Lab: Vector Analysis I, Salamanders and Lakes

Learning Objectives

• Use the "Select by Location" , "Buffer" , Dissolve, Clip and Union tools to solve spatial analysis problems.

Data

Data for Arcata Community Forest

Lab Preparation

To setup for this assignment:

1. Create a folder structure on "D:" as we have done before:
   1. 1_Originals
   2. 2_Working
2. Download the data above, unzip it, and check it all for spatial reference and other issues. (Hint: Check the Boundary_metadata.txt file).

Warning: ArcGIS will project on the fly for visualization but it will not perform datum transformation. This means that if you do not project the boundary file to the correct spatial reference (NAD 27, State Plane California I, US Feet), your analysis will be incorrect.

Skill 1:

The first section of today’s lab presents you with a fictitious scenario to introduce you to some of the spatial analysis tools available in ArcMap. For this lab, you'll be using the data provided except for the "lakes_part2.shp" file.

Let’s suppose that an HSU wildlife biologist discovers an endangered salamander species living in the Arcata Community Forest. This salamander has some specific habitat requirements; It only lives within a specific vegetation type (delineated by the salamander_vegetation layer). And it only lives within 50 yards of a creek or lake. Hence, this salamander’s survival requires that trails, fire roads, and power lines within 50 yards of any creeks or lakes within this vegetation type be closed to public access.

Your mission is to generate a map communicating which sections of trails, fire roads, and power lines (if any) will need to be restricted from public access in order to ensure the well being and survival of this species.

1. Create a layer for streams within the forest boundary
2. Use a "Buffer" to create a layer that contains the area that is within 50 yards of a creek or lake that might contain salamanders
3. "Clip" the trails, fire roads, and power line layers to the area in the buffer layer.

Creating a layer for streams within the forest boundary

To start, we'll select the features that are within the Salamander habitat and then create a new layer from the selection.

1. Before starting, make sure you do not have anything selected by selecting "Clear Selected Features" from the "Selection" menu in the ArcMap menu bar. If "Clear Selected Features" is grayed out, then you do not have a selection.

1. Open the "Select by Location" dialog by selecting "Select by Location..." from the "Selection" menu.

2. The first popup allows you to change the "Selection Method"

• "select features from": Create a new selection
• add to the existing selection
• remove from the existing selection).
• select a new selection from the existing selection

You'll typically leave this on the first setting.

3. The "Target layer(s):" specifies the layer or layers that you want to create a selection in. Make sure "creeks" is checked for this exercise.

4. The "Source layer" is the layer that will be compared with the "Target layer(s)". Select the "boundary" layer for this exercise as we want to select the creeks that are within the forest.

5. There are a number of options for how to "Select by Location" in the next popup, "Spatial selection method...". There is a help page from Esri that shows all the options but lets take a look at a few of them here. The graphics below were created with just the "boundary" and "creeks" layers. Take a look at them and decide which method we should use.

Spatial selection methods:

"are within the source layer feature"

"intersect the source layer feature"

Note that "within" in this case, means "completely within" and we want creeks that may be partially outside of the boundary or "intersect" with the boundary so we need to use the "intersect the source layer feature" method.

6. After selecting the correct creeks, right-click on the "creeks" layer and select "Selection -> Create Layer from Selected Features". A new layer, "creeks selection" should appear in the "Table of Contents". You can now turn off the original "creeks" layer.

7. Note that you were not prompted for a file name when you created the new layer and it has not been saved to disk yet. Right click on the layer and select "Data -> Export Data". Save the new layer to a file with an appropriate name like "creeks_in_boundary.shp".

8. ArcMap may ask you if you want to load the new layer, go ahead and load it and then "Remove" the other one "creeks selection". You now have a new shapefile that you can use for analysis without having to manage always selecting the features that intersect with the boundary.

Creating a layer with all the area within 50 yards of a creek or lake

To find the portions of trails, roads, and power lines that are within 50 yards of a creek or lake, we need to "Buffer" the creek and lake layers to 50 yards.

1. From the "Geoprocessing" menu, select "Buffer".

2. If the window that appears does not look like the one below, click "Show Help". All tools have these small help panels and additional help available under "Show Help". There is a great deal of information in the ArcMap help and also online.

3. Select the "creeks_in_boundary" layer for the "Input Features".

4. Set the "Output Feature Class" to a new shapefile in your working folder.

5. Set the "Linear unit" to "yards" and the text box to the left of the units to "50".

6. You can explore other options layer when you have time, for now, select "OK" and a new layer similar to the one below should appear.

7. Now execute a "Buffer" operation on the "lakes_part1" layer.

8. We really want to have one layer that represents all of the salamander habitat so we need to "union" the layers together. Select "Union" from the "Geoprocessing" menu.

