Spatial Analysis: Geoprocessing in ArcGIS Pro

2023-02-09

Recap

What is Spatial Analysis in GIS?

Image source

First step: Overlay spatial features/data to tell a story, make an argument, solve or understand a problem, etc about locations.
Second step: Share your spatial analysis in reports, maps, tables, and charts through visual communication.

Spatial Analysis in GIS

determine relationships
detect and quantify patterns, assess trends
make predictions and decisions.

Spatial Analysis Workflow

Think about your final project!

Frame the question you want to answer (proposal)
Find and prepare the data to make it ready for analysis
Explore the data both with and without mapping
Perform the spatial analysis, using the right tool or set of tools to answer the question
You may want to make the analysis easy to repeat or automate using modeling and scripting
Share your results to communicate findings or allow others to repeat the process

Spatial Analysis in ArcGIS Pro

The basics

Query, also known as a phenomenon-based search or a containment search within a spatial region
Extract and overlay data.
Add and calculate attribute fields.
Summarize and aggregate data.
Calculate statistics.
Model relationships and discover patterns.

Spatial Analysis in ArcGIS Pro

The powerful extensions

Spatial Analyst — Perform interpolation, overlay, distance measurement, density, hydrology modeling, site suitability, and math and statistics on cell-based raster data.
Network Analyst — Measure distances and travel times along a network to find a route between multiple locations, create drive-time buffers or service areas, and find the best locations for facilities to serve a set of locations.
Geostatistical Analyst — Analyze and predict the values associated with spatial or spatiotemporal phenomena.

Spatial Analysis in ArcGIS Pro

The powerful extensions

Business Analyst — Analyze market trends, including customer and competitor analysis, site evaluation, and territory planning.
Image Analyst — Interpret and exploit imagery, perform feature extraction and measurement, and perform classification and object detection using machine learning.
3D Analyst — Analyze and create 3D GIS data and perform 3D surface operations using rasters, TINs, terrains, and LAS datasets (lidar).

The Periodic Table for Spatial Analysis

Geoprocessing

Geoprocessing is a framework and set of tools for spatial analysis. It is used to automate GIS tasks and for modeling and spatial analysis.

It Manipulates and alters the input geographic data to create an output data (derived data) for spatial analysis.

Geoprocessing

It can be done:

Interactively (through Arc toolbox)
Using a model (ex. Suitability Analysis)
Command Line
Python Script

Geoprocessing Operations We learn today

Dissolve (spatial data reclassification)
Merge
Overlay toolset
- Union
- Intersect
- Spatial join
Extract toolset
- Clip
Proximity toolset
- Buffer

Dissolve and Merge

Dissolve eliminate boundaries by aggregating features with a shared attribute.

Merge is used when you have two or more geographically adjacent layers, each containing the same type of features, and you want one layer that contains all their features.

For instance, if your database of census tract boundaries was dissolved by city council districts, your result would be a layer of tracts whose individual boundaries have been eliminated, and the tract polygons merged together based on council district. The result: All the tracts located within a particular council district become a single polygon. By dissolving tract boundaries, we can easily obtain summary statistics on the tracts within the jurisdiction of each council district—not only a count of the number of tracts, but also sums, minimums, maximums, averages, standard deviations, and other calculations of any field that was part of the original attribute table for tracts.

Each layer to be merged must have at least some of the same fields and be represented by the same type of geographic entity (point, line, or polygon). If one layer has additional fields or different fields from the initial datasets, these fields will not appear in the new layer.

You might use the merge operation if you had a street dataset for a city, and you wanted to create a seamless database of adjacent suburban street layers by appending the suburban street layers to the urban street layer, thus creating a metropolitan area street database.

Dissolve: inside the attribute table

Overlay toolset is used to answer

For example:

What land use is on top of what soil type?
What parcels are within the 100-yard floodplain? (“Within” is just another way of saying “on top of.”)
What roads are within what counties?
What wells are within abandoned military bases?

Overlay Toolset

Union

Computes a geometric union of the input features. All features and their attributes will be written to the output feature class.
All input feature classes and feature layers must have polygon geometry.

