digitizing it.
real world GIS world
DATA GEOINFORMATION
Figure 2.1: Representing relevant aspects of real- world phenomena inside a GIS to build models or simulations.
In order to better understand both our representation of the phenomena, and our eventual output from any analysis, we can use the GIS to createvisualizations
from the computer representation, either on-screen, printed on paper, or other- wise.1 It is crucial to understand the fundamental differences between these notions. The real world, after all, is a completely different domain than the ‘GIS’ world, in which we build models or simulations of the real world.
Given the complexity of real world phenomena, our models can by definition never be perfect. We have limitations on the amount of data that we can store,
limits on the amount of detail we can capture, and (usually) limits on the time Complexity we have available for a project. It is therefore possible that some facts or relation-
1It should be mentioned here that illustrations in this chapter—by nature—are visualizations
ships that exist in the real world may not be discovered through our ‘models’. Any geographic phenomenon can usually be represented in various ways; the choice of which representation is best depends mostly on two issues. Firstly, what original, raw data (from sensors or otherwise) is available, and secondly,
what sort of data manipulation is required or will be undertaken. Key aspects Representation of phenomena of data acquisition and preparation are discussed in Chapter5. This chapter will
examine various types of geographic phenomena in more depth, and the types of computer representations available for them.