FANR 3800 Lab 2 Supplementary Document: Forest Planning for Sensitive Wildlife Species
(modified from ESRI Spatial Labs 2013 document prepared by Alexandra Locher)
You are introduced to a typical GIS analysis in lab 2. There are quite a few steps so you must take extra measures to organize and name your intermediate outputs appropriately.
GIS methods in this lab:
- Create an event theme
- Save vector layer (or event theme) as new shapefile
- Project spatial data
- Spatial join (Join by Location)
- Summarize tabular data
- Add Field and Field Calculator
- Calculate geometry
- Attribute query (Select by Attribute)
- Create selection layer
- Near (Analysis)
- Buffer (Analysis)
- Clip (Analysis)
Before you begin:
- Set up your lab2 project workspace on the E:\ drive
- Download the lab 2 data (Lab2.zip) from ELC and copy the compressed file to your project workspace
- Extract the Lab2.zip file into your project workspace
- You should see 1 “Data” folder and 1 PDF
- Save your project to your project workspace (File>Save As)
Create an event theme
An event theme is a GIS layer you create from a list of XY coordinates. Even though you see the event theme points on displayed on the map canvas, they are not permanent until you save them as a shapefile .
Create an event theme in 2 steps:
- Load a table document to your project
- Right-click on the table layer in the TOC > Display XY Data
In this case, make SURE you (hit the Edit button to) change the coordinate system to match the coordinate system shown in your table document
Save vector layer as new shapefile
You can save any existing vector layer out to a new shapefile by doing the following:
Right-click in the TOC on the layer you want to save > Data > Export Data
- Notice, you specify the “.shp” extension and the path has no strange characters
Use the PROJECT command to mathematically transform your layer to a new coordinate system
It is important to ensure all GIS layers are using the same coordinate system before you perform any analyses. The GIS command to project a layer from one coordinate system to another is called PROJECT. The ‘output coordinate system’ will vary depending on where on the face of the Earth you are working. In the screenshot below, I am mathematically transforming my owl_locations point layer from the geographic Lat/Long to a coordinate system designed for use in northern Lower Michigan. When working in Georgia, you might use UTM/Zone17N/NAD1983/Meters or Ga State Plane/NAD1927/Feet.
Joining information based on location (Spatial Join)
In this week’s lab, you have a polygon layer with a land cover attribute and you have points that represent nest locations. You will use the spatial join to populate the point’s attribute table with the land cover attributes into in which each point falls
After your spatial join, open the point layer’s attribute table. You should notice each point is now attributed with the land cover type in which it falls.
Summarizing a table based on a categorical variable
This method is one of my go-to steps when I need to figure out, for example, how many acres of each land cover type I have on a property. The step is straight forward:
- Open the layer’s attribute table
- Right-click on a categorical variable > Summarize
- Select your summary variable(s) and type(s)
- Before you hit OK, give your output table a “.dbf’ extension and ensure you are saving this in your project directory
Add a new field and then use the Field Calculator to calculate a new field
You can add a new field to any attribute table by
- Open the attribute table
- Select ‘Add Field’ under the Table Options button
DO NOT USE SPACES WHEN NAMING YOUR FIELD!!!!
Field Types I use often are: Short Integer, Long Integer, Float, Text
When using the Field Calculator, you must adhere to “Please Excuse My Dear Aunt Sally” rules.
Calculating Geometry (recall the inherent measurements)
The typical workflow is to
- First open your attribute table and add a new field as a FLOAT type and then
- right-click on that field > Calculate Geometry
We will address attribute queries in a week or two. For the time being, concentrate on the query format. Follow the steps below closely. At this time, don’t try to manually type the query string.
For this lab,
- Open the Select By Attributes dialog
- Put your cursor in the box at the bottom of the dialog
- Double-click on cover_type,
- Click on the “Get Unique Values” button in the middle of the dialog
- single-click on the “=” sign,
- double-click on the ‘herbaceous openland’ entry in the unique values window
- single-click on the “And” button
- double-click on the “Hectares” entry in the top window
- single-click on the “>=” button
- manually type 0.405 at the end of the string
Create a Selection Layer
When you create a Selection Layer, ArcMap redisplays a layer’s selected features. This is useful when you must select a group of features and then ‘do something with them.’ Once you have your features selected, right-click on the layer in the TOC > Selection > Create Layer from Selected Features. You are not creating a new shapefile when you do this, you merely are redisplaying a subset of polygons.
The NEAR (ANALYSIS) tool allows the user to calculate the distance from all features in one layer to the nearest feature in another layer. The output is saved in the attribute table of a new layer.
The BUFFER (ANALYSIS) tool allows the user to create a, you guessed it, buffer of a user-specified distance around features in a layer. The results are saved as a new polygon layer.
Think of the CLIP (ANALYSIS) command as a ‘cookie-cutter’ operation where the features in the input layer are cut by the features in the clip layer. Again, this creates a new layer.