Chapter 7: Mapping Change
GIS lets you map where things move or the changing conditions in a place over time. (149)
Why Map Change?
- People map what’s changed to anticipate future conditions, to decide a course of action, or even to evaluate the results of a certain policy or action.
- You can gain insight to things by mapping how they behave
- a meteorologist studying the path of hurricanes to predict where and when a future one can occur.
- Wildlife biologist studying the movement of bears over a 24- hour period
- Anticipating future change is another reason to map.
- A police chief studying how crime patterns from month to month, or year to year to assign officers to specific places.
- Transportation planner looking at trends in traffic flow to see where lanes need to be added in the future.
- You can see the impact of conditions by mapping them before they happen.
- Map Gallery: Maps are being created by biologists in Orlando, Florida at the St. Johns River Water Management district to map change from 1941-1987. These maps allowed the biologist to compare and understand how the historical vegetation patterns have changed and how the current conditions have developed. This information that is formed in these maps allows the district to restore the wetlands to create more waterfowl habitat, provides more recreation, and even includes water quality.
Defining Your Analysis
- You can map change by showing the location and conditions of features of each date. You can also map the difference in a value between two and more dates.
- Knowing the type of change and type of features you are working with, it will help you decide how to map the change
· The types of change come in 2 forms:
- Change in location- allows you to show how features behave so you can predict how they move.
– Change in character or magnitude- shows how conditions in a given place have changed.
- Discrete features: you can map features that physically move, even as they move through space. They can be individual features such as a hurricane, vehicle, or an animal.- Measuring time: Time will affect the geographic patterns you see on your map
- There are 3 types of time patterns:
1. A trend: change between two or more dates at a time. Trends indicate whether something is increasing or decreasing.
2. Before and after: conditions of something before an event, and after the event
3. A cycle: change over a period time; day, month, year.
- Snapshots show the condition at any given moment- it is used to map the phenomena.
- Summarizing is used for mapping DISCRETE events in an actual place that are not continuous in time.
- 911 calls made in a given month.
- Mapping trends needs you to determine the interval, the number of dates and the duration. The duration DIVIDED by the number of dates yields the interval ( 155).
- The interval does not necessarily need to be determined if there only has been a few dates collected—but you if you have a range of dates, you can chose the interval.
- The number of dates you use depend on the consistency of the change you are mapping. You have to really look at the date to determine what interval is best, because if you have fewer dates with a wider interval you may miss some changes.
- When mapping cycles , you can show the change in a snapshot or summarize over a period, depending on your data. ( discrete or continuous).
- For discrete you summarize the events.
Creating a Time Series
Creating a set of time series map is very similar to making maps to show where features are or where the most and least are.
- A time series is effective for showing the patterns movement if you are tracking many individual features. You want to include some stationary features, too.
- Time series maps are very good at showing the change in magnitude for DISCRETE areas and surfaces, especially in a large change.
- Displaying tables and charts around your map also helps show change. Also, you can combine tables by using the category field to join them and create charts.
Creating a Tracking Map
- A tracking map shows the position of a feature or features at several dates and times. It is useful for showing incremental movement of discrete features.
- Mapping the movement of linear features is extremely helpful too. To do that you draw them using different colors for each date or time, and label each.
- Linear features are more often mapped for a before and after event.
- To map the movement of a contiguous feature, you draw the boundaries of the area at each time or date. Using different colors, you alternately shade the areas to distinguish different patterns. You can be creative with how you want to shade them.
Measuring and Mapping Change
- To measure and map change you calculate the difference in the values between the two dates and map the features. With discrete features- the values you are mapping are stored in the layers data table.
- Calculating change as a percent lets you compare the relative rather than the absolute.
- For individual locations- use colors or symbols to represent the calculated value
- Like a store mapping the stores by percentage in sales year by year- you could map the trend using a trend line.
- Use graduated symbols for areas such as parcels
- Calculating change for data summarized by data- it is the same as calculating discrete features. Once you calculate the change, you make the maps based on the value. Use graduated colors to show the change.
- You can create charts and trend line charts to show the changes of different periods.
- Some features may have negative values. When making the maps, set data values that are negative, and set that as one color ( like a blue)
- By using vector and raster data you can create a map of areas that have changed
.- When using vector data, you overlay the areas to create a new layer containing category codes for both dates.
- Raster data, you create a new layer of the two inputs.- When you have continuous numeric values for two date, you want to calculate change.
- TO create a change map between the two surfaces you subtract the layers. The map shows where the most and least have changed.
