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Application of GIS technology for Coastal Zone Management: a hydrographer perspective
Why GIS for CZM?
Since the coast all around the world are fast developing and firm management policies have to be established. However, for any management of the shore to be effective, it is necessary for the policies to be based on informed decision-making. This in turn requires ready access to appropriate, reliable and timely data and information, in suitable form for the task at hand. Since much of this information and data is likely to have spatial component, one branch of information technology with apparent potential for contributing significantly to coastal management in a number of ways. These include:
- The ability to handle much larger databases and to integrate and synthesise data from a much wider range of relevant criteria than might be achieved by manual methods. This in turn means that more balanced and coordinated management strategies may be developed for considerably longer lengths of coast.
- GIS encourages the development and use of standards for coastal data definition, collection and storage, which promotes compatibility of data and processing techniques between projects and departments, as well as ensuring consistency of approach at any one site over time.
- The use of a shared database(especially if the access is provided via a data network) also facilitates the updating of records, and the provision of a common set of data to the many different departments or offices that might typically be involved in management of a single stretch of coast. A shared database implies reduction or elimination of duplicated records, and thus the potential for significant economic savings as well as improved operational efficiency.
- Provides efficient data storage and retrieval facilities.
- GIS also offers the ability to model, test and compare alternate management scenarios, before a proposed strategy is imposed on the real-world system. Computer technology allows the consideration of much more complex simulations; their application to very much larger data bases: and also enables compression of temporal and spatial scales to more manageable dimensions.
Some Aspects of Accuracy in gis
Map accuracy is relatively a minor issue in cartography, and the map user are rarely aware of the problem. But when the same map is digitised and input to GIS, the mode of use changes. The new uses extend well beyond the domain for which the original map was intended and designed. Therefore, accuracy problem in GIS requires consideration of both object oriented and field oriented views of geographic variations. Moreover, the machine used to make measurement in GIS (Digital computers) are inherently more precise than the machine of conventional map analysis. Error analysis in spatial data base is very important with a direct bearing on the accuracy on GIS and hence require due consideration. One of major requirements in the Digital Topographic DataBases (DTDB) of a large country is consideration of a variety of features. Digital Stereo photogrammetry may be useful in extraction of cartographic features with a greater accuracy. The Digital Elevation data in the form of contours, thus digital monoplotting can be considered as a quicker method of plotting the data for maintenance of DTDB. Some of the other factors that also must be considered are:
- The type of spatial data
- The scale and resolution of the spatial data
- The type of map projection
- The measuring unit
- Horizontal and vertical datum for Geographic co-ordinates
- Metadata
The thorough explanation of the complexities of these important factors is well beyond the scope of this paper, however they are briefly summarized below:
Scale
A map scale is a representation of the ratio between a measurement of distance on a map to the same distance in the real world. For example, a map feature that is 2 centimeters long on a 1:50,000 scale map represent an object that is 100,000 centimeters or 1000 meters in the real world. The larger the scale factor becomes, the smaller the map scale becomes, and the opposite is true as well. As a general rule, as the scale of the map increases, there will also be an increase in the accuracy of the map, and there will also be an increase of the amount of geographic features that can be displayed on the map. When using spatial data for mapping and measuring purposes, the largest scale map and spatial data that cover a selected study area should be obtained for use in a geographic information system.
Spatial Resolution of Data
The degree of resolution of spatial data in important for the user of geographic information system. The spatial resolution of GIS data is also a function of generalisation since small scale map do not usually contain as much geographic details as larger scale map and as a result, the data is most often recorded and stored with less spatial precision. For vector data, spatial accuracy is a function of how many nodes and versions are used to record geographic feature at a particular map scale. The closer the vertices to each other, then the degree of resolution of the spatial data also increases. The farther apart the vertices are from each other, then the reliability of the spatial data to represent the geographical features in their true shape and size will diminish accordingly. This is especially true for rivers and coastlines when their shapes and orientation can constantly change across the earth’s surface.
Projection
The earth is a sphere that is slightly flattened in the Polar Regions, and to draw a large portion of the earth’s surface onto a flat piece of paper has always been a problem. There are over 250 different map projections and each one has its own particular strength and specific uses. Some projections are better for the accurate representation of certain regions of the world, which other projections are designed for specific purpose such as showing true direction for Navigation (but not true distance) such as with the famous Mercator Projection. All map projection will distort the earth’s sphere to a certain degree especially regarding the representation of angles, area, distances and directions.
The Measuring Unit
Different maps can be based on different types of map measuring units. Spatial data can be stored in various types of units such as decimal degrees, metres or feet. It is important to know what unit the spatial data uses so that the correct scale can be applied to the map. If incorrect measuring unit is applied to spatial data that is loaded into a GIS, then a false map scale will be the result. If the map scale is incorrect, then all the measurements using a GIS will be wrong.
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