9. Add the layers for the buffered creeks and lake as "Input Features".

10. Give the output a good name like "water_buffer.shp" as this represents all the water features buffered.

11. You should now have a layer that looks similar to the graphic below:

Note: that the ArcMap "Union" tool does not perform a logical union on the two layers, it just combines them into a new layer.

12. We now have a large number of little polygons overlapping with each other and this is just messy. Let's "dissolve" them together by selecting the "Dissolve" tool from the Geoprocessing menu.

13. Select the "water_buffer" layer as the "Input Features" and give the output an appropriate name like "water_buffer_dissolve.shp".

14. Your new layer should look something like the following:

Note: There are parts of this buffer zone that fall outside the salamander vegetation layer. Again, this is OK -- if a trail, fire road or power line runs through one of these areas, it should be closed from public access as well (since we will assume salamanders may venture out into these areas).

Finding trails, fire roads, and power lines that are within 50 yards of salamander creeks

Now we need to identify any segment (i.e., any part) of a trail, power line, or fire road that fall within this 50 yard buffer zone (since these will need to be closed to public access to help protect this species). Remember that a select by location would not be the appropriate tool to use in this case, because we need to identify just the parts (i.e., segments) of trails, power lines and fire roads that fall within this buffer zone (we don’t necessarily want to close the whole thing, just the parts that fall within salamander habitat). So, instead of the select by location tool, you will need to use the "Clip" tool.

1. Select "Clip" from the "Geoprocessing" menu.

2. Select "fireroads" as the "Input Features". This is the layer that will be clipped to produce the new layer.

3. Select your layer with the buffered water areas as the "Clip Features". This is the layer that will be used to clip the "Input Features" layer. Think of this layer as being a "template" that you are going to use to "clip" out parts of the "Input Features" that are outside the "template".

4. The final result should look like the following if you turn off all the layers except the forest boundary and the clipped roads.

5. Now repeat this process for the trails and power line layers.

6. It would be nice to know the total amount of fire roads, trails, and power line that we needed to close. For the fire road layer, open the Attribute table.

7. Notice that a "LENGTH" field already been created. Right click on the name of the field and select "Statistics".

8. A dialog should appear with a histogram and summary statistics for the attribute. Note the "Sum" as this is the total length of the polylines for the old file (before the clip).

Note: It is very important to note that these lengths are NOT correct. Geometry fields in shapefiles are not automoatically updated after running processes on them and must first be updated. Right click on the length field and choose “Calculate Geometry.” On the top menu choose Length and for the units choose Feet (US).

9. Calculate the total length again and notice the difference in the sum. Remember to always update the geometry for all layers when any type of analysis has been run, otherwise all calculations will be erroneous.

10. Compute the total length for the trails and power line layers (remember to first update the lengths).

Exercise 1.

Repeat the process above except try a buffer distance of 25 yards instead of 50 yards.

Exercise 2: Variable Width Buffers

There are many applications for fixed width buffers but in some cases we will want to vary the width of the buffer based on the characteristics of the spatial data or the area surrounding it (Polyakov 2005). For this exercise, you'll be creating a map with different sized buffers around each lake, based on the area of the lake.

Let's say that we want to create conservation zones around each lake based on the area of the lake. We would want the distance to be larger on larger lakes to make sure they are protected while using a smaller distance on smaller lakes. The formula provided is:

• Buffer Distance = Area of the Lake * 10,000

To complete this exercise:

1. Load the "lakes_part2.shp" file into ArcGIS.

2. Add an attribute of type "Double" to the file.

3. Use the equation above to compute the buffer distance.

4. Use the "Buffer" tool to add buffers but this time, select "Field" for the "Distance" and select the attribute you added as the "Field".

5. You should see different buffer widths around each lake. From this, create a simple map for inclusion in your report.

Turn In

A short report that includes your results from Exercise 1 and Exercise 2.

1. For Exercise 1, include maps of the roads, trails, and power lines that will need to be shut down to protect the salamanders for both the 50 and 25 yard values. The maps should also show the salamander habitat based on the buffered lake and creeks. Also include a table summarizing the lengths of the trails, roads, and power lines that need to be closed.
2. For Exercise 2, just include a map of the buffered lakes with an appropriate caption and supporting text.

Note: Please take time to provide a good looking, simple, site/locator map. Make sure all the text in your maps is readable and above 9 point. You do not need to include your name or the data in the map as your name and the date will be on the overall document. You can also move the information on spatial reference and data sources to the caption to simplify the map. Also remember to define all acronyms before using them and spell check your document before turning it in. We will begin deducting more points if these items are not addressed in this and future labs.

Citations

Polyakov, V., A. Fares, M. H. Ryder, Precision riparian buffers for the
control of nonpoint source pollutant loading into surface water: A review, 2005, NRC Research Press