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Freshwater Management Series No. 5

Guidelines for the Integrated Management of the Watershed
- Phytotechnology and Ecohydrology -


A. Introduction

GIS is an acronym for Geographical Information Systems, a computer program designed for processing and analysing geographically-located data. This type of data means that we have not only a value for a given data point but also a location for that quantity. The location information is expressed usually in geographic co-ordinates. The word "system" in the context of GIS should be understood to be a combination of these data, the computer program, and the users who process the data and interpret the results. No machine can replace a human in its ability to interpret and understand geographical data, but the computer can contribute immense power for the solution of simple arithmetic computations, thereby making it a useful tool. A characteristic feature of geographical data sets is their large volume. They describe the distribution of certain characteristics in space, and, potentially, in time. These data sets contain or carry information, but they commonly have a simple structure. Information, which is a basis for analysis, requires that data be processed and enhanced. Geographic data sets are continuously changing, and older data may become outdated. The rate at which data sets become outdated depends on process dynamics, and on access to new measurements, observations, etc. The simplicity of data entry, processing, and printing within GIS programs, therefore, adds value in that updating of the geographical information becomes an ongoing process, with the result that computer maps do not age as quickly as analogue paper maps.

An important issue in classifying GIS programs is the volume of data to be processed. Major institutions and organisations may have large collections of spatial data covering vast areas. However, for small projects, we do not need large data sets or very sophisticated tools. Notwithstanding, the value of using GIS in these smaller applications is that data, once processed, can be refreshed and accessed by multiple users. For scientific purposes, programs with analytical tools may be of greater use, while, for nature resources management applications, data accessibility, maintenance, security, standardisation, and compatibility may be more important. In ecohydrological studies, more attention should be paid to the first group of programs because of the complex questions we want to ask in order to understand the relationships between landscape components.

GIS operations are similar to other computer program operations. GIS perform following tasks:

  • organisation of data collections in a structured way,
  • provision of storage and access to data,
  • data processing and analysis,
  • presentation of the results in the form of maps, graphs, and tables.

Some of these tasks can be performed by other commonly-used programs like data base programs, computer aided design (CAD) programs, statistical packages, and graphical tools. However, the GIS programs merge the abilities of these individual programs, and add a large library of procedures designed to analyse spatial data (e.g., functions to interpolate, generalise, and conduct neighbourhood analyses) and provide a mechanism to visualise the results. In ecohydrological studies, GIS play an important role in allowing users to integrate various data and thematic layers. Important features include the ability within the GIS program to perform scenario-based, cartographic modelling, ands to reconstruct past, and forecast future, situations in the environment.

B. Spatial data co-ordinates

A common feature of all data processed in GIS programs is their use of co-ordinates describing their location in space. There are many systems by which an object may be located in space; for instance: objects can be located by geographical name, typology, relative co-ordinates, or cartographic co-ordinates. From the point of view of accuracy and unique identification of an object, the optimal means of describing its location is by cartographic orthogonal co-ordinates (x, y, z) which can be obtained from topographic maps or calculated from geographic co-ordinates (φ, λ, h) obtained from a global positioning system (GPS) receiver. Effectively, this means that information sources for a GIS are often both analogue topographic maps and thematic maps. Data locations generally are described in a system of orthogonal cartographic co-ordinates, making it possible to perform cartometric calculations of length, area, or volume.

The cartographic co-ordinate-based location of an object is described by eastings and northings. These provide a co-ordinate pair (x, y) comprised of even numbers of digits. To represent the spherical surface of an ellipsoid as a two-dimensional plane map, mathematical transformations which describe the cartographic projection are commonly used. These transformations can be generally expressed by two equations :

X = f1(φ, λ)
Y = f2(φ, λ)

where: (x, y) - orthogonal plane cartographic co-ordinates, and
(φ, λ) - geographic co-ordinates.

Every cartographic projection has its own equations, f1 and f2. GIS programs have an ability to do conversions between various cartographic projections. In every project, however, it is important to select a single working projection which will be standard for all processed maps. To convert (x, y) or (φ, λ) co-ordinates to the working projection co-ordinate system (x', y'), we need to know parameters of the equationsf1 and f2.

There are over 400 different cartographic projections. The choice of cartographic projection depends on the purpose of the map being produced. Conformal projections are used for topographic and hydrographic maps, while thematic maps in atlases usually are based on equidistant or equivalent (equal area) projections. Valuable sources of information for GIS are topographic maps, because they contain detailed information on many objects, are accurate, cover a wide range of scales, and have a co-ordinate grid system. Topographic maps are made using geodetic, topographic, and photogrammetric survey data. Twenty-seven different cartographic projections are used for topographic maps, with those that are most frequently applied being the transverse Mercator, Gauss-Krer, and quasi-stereographic projections.


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