Analysis Tools in ArcGIS Online

The objective of this tutorial is to demonstrate how to use a few basic spatial analysis tools in ArcGIS online as a way to answer research questions and explore interactions between humans and the environment in your watersheds of interest. It will also show how to import tabular data from the web into ArcGIS Online.

So what is spatial analysis? ESRI defines it in the following way (emphasis added):

Spatial analysis is the process of examining the locations, attributes, and relationships of features in spatial data through overlay and other analytical techniques, in order to address a question or gain useful knowledge.
Spatial analysis extracts or creates new information from spatial data.

A number of spatial analysis tools are available in ArcGIS Online, and this tutorial will introduce you to a few of them. They are:

1. Overlay Layers

"Overlay combines two or more layers into one single layer. You can think of overlay as peering through a stack of maps and creating a single map containing all the information found in the stack, for example:

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

2. Create Buffers

"A buffer is an area that covers a given distance from a point, line, or area feature.

Buffers are typically used to create areas that can be further analyzed using other tools such as Overlay Layers. For example, if the question is "What buildings are within one mile of the school?", the answer can be found by creating a one-mile buffer around the school and overlaying the buffer with the layer containing building footprints. The end result is a layer of those buildings within one mile of the school."

3. Trace Downstream

"Determine the flow paths in a downstream direction from the locations you specify."

You can use this tool to determine the path of water from a particular spot. For example, you can see where runoff from a housing development would go or identify impacted streams from a contaminant spill.

4. Calculate Density

"The Calculate Density tool creates a density map from point or line features by spreading known quantities of some phenomenon (represented as attributes of the points or lines) across the map. The result is a layer of areas classified from least dense to most dense."

Once the file is open in Excel, notice that there are 5 rows of text above the actual table of attribute information. Also, the position coordinates (Latitude and Longitude) are stored in separate columns. We will have to get rid of this unnecessary text and combine the Latitude and Longitude using the concatenate function in Excel, represented by the symbol &, before importing the data.

2.1.5. Open the file in Microsoft Excel.

5. Derive New Locations

"Creates new features in your study area that meet a series of criteria you specify. These criteria can be based on attribute queries (for example, parcels that are vacant) and spatial queries (for example, within 1 mile of a river)."

1. Getting Started

1.1 First, search for and add the following layers to your demo map.

- USA States (Generalized) by esri

- ESRI_Census_USA by esri

- Pickens_Superfund by GIS_Specialist_Clemson

- EPA_Superfund_Sites_in_HuronRiverWatershed by GIS_Specialist_Clemson

- HuronRiverWatershed by GIS_Specialist_Clemson

- HuronRiverLine by GIS_Specialist_Clemson

- PermittedPolluters by GIS_Specialist_Clemson

- 12MileCreek_Watershed by GIS_Specialist_Clemson

2. Mapping toxic chemical releases in South Carolina

The US Environmental Protection Agency (EPA) has a database called the Toxic Release Inventory (TRI) which contains information about toxic chemical spills across the country. Let's start the tutorial by answering the simple question:

Of all the 2014 TRI spills, which occurred in South Carolina, where were they located, and what was the primary chemical released?

2.1. Importing data from a file

Spatial data can be imported into ArcGIS online if the attributes have an address, a country, or position coordinates. On the US National Institutes of Health (NIH) website there is an interactive map which shows EPA Superfund and TRI sites. Information about the Superfund and TRI sites can be downloaded as a delimited (CSV) file which can then be imported into ArcGIS online, though we will have to do a bit of manipulation of the file in Excel before we upload it.

2.1.1. Navigate to the TRI webmap application: http://toxmap-classic.nlm.nih.gov/toxmap/main/index.jsp

2.1.2. Click on the TRI Facilities tab.

2.1.3. Use the Zoom To box to zoom in on South Carolina. The map should show South Carolina and the near surroundings.

2.1.4. Click the Save button next to the letters TRI, and in the window that opens, click Download Now to save the site

information to a CSV file.

2.1.6. Select and delete the first 5 rows in the spreadsheet. Row 1 should now have the column titles (FAC_ID, NAME, ADDRESS, CITY, etc.) in it.

2.1.7. In cell X1, enter the name Location as the column title.

2.1.8. In cell X2, enter the formula: =J2&" "I2 (note the space in between the quotation marks)

2.1.9. Copy the formula to the other rows.

2.1.10. Save the file, making sure it is still saved as a CSV file. (Note: if this was unsuccessful or you do not have Microsoft Excel, the file can be downloaded directly on the bottom of the page).

