G 424/524 GIS for the Natural Sciences
D. Percy
e-mail: percyd@pdx.edu

A first cut at analysing the distribution of points

You are going to compare the distribution of points with a theoretical Poisson distribution (random) and use a Chi-Squared statistical test to see if there is a significant difference.

These are the step-bystep instructions:

1. Download the GenerateGrid script from www.esri.com/arcscripts. Save it to your assignment 5 directory.
2. Follow the instructions for installing it.Make sure you turn on Scripting Runtime
3. Use the this script to create a grid over the state: Tools-> Macros->Macros->modGen.GenerateGridUI, enter number of rows and columns in subsequent dialog boxes.
4. Use the Selection-> Select by Location function to do a spatial query of all grids that intersect the state (assuming that you drew your box larger than the state). Your screen might look like this. Save this as a new theme. We won't worry about the boundary problem.
5. Choose the oregon_earthquakes theme and do a spatial join to the intersected grids theme (from the previous step).
6. Open the table (Join_Output), right-click on the field Name and choose Summarize. Accept the defaults in the dialog box. This gives you a table of how many earthquakes occured in each grid cell.
7. Summarize this table in the same way that you just did, but this time summarize on the field Count_Name. This gives you a list of how many cells had 1 earthquake, how many 2 earthquakes, etc. It's a little confusing because the fields are both named variants of Count. Just remember that the first count is "how many earthquakes in a grid", the second is "how many grid cells are like that". Your screen might look like this.
8. Export the summarized table to DBF so that you can import it to Excel. (actually, you can just open it directly in Excel without exporting, just keep track of the filename, like Sum_output2.DBF). In Excel, change your filetype to Dbase in the Open Files dialog box.
9. The number of cells that had zero earthquakes is equal to the total cells minus cells that had quakes. Duh :-)
10. Download my Poisson example spreadsheet. For a quadrat analysis you need to have at least 5 events in a category. You will probably combine all of the categories above a certain value. For my 50 x 50 example I combined everything greater than 5. Remember that the sum of all probabilities in the Poisson distribution is 1. The sum of the Expected and Observed should both be the total number of quadrats that intersected Oregon.
11. Alter the spreadsheet to accomodate the grid size that you used. Your Chi Square value should be VERY large, since this is obviously a clustered data set!