The union function is one of the most powerful of the geoprocessing tools. It will overlay two datasets, producing a new dataset that has the combined attributes of both, whether or not they overlap in extent.

Suppose we had a dataset of watershed or drainage basin boundaries within a municipality, and another dataset of property lots.

For example, we want to be able to see which properties are contained within each watershed, including those properties that could be split between more than one watershed; we also want to see if any watersheds are split by any property lots.

The union function will preserve the attributes of both datasets. Each resulting polygon will contain the attributes (data fields) of both the property lot and the watershed it belongs to.

Our new data layer will show the total extent of both input data layers; it will contain areas meeting either criteria: belonging to the watershed dataset or the property lot dataset.

Overlay Toolset

Intersect

Computes a geometric intersection of the input features. Features or portions of features that overlap in all layers or feature classes will be written to the output feature class.

Overlay Toolset

Spatial Join

Joins attributes from one feature to another based on the spatial relationship. The target features and the joined attributes from the join features are written to the output feature class.

The spatial join joins two layers that have different spatial boundaries or properties, such as a point layer and a polygon layer, or a line layer and a polygon layer. This spatial join function joins two sets of spatial data together to assign the attributes of one to the other.

Suppose we had a point shapefile locating all automobile thefts in a city, and wanted to find out the neighborhood characteristics where the crimes were most prevalent. We could perform an analysis of demographic information and other indicators of the census tracts where the thefts occurred, but the auto theft database does not contain data giving the census tract number. To do the demographic analysis, we would first have to know the census tract number for each of the auto theft locations. This could be accomplished with a spatial join.

By spatially joining the two databases and using an overlay process, the software would calculate where each auto theft is located by census tract, and append the tract number and all other attributes of the census tract data to the auto theft database.

Furthermore, in a spatial join, the software actually performs a type of overlay analysis with the point and the polygon shapefiles, and determines within which polygon each point lies. When the spatial join was completed in this example, the data could be summarized and you could determine how many auto thefts occurred in each census tract. From there you could develop rates per census tract, and examine those rates in light of demographic, socioeconomic, or other characteristics of the tract.

Overlay Toolset

Spatial Join

Spatial Join Relation: One to One and One to Many

One-to-one operation: If multiple join features are found that have the same spatial relationship with a single target feature, the attributes from the multiple join features will be aggregated using a field map merge rule.

One-to-many operation: If multiple join features are found that have the same spatial relationship with a single target feature, the output feature class will contain multiple copies (records) of the target feature.

Overlay Toolset

Spatial Join Relation: One to One and One to Many

Extract Toolset

Clip

Extracts input features that overlay the clip features. Use this tool to cut out a piece of one dataset using one or more of the features in another dataset as a cookie cutter.

Note: The attributes of clipping feature are ignored.

Proximity Toolset

What’s near what?

How close is this well to a landfill?
Do any roads pass within 1,000 meters of a stream?
What is the distance between two locations?
What is the nearest or farthest feature from something?
What is the distance between each feature in a layer and the features in another layer?
What is the shortest street network route from some location to another?

Proximity Toolset

Buffer

Creates buffer polygons around input features to a specified distance.

Proximity Toolset

Buffer

Buffer Applications:

Protected zones around lakes and streams
Zone of noise pollution around highways
Walking distances to grocery stores.

Things to Watch When Using Geoprocessing Tools

Organizing files: Output to different folders than your input layers are stored

Speed & space: Geoprocessing functions take up a lot of space (check out pairwise function)
Naming files: Do not use default name (“clip_output”, etc.)!!! If a process fails, try again using a different output file name
Same projection (GCS & PCS): Shapefiles (and data frame!) must be in the same map projection/coordinate system
Geometry features: When using the spatial overlay tools (i.e., Union, intersect), ArcGIS DOES NOT recalculate area, length, or perimeter

References:

Spatial analysis in ArcGIS Pro

An overview of the Analysis toolbox

Maantay, J., & Ziegler, J. (2006). GIS for the Urban Environment. Redlands, CA: Esri Press.