2.1.11. In ArcGIS online, go to Add > Add Layer From File and select the CSV file you saved above. The website should read in the locations automatically.

2.1.12. When prompted, choose to display the file with a Single Symbol based on Location.

2.2 Using Derive New Locations and Overlay Layers to trim one layer based on another

We imported all of the TRI data to the map, but we want to limit this layer to data from South Carolina only. One way we can do this is to first create a polygon representing the state of South Carolina from the USA States (Generalized) layer using a logical expression.

2.2.1. Click Analysis > Find Locations > Derive New Locations. Click on the green

Add Expression button.

2.2.2. Fill in the dialog as shown in the figure to the right to create the expression:

Select from the USA States (Generalized) layer where the

STATE_NAME attribute is the unique field South Carolina.

2.2.3. Set the Result Layer Name to SouthCarolina. The South Carolina border layer is added to the map.

Next, use the Overlay Layers analysis tool to select information in the Overlay layer (the uploaded TRI points) based on the boundary of the Input layer, South Carolina, using intersection to trim the data

2.2.3. Click Analysis > Manage Data > Overlay Layers

. 2.2.4. Enter SouthCarolina as the input layer in (1).

2.2.5. Enter SouthCarolina_TRI_raw as the overlay layer in (2).

2.2.6. Name the layer SC_TRI_clip in (4). Click Run Analysis.

The SC_TRI_clip layer answers our original question: it contains only the locations of toxic chemical releases in SC and by changing the visualization style of the layer, the primary chemical released can be shown on the map. (Refer to the Map Basics tutorial for review of this concept).

3. Predicting particle transport pathways and affected areas of a watershed

The potential pathways of particles can be evaluated using the Trace Downstream tool which uses local topography to determine flow paths. Though actual flow pathways can be complex, this tool allows for identifying where particles from a source location are likely to go. This is a useful tool, for instance, the path of surface runoff from a site, a contaminant spill, or sediment from a new housing development can be predicted and impacted areas identified.

3.1 Using the Trace Downstream tool to evaluate the movement of PCB's in Twelve Mile Creek

The Twelve Mile Creek arm of Lake Hartwell is contaminated with polychlorinated biphenyl's (PCBs). These compounds are non-volatile, stable, hydrophobic molecules often used in industrial processes but are carcinogenic in animals, likely cause cancer in humans and, due to their stability, tend to accumulate in fatty tissues in organisms and accumulate with increasing trophic level. Sangamo Weston, Inc. operated a plant on Twelve Mile Creek near Clemson which leaked PCB's to surface and ground water, affecting drinking water supplies and fish populations. Sangamo Weston is a Superfund site. We can predict the path of PCB's from the site from the local topography and use the Trace Downstream tool to verify. Our question to be answered is:

What is the flow path of PCB's from the Sangamo Weston site?

Based on the topography of the area surrounding Sangamo Weston, which way do you think PBS's would be transported?

3.1.4. Save the layer as SangamoDownstream and click Run Analysis. Zoom to the new layer to see the entire flow path

from the site to where it reaches Lake Hartwell. Does it match your prediction?

3.1.3. Draw a point at the location of Sangamo Weston using the Pickens_Superfund layer as a guide.

3.1.2 Click Analysis > Find Locations > Trace Downstream

3.1.1 Zoom to the Pickens_Superfund layer which contains the location of Sangamo Weston, Inc.

3.2 Using the Trace Downstream tool to predict flow from Superfund sites in Michigan

I grew up in the Metro Detroit area where the landscape is fairly flat and was shaped by glacial activity and lakes, streams, and wetlands are abundant. I grew up on a lake near two other Superfund sites, Rasmussen's Dump and Spiegelberg Landfill. The landfills are surrounded by fairly flat land, so tracking particle flow from these sites is not as intuitive as the Sangamo Weston site. There are also several lakes and wetlands around these landfills which could be impacted. So let's answer the question:

Where would contaminants from Rasmussen and Spiegelberg Landfills be likely to go? What are the affected water bodies?

4. Locating permitted polluter facilities near a stream with Create Buffer and Overlay Layers

Interestingly, the particles from each landfill would flow in a different direction. Flow from Rasmussen's Dump would likely head east toward the small Mohican Lake, and flow from Spiegelberg Landfill would head south and west. This was not obvious from studying the topography of the surroundings. This information could potentially guide monitoring strategies for evaluating contaminant transport from these sites.

3.2.2. Open the Trace Downstream tool. Select the EPA Superfund Sites in Huron River Watershed as the starting

locations and save the layer as MI_Superfund_Downstream. Run the analysis.

