Geographer's WorkBench G.E.M. Geotechnologies 2001 Mapping Classification techniques Groups of Features with Similar Values

Geographer's WorkBench G.E.M. Geotechnologies 2001 Natural Breaks Finds groupings and patterns inherent in your data. Values in a class are likely to be similar Data values that cluster are placed into a single class Class breaks are defined where there is a gap between clusters of values

Geographer's WorkBench G.E.M. Geotechnologies 2001 Natural Breaks How it Works ; GIS determines high and low value for each class Uses mathematical procedure to test class breaks Picks class breaks to best group similar values & maximize differences between classes What it is Good for: Mapping data values that are not evenly distributed Disadvantages Data sets are map specific,therefore hard to compare to other maps Choosing optimum number of classes is difficult

Geographer's WorkBench G.E.M. Geotechnologies 2001 Notice how the population has been separated in this Natural Breaks layout. The division takes place where there is an obvious difference in population. Natural Breaks

Geographer's WorkBench G.E.M. Geotechnologies 2001 Quantile Each Class has an equal number of features in it. How it Works GIS orders the features, based on attribute value,usually from low to high. Sums the number of features Divides by the total number of classes you have pre chosen Fills all the classes with an equal number of features

Geographer's WorkBench G.E.M. Geotechnologies 2001 Quantile What is it good for: Comparing areas that are roughly the same size Mapping data in which the values are evenly distributed Emphasizing relative position of a feature among other features IE top 20 % Disadvantages: Features with close values may end up in different classes Can exaggerate the difference between features Conversely few widely ranging value may end up in same class

Geographer's WorkBench G.E.M. Geotechnologies 2001 This is very similar to Natural Breaks, however the categories are quite different. The groupings are more equal in their amount of population than Natural Breaks. Quantile

Geographer's WorkBench G.E.M. Geotechnologies 2001 Equal Interval Each class has an equal range of values – that is the difference between the high and low value is the same for each class How it works: GIS subtracts lowest value in data set from highest Divides that number by the number of classes you have chosen adds that number to the lowest data value to get maximum value for the first class Then adds to each maximum value to set breaks for rest of the classes

Geographer's WorkBench G.E.M. Geotechnologies 2001 Equal Interval What it is good for: Presenting information to a non-technical audience Equal intervals are easier to interpret since the range for each class is equal Good with familiar values- % Good for mapping continuous data – precipitation, temperature Disadvantages: Problem is data is clustered rather than evenly distributed There may be too many features in one or two classes and some classes with no features

Geographer's WorkBench G.E.M. Geotechnologies 2001 With Equal Interval the population has been divided so there is equal numbers for each category. This is quite different from the previous two. Equal Interval

Geographer's WorkBench G.E.M. Geotechnologies 2001 Standard Deviation Each class is defined by its distance from the mean value of all the feature How it works: Through a formula it measures the average amount that data varies from the mean GIS creates class breaks above and below the mean based on the number of standard deviations you specify such as ½ or 1 standard deviation.

Geographer's WorkBench G.E.M. Geotechnologies 2001 Standard Deviation What it’s good for: Seeing which features are above and below an average value Displaying data that has many values around a mean and few further from the mean – Bell Curve or normal distribution Disadvantages: The map doesn’t show the the actual values of the features, only how far their value is from the mean Very high or low values can skew the mean so that most features will fall in the same class

Geographer's WorkBench G.E.M. Geotechnologies 2001 With Standard Deviation the population has been plotted by how far away from the mean each province is. Here we can see what provinces are above or below the mean easily. Standard Deviation

Geographer's WorkBench G.E.M. Geotechnologies 2001 Choosing a Classification Scheme You need to know how data values are distributed Methodology: Create bar graph Set horizontal axis to be attribute levels Vertical axis should represent # of features having a particular value Choose proper classification scheme

Geographer's WorkBench G.E.M. Geotechnologies 2001 Choosing a Classification Scheme If your Data is unevenly distributed many features have the same or similar values, there are gaps between groups of values NATURAL BREAKS

Geographer's WorkBench G.E.M. Geotechnologies 2001 Choosing a Classification Scheme If your data is evenly distributed You want to emphasize the relative difference between features QUANTILE

Geographer's WorkBench G.E.M. Geotechnologies 2001 Choosing a Classification Scheme If your data is evenly distributed You want to emphasize the difference between features EQUAL INTERVAL Or STANDARD DEVIATION