Overall: Finding what is inside can let you see activities that occur in a certain area or summarize information for several areas for comparison.
To find what is inide you can draw boundary lines on top of a feature, use area boundary to select the feature inside and list or summarize them, or combine the area boundary and features to create summary
Finding what’s inside a single area
Service area around a central facility
A buffer that debines a distance around some feature
An administrative or natural boundary
Area you draw manually
The results of a model, such as boundaries of floodplain modeled in GIS
Finding what’s inside multiple areas
Contiguous – zip code/watersheds
Disjunct – state parks
Nested – 50 year old floodplains/area 1 or 2 miles within store
Discrete features: identifiable features, can list or count them or summarize a numeric attribute associated with them.
Continuous features: seamless geographic phenomena, can summarize features
Drawing areas and features: good for finding out whether features are inside or outside an area
Selecting the features inside the area: good for getting a list or summary of features inside an area
Overlaying the areas and features: good for finding out which features are inside which areas, and summarizing how many or how much by area
Statistical summaries include:
Count – total number of features inside area
Frequency – number of features with a given value, or within range of values, inside area, displayed as a table
Sum – overall total
Average – mean
Median – middle of range of values
Standard deviation – average amount of values are from the mean
Vector or a raster method to overlay areas with continuous categories or classes
Vector – GIS splits category or class noundaires where they cross areas and creates a new dataset with the areas that result, then use data table for new layer to summarize the amont of each category in area
Raster – GIS compares each cell on the area layer to the corresponding cell of the layer containing the categories
Mitchell ch. 6: Finding What’s Nearby
Overall: Finding what is near lets you see what is within a set distance or tavel range of a feature. Can monitor events in an area or find an area served by a facility or the features affected by an activity
Travel range is measured using distance, time, or cost.
To find whats near you can measure straight-line distance, measure cost/distance over a network, or measure cost over a surface
Can calculate distance assuming the earth is flat (planar method) or curved (geodesic method)
Planar – good when area of interest is small like city, county, state
Geodesic – good for large areas such as large region, continent, or whole Earth
Once you have identified which feature are near source you can get a list of the features, a count, or a summary statistic
Summary statistics can be: a total number, an amount by category, a statistical summary (mean, maximum, minimum, or standard deviation)
If you are specifying more than one range you can create either inclusive rings or distinct bands
Inclusive rings are useful for finding out how the total amount increase as the distance increases
Distinct bands are useful if you want to compare distance to other characteristics
Ways of finding what is nearby
Single-line distance: defining an area of influence around a feature, and creating a boundary or selecting features within the distance
Distance or cost over a network: measuring travel over a fixed infrastructure
Cost over a surface: measuring overland travel and calculating how much area is within a travel range
To create buffer you specify the source feature and the buffer distance, GIS draws line around feature, can have several source features with buffers around them
If you are finding individual locations near a source feature and you can GIS calculate the distance between each location and the closest source
Mitchell ch. 7: Mapping Change
Overall: GIS lets you map where thing move and knowing what’s changed can help you understand how thing behave over time, anticipate future conditions, or evaluate the results of an action or policy
Mapping change in character or magnitude shows you how condition in a place have changed. Change can be a feature or can be associated with a quantity with each feature
Can map discrete features that physically move or events that represent geographic phenomena that change location
Discrete features – change in character or in the quantity of an attribute associated with them
Data summatized by area – totals, percentages, or other quantities are associated with features within defined areas
Continuous categories – show the type of feature in a place, such as each land cover type
Continous values – these quantities are continuous, such as air pollution levels
Can map time 3 differnet ways:
A trend – change between two or more dates or timess
Before and after – conditions preceding and following an event
A cycle – change over a recurring time period, such as a day, month, or year
Snapshots show the condition at any given moment and are used to map phenomena that are continuous in time
Summarizing is used for mapping discrete events in a particular place that are not continuous in time
Duration divided by the number of dates yields the interval
Number of dates you use depends on the consistency of the change
When calculating change in magnitude you subtract the numeric values associated with each feature
Can also calculate percentage to show which feature changed the most relative to their original value
To measure change in type or category you sum the land area of each category and calculate the the actual or percentage difference between the dates
3 Ways to map change:
Time series – Strong visual impact if change is substantial; shows conditions at each date/time
Tracking map – Easier to see movement and rate of change that with time series, especially if change is subtle
Measuring change – Shows actual difference in amounts or values
1 thought on “Jocelyn Weaver – Week 3”
great notes. hopefully much of this (or at least some of it) will start to make more sense when you work thru the software tutorial, and put some into practice in the “exam.”
great notes. hopefully much of this (or at least some of it) will start to make more sense when you work thru the software tutorial, and put some into practice in the “exam.”