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GEOB 270 Lab 2

Spatial Analysis

We’re going to look at two methods for overlaying a raster image and a vector layer: 1) Using the Zonal Statistics tool to overlay a raster layer, 2) Converting the raster to a vector and doing some polygon overlays. Method 1 is slightly faster and more accurate, but can be applied in a more limited number of circumstances. Method 2 induces some error when converting between data types (we will discuss more in lecture) and requires more steps, but it is more flexible.

Method 1) Zonal Statistics

We are going to overlay the vector data on the raster data to measure the mean NDVI value for each DA & CT using the Zonal Statistics as Table tool. Then we will Join the resulting output table to display the mean NDVI values per DA & CT.

1) Calculate the Zonal Statistics

  • Find the Zonal Statistics as Table tool in the Geoprocessing pane.
  • Choose Van_DA_2016 as the feature zone data.
  • Set DAUID as the Zone Field
  • Set your NDVI layer as the Input value raster.
  • Select All statistics types
  • Run

2) Join the Zonal Statistics table with the Van_DA_2016

  • Right click on Van_DA_2016 and choose Joins and Relates > Add Join
  • Set DAUID as the Input Join Field
  • Choose the Zonal Statistics table (mine is named ZonalST_Van_DA_1) as your input.
  • Make sure DAUID is selected as the Join Table Field as well
  • Click OK

3) Inspect the Join

  • Open the attribute table and note the new columns.
  • Change the symbology, and choose Mean as the Field.
  • Zoom in, turn Van_DA_2016 layer on/off, compare to the classification, NDVI layer, and base map. Confirm the values make sense given the inputs.

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Question 7)

What is the DAUID of the DA with the highest Mean NDVI value? Hint Double clicking on Green Veg Area in the attribute table allows you to sort in ascending/descending order.

Method 2) Raster to Polygon Conversion

We are going to convert the Classification raster layer to a vector data and select just the Green Vegetation. We can then overlay the resulting vector layer with the Van_DA_2016 layer using an intersect.

1) Use the Raster to Polygon Tool

  • Set Classification as the Input raster
  • Choose Value as the Field

2) Set the Symbology to Unique Values

  • Choose gridcode as the Field
  • Zoom into somewhere of interest (eg. I chose Trout Lake), toggle the newly created vector layer on and off to see if it makes sense.

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Intersect

The Intersect tool is one of the most useful vector - vector overlay operations. It will combine the feature classes where they overlap and exclude all other areas

1) Run the Intersect.

2) Change the symbology and open the attribute table to confirm the results look like what we’d expect.

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Add & Calculate Field

Adding new fields to our attribute table allows us to perform calculations or copy a subset of our data to a new column.

1) In the Van_DA_2016_Intersect layer, create a Green Veg Area Field.

  • Right click on Field in the attribute table.
  • Name the field Green_Veg_Area and give it an alias without the underscores.
    • Arc doesn’t allow spaces in column names.
  • Make sure the data type is Double.
    • Double is a type of data that allows for decimals.
  • Make sure to save the field.
  • Close the field window and go back to the attribute table.

2) Select only the green vegetation areas.

  • Choose Select by Attribute: Where gridcode is equal to 3.
    • Select by attribute allows us to select rows/objects with a certain attribute.
    • It relies on something called a Structured Query Language (SQL).
    • We are selecting all rows “Where” our conditions are met.
    • Our condition is that grdicode (attribute from the NDVI layer representing vegetation category) is equal to 3 (green vegetation).

3) Calculate the Green Veg Area.

  • Right click on Green Veg Area and choose calculate field.
    • This allows us to define a function and apply it.
  • Set the expression to: Green_Veg_Area = Shape_Area as the.
    • Note you only need to complete the right side of the equation.
    • This will simply copy the shape area for the green vegetation areas, we will work with a mathematical expression on the next page.

4) Assign all other shapes a zero.

  • We can quickly invert our selection.
  • Calculate the field again, but with Green_Veg_Area = 0
    • We have selected girdcode 1 & 2 (Not vegetation) so they all get zeros.

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Summary Table

Summarizing by a field (eg. DAUID - the Dissemination Unit ID) allows us to get statistics of interest for specific columns.

1) Get the sum of Green Veg Area by DA.

  • Right click on DAUID and click Summarize.
  • Set Green Veg Area as the Field and choose Sum as the statistic type.
  • Make sure DAUID as the Case Field.
  • The resulting table will show the total of just the green vegetation area per DA, and can be joined to the Van_DA_2016 layer.

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Join Summary Table

Do a Join to add the Summary Table output to Van_DA_2016. Following the same steps as above but use the Summary Table you just generated as the Join Table.

Question 8)

What is the DAUID of the DA with the highest Green Veg Area?

Question 9)

Is the DA with the highest mean NDVI value the same as the DA with the greatest area of green vegetation? Y/N

Question 10)

Create a scatter plot, with the Mean NDVI value (Output from Method 1) on the X-axis Green Veg Area Sum (Output from Method 2) on the Y-axis. What is the R2 value? How strongly related are these two measures of greenness?

Question 11)

Change the Y-axis to Income and leave the X-axis as Mean NDVI, note the R2 score. Now change the X-axis to the Green Veg Area Sum and note the R2 score. Which variable has a “stronger” relationship with income?

Question 12)

Are either of these variables strongly linked to income?  Why?