Mitchell Chap 3

Mapping the Most and Least

WHy map the most and least? – to find places that meet their criteria and take action, or to see relationships. Quantities associated with bothare key. Mapping with quantities adds another layer of information. More dense areas can be designatesd the larger dots. By mapping the pattens of features with the similar values, you’ll see where the most and least are.

Discrete features can be individual locations, linear features or areas. Continuous phenomena can be defined areas of a surface of continuous values.  Areas can be displayed using colors, surfaces with graduated colors, contours or 3-D.  Data summarized by area is displayed by shading each area by its value, or with charts.

Quantities can be counts or amounts , ratios, or ranks. Counts show total numbers, the actual number of features on the map. Can be used in discrete and continuous maps, not so much in summarized by area. Ratios show the relationship between 2 quantities, they even out the differences between large and small areas. Proportions show you what part of a whole a section represents. Ranks create an order from higher to lower. Relative values instead of discrete ones. Ranks can be given texts as well.

 

Creating Classes- Assign each individual value its own symbol or group values into classes. Trade off between accuracy and generalizing to create patterns. Counts, amounts and ratios work well in classes.  When mapping ranks assign one symbol to each rank. Features with similar values assign same symbol to see patterns. Create classes manually if looking for features with meet specific criteria, or comparing features to specific meaningful value.

Used standard classification scheme if u want to group similar values to look for patterns in the data. Classes are based on how data values are distributed. Goodway to seeing how data values are distributed is to plot them on a chart. Natural Breaks: based on natural groupings of data values, breaks set where there is a jump in values. Quantile: each class contains an equal number of features. Block groups with similar values are forced in to adjacent classes, and the block group at the high end are lumped into classes.  Equal Interval: difference between high and low value is equal for every class. Standard Deviation: Features are placed in classes based on how much their values vary from the mean. GIS adds or subtracts the SD to or from the mean to get the class breaks.

Choosing Schemes: If data unevenly distributed use natural breaks. If data evenly distriubted, emphasize the difference between features, use equal interval or SD. If data is evenly distributed and you want to emphasize the relation difference between features use quantile. Outliers can screw up maps so beware of them. Rounding minimum and maximun scales makes it easier for reader to see and interprate.

Making a Map=Graduated symbols, Graduated colors, Charts, Contours and 3-D persepctive views. Page 56-57

Graduated Symbols- to map discrete locations or lines. Points are draw at locaton of feature and magnitude of feature is descripted by size of point.

Graduated Color- to map descrete areas, data summarized by area and continuous phenomena. Assign shades of one or two colors to a class. if you have fewer than 5-6 classes use one color and vary the shade. Darker colors mean more. Red usually means “hot spot”.

Charts- to map data summarized by area or discrete locations. You can show patterns of quantities and categories at the same time. Useful for a quick study of patterns. Pie charts or bar charts can be placed on the map to show percentages of areas.

Contuour Lines- to show rate of change in values across an area for spatially continuous phenomena. Contour lines are drawn at an interval that you chose. The interval determines the number of line adn the distance between them. Contour lines should eblabeled with their value. Use a bold line for every 5th interval.

3-D Perspective- used for contiunous phenomena. Difficult to determine actual value of areas. Viewer location, make sure the taller features are in the back so the smaller ones can be seen. Z-factor can determine the variation between values. Light source and Z factor can determine how shadows develop in map. For light source you choose direction and angle.

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