3.2.1 Zoom to the EPA Superfund Sites in Huron River Watershed layer.

In many cases, you may want to find features within a certain distance of another feature. This can be accomplished using a combination of the buffer tool and overlay. In this scenario, lets assume that a contaminant was newly discovered in the Huron River, and, based on the chemical, you have a hunch that it came from a facility permitted to discharge into the river and not from a rogue agent, but you need to find out which of these facilities are nearest the river to begin your investigation. Here we generate a list of potential point source polluters using buffer and overlay. Our research question:

Which EPA registered polluting facilities are within 1 km of the Huron River?

From these data, we now have a list of facilities very near the Huron River which we could begin to investigate as the source of the contamination.

4.5. Save the output with a meaningful name.

4.4. Go to Analysis > Manage Data > Overlay Layers. Use the HuronRiverBuffer as the input layer and PermittedPolluters as the source layer.

4.3. Turn off the Use Current Map Extent. Save the result as HuronRiverBuffer and run the analysis.

4.2. Go to Analysis > Use Proximity > Create Buffers. Use the HuronRiverLine as the input and enter a distance of 1 km.

I've already restricted the EPA Permitted Polluter data for the state of Michigan to the watershed similar to our SC-only TRI spills in Section 2.2. The HuronRiverLine is taken from an overlay of the USA Streams and Rivers layer from ESRI and the watershed boundary we previously created.

4.1. Turn on the PermittedPolluters and HuronRiverLine. Turn off the other layers.

5. Calculating population density

Each person living in a watershed makes an impact on it by consuming resources, polluting air and water, clearing forested land and filling wetlands, even by fertilizing the lawn. Thus, the population density within a watershed can be a broad indicator for the amount of disturbance that can be expected. The USA Census provides data about population, demographics, and socioeconomic information. The Calculate Density tool can be used to determine the population density from the Census data points.

5.1 Trim census data using Overlay Layers and calculate population density in the Twelve Mile Creek watershed

Let's consider the Twelve Mile Creek again for calculating population density. The ESRI Census USA layer contains population information for the country consisting of millions of points. We are going to use Overlay Layers to limit the census data to our watershed. Note that the Overlay Layers tool determines the credit cost for the analysis from the number of points on the current map extent, so using the census points can eat up a lot of credits in a densely populated area.

5.1.5. Shrink your window until the watershed just fits into the screen. Click on Show Credits again.

5.1.4. Click on Show Credits. When I am zoomed to the extent of the watershed, this tells me that there are 2,891 records, or points, in the map area that it has to include in the analysis, at a cost of 2.891 credits. Note that yours may be slightly different based on monitor size, map extent, etc.

5.1.3. Choose the watershed boundary as the input layer and the ESRI_Census_USA-Census Block Points as the overlay layer. Rename the output to something more meaningful.

5.1.1. Turn off previous layers and turn on the 12MileCreek_Watershed and ESRI_Census_USA layers. Zoom to the 12MileCreek_Watershed layer.

Zoom in to see the individual census points, and then zoom to the watershed again.

5.1.2. Go to Analysis > Manage Data > Overlay Layers.

Now that we have census data limited to the watershed boundary we can determine the population density in the watershed.

5.1.6. Click Run Analysis. Change the display style of the points if you wish.

This has reduced the number of points to 1,721 and the cost to 1.721 credits (again, yours may vary slightly). While this is not a huge difference, keep in mind that analyses on a large number of points can eat up a lot of credits, and you can always check the credit cost of an operation before you run it.

5.1.7. Go to Analysis > Analyze Patterns > Calculate Density.

5.1.8. Use the census layer you created in step 1 and select the attribute POP2000 as the count field.

5.1.9. Expand the Options dropdown and change the output area units to square miles.

5.1.10. Save the layer as PopDensity_12MileCreek. Check the number of credits required (for me it's 1.108, yours should be similar). Run the analysis.

The population density is highest in the cities, as would be expected, and fairly low elsewhere in the watershed. Pickens has the greatest density and the areas of Easley, Six Mile, and Norris are easily seen.

Summary

The tutorial has demonstrated how to perform a variety of spatial analytics in ArcGIS Online. These include finding data specific to a particular area (state borders, watersheds, near streams) using overlay, derive new locations, and buffer operations. The likely flow paths of particles were evaluated by tracing downstream from Superfund sites. Tabular data with latitude & longitude information were imported directly into the GIS. These few tools can facilitate a variety of interesting analyses.