W F 10: Notes: Mitchell Chp. 5-7

Chapter 5:  Finding what’s inside

Why map what’s inside?

To monitor what’s occurring inside, or to compare several areas based on what’s in each (to decide if need to take action)

Defining your analysis

3 ways to find what’s inside:

1. overlay area on top of features
2. use area boundary to select features inside and list/summarize them
3. combine area boundary and features to create summary data

Are you finding what’s inside a single area or multiple areas?

Find what’s inside a single area:  lets you monitor activity in the area

Find what’s inside multiple areas:  lets you compare the areas

1. contiguous areas = next to each other (zip codes, watersheds)
2. disjunct areas = separated (park)
3. nested areas = within each other (stores within 1, 2, and 3 miles of campus)

Are the features inside discrete or continuous?

discrete = unique, identifiable features (locations, roads, land parcels)

continuous = seamless geographic phenomena (vegetation type, temperature, etc)

Three ways of finding what’s inside

1. Drawing the area and the features inside
2. Selecting the features inside the area (specify the area and the layer with the features, and GIS selects the subset of the features inside the area)
3. Overlaying the areas and features: creates new layer with both area and features present

Chapter 6: Mapping what’s nearby

Why map what’s nearby?

1. find out what’s happening within a set distance from a feature:  identifies the area affected by an event/activity
2. find out what’s within traveling range:  help define area served by a facility, help delineate areas that are suitable for a specific use

• • Example:  fire department mapped areas within 5, 10, and 15 minutes of stations, decide where to build new stations
  • Example:  map which stores customers are near so company can target ad sales

Defining your analysis

Is what’s nearby defined by a set distance or by travel to/from a feature?

1. Define what’s nearby based on set distance: use if there is no movement between features; used to measure an area of influence; use straight-line distances
2. Define what’s inside based on travel to/from a feature: use if there is movement between features (people traveling to store, etc)

Are you measuring what’s nearby using distance or cost?

1. Measure what’s nearby with distance
2. Measure what’s nearby with cost (time, money, effort, etc): more precise way to measure, but requires more data preparation and processing

The information you need from the analysis

Do you need a list, count, or summary?

1. a list: all the features that are near the source
2. a count: the total number or count by category of all the features near the source
3. a summary statistic = total amount (ex. number of acres of land within a stream buffer) or amount by category (ex. number of acres of each land type within a stream buffer)
4. a statistical summary: average, minimum, maximum, standard deviation, etc

How many distance or cost ranges do you need?

1. single range
2. several ranges:  can have inclusive or distinct rings

• • inclusive rings: useful for finding how the total amount increases as distance increases
  • distinct bands: useful to compare distance to other characteristics (ex. find if people living in each distance band spend different amounts of money)

Three ways to find what’s nearby

1. Straight-line distance: good for creating boundary or selecting features at set distance from source

• pros:  quick and easy
• cons:  only gives rough estimate of travel distance

1. Distance or cost over a network: specify source locations (ex. fire station) and a distance or travel cost along each linear feature (ex. roads), GIS then finds which segments of the features (ex. roads) are within the distance or cost; good for finding what’s within a travel distance or cost over a fixed network

• pros:  gives more precise travel distance over network
• cons:  requires accurate network layer (ex. network of roads)

1. Cost over a surface: specify the locations of the source features and a travel cost, GIS creates new layer showing travel cost from each source;  good for calculating overland travel cost

• pros:  can measure overland travel cost
• cons:  need data preparation to build cost layer

Tip:  Use straight-line distance to define area of influence or get quick estimate of travel range; use cost or distance over network if measuring travel over fixed infrastructure; use cost over a surface to measure overland travel

Using straight-line distance

1. create a buffer

• specify source feature and buffer distance
• can create buffers around multiple features to see where they overlap
• find features within distance of multiple sources

1. select features to find within a given distance

• doesn’t create a boundary around source feature (it simply selects the features within a given distance)

1. calculate feature-to-feature distance

• useful to find exactly how far each feature is from source

1. creating a distance surface

• use graduated colors to show graduation

Measuring distance or cost over a network

• identifies all lines of a network (streets, rivers, etc) within a given distance

• each segment in a network is tagged with a measure of the cost to travel that segment (money, time, distance)

Calculating cost over a geographic surface

• find out what’s nearby when traveling overland
• shows rate of change as you travel