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A GIS-based modeling of environmental health risks in populated areas of Port-au-prince, Haiti

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par Myrtho Joseph
University of Arizona - Master in Natural Resources Information System 1987

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2.9 Classification methods

Classification procedures are utilized in various map production software to facilitate user interpretation (Longley et al. 2005). However, the statistical algorithm used to classify a range of continuous values can strongly influence the visual impression (Evans 1977), the analysis (Smith et al. 2007) and consequently the conclusions of a study. Based on the way a thematic map is created, the characteristics of the original data might be overlooked, or there might be a risk of misjudgment about the characteristic of the original data (Osaragi 2002). Natural breaks (Jenks), Quantile, Equal Interval, and Standard Deviation classifications are among the most popular used in GIS software (Osaragi 2002, Longley 2005). ESRI (1996) provides a conceptual framework of the different classification techniques along with some of their advantages and drawbacks.

In the quantile technique, an equal number of features is allocated to each class. While this arrangement is suitable for linearly distributed data it can be misleading since comparable values can be grouped in adjacent classes or diverging values can be put in the same category. On the other hand, the natural breaks method, by looking at big jumps between values overcome this weakness and ensures that similar values are placed in the same class. The equal interval scheme divides the range of values into equal-length sub-ranges and helps determining the number of intervals into which the values are distributed. The algorithm used in the geometric interval insures that there is a good distribution of values in term of quantity between classes. Likewise, this technique makes reliable the change between intervals. This approach is deemed convenient to accommodate continuous data and can generate cartographically comprehensive results. Finally, the Standard Deviation method shows the extent to which an attribute's values depart from the mean of all the values.

A study conducted by Brewer and Pickle (2002) in which they asked the respondents to evaluate seven classification methods recognized the quantile technique as the best for conveying patterns of mapped rates. To investigate the characteristics of different classification algorithms, Osaragi (2002) applied them to seven different datasets. The results suggest that the Natural Break method can be applied to different types of data for its relatively lower loss of information compared to the other, but it is not suitable for data with unclear division. Osaragi recommends examining the distribution of data before choosing a particular method. Alternatively some cartographers suggest to generate several maps for one dataset to allow the reader to compare them (Dramowicz and Dramowicz 2004). The present study compares the classification performed by four of these techniques on the spatial dataset used.

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9Impact, le film from Onalukusu Luambo on Vimeo.

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