New tool - Patch and Gap Metrics Toolbox for ArcGIS

Forest ecologists, vegetation ecologists, and others are frequently interested in characterizing the structure of patches and gaps on the landscape. Typically, data, such as tree crown size, are collected in quadrats. In order to synthesize these data for each quadrat I developed the Patch and Gap Metrics Toolbox for ArcGIS. This tool takes polygons of quadrats combined with polygons of representing tree crowns, shrub crowns, or some other patch on the landscape, and calculates the number of patches and area of those patches as well as gaps. There is a version of the tool that allows the user to specify a radius to filter the gaps by in order to ensure that only large gaps are included in the output. You can download the tool by clicking HERE.

The Patch and Gap Metrics Toolbox only requires two input layers: 1) a polygon shapefile of patches (need not be dissolved) and 2) a polygon shapefile of quadrats. The quadrats can be any shape or size and can even be overlapping. In addition to this the quadrat polygons need a quadrat ID field upon which dissolving can be based on. Finally, for the version of the tool that accommodates additional gap size criteria there is a parameter that specifies the radius of gaps to be considered. For example, a value of 6 would eliminate any gaps with a diameter less than 12.

Above: There are two inputs required by the Patch and Gap Metrics Toolbox. The picture on the left shows a polygon shapefile representing the patches. Note that adjacent patches do not need to be merged. The tool will do this automatically. The picture on the right shows overlapping quadrats. Quadrats need not be overlapping.

The main output of this tool is a point shapefile representing the centroid of each quadrat that is attributed with the following fields:

COUNT_SHAPE – Count of the number of patches
SUM_ SHAPE – Total area of the patches in the quadrat
MEAN_ SHAPE – Average size of the patches in the quadrat
STD_ SHAPE – Standard deviation of the patches in the quadrat
MIN_SHAPE – Minimum number of patches in the quadrat
MAX_SHAPE - Maximum number of patches in the quadrat
SUM_ SHAPE1 - Total perimeter of patches in the quadrat
COUNT_SHAP_1 – Count of the number of gaps
SUM_ SHAP_1 - Total area of the gaps in the quadrat
MEAN_ SHAP_1  - Average size of the gaps in the quadrat
STD_ SHAP_1 – Standard deviation of the patches in the quadrat
MIN_ SHAP_1 - Minimum number of patches in the quadrat
MAX_ SHAP_1 - Maximum number of patches in the quadrat
SUM_ SHAP_2 – Total perimeter of gaps in the quadrat

In addition to attributing each quadrat centroid with the above values there are also two additional outputs. In the image below the darker green polygons with red outlines show patches as generated by this tool. The tan polygons with purple outlines show the gaps using a 6 meter radius filter. The remaining light green areas are classified as neither patch nor gap.

New tool - Create quadrats along transects tool

I got a request from a student recently who was interested in creating points so that he could visualize his quadrats on a map. With the advent of high resolution imagery it is now easy to get a sense of the layout of someone's field site. Overlaying quadrat and transect locations may also help find errors in the data. For example, if a forested plot shows more trees than are visible in an aerial photo this might serve as a red flag that there has been an error in data entry. By overlaying quadrats and transects on aerial photographs you may also be able to understand whether your random/stratified random/regular points/lines are representative of the site as a whole. I encourage field ecologists to take the extra steps to make their data spatially-explicit. This can be as simple recording a single set of XY coordinates + transect line distance + bearing and the distance of start/stop locations (quadrats) along the line. Consider also building map books so that you can quickly visualize what your field site looks like from a bird's eye perspective.

The new tool is called Split Straight Lines at Irregular Distances and runs in ArcGIS 10.1 (should work for other versions of ArcGIS as well). The tool also goes by the name Create Plots along Transects. To run this tool you'll need the following: 1) A point shapefile representing the start end of the transect attributed with XY coordinates, line distance, bearing (polar coodinates in degrees from north), and a unique ID field (FID will do). Make sure that you've declinated the bearing to account for the difference between magentic and true north.
2) A table with unique IDs for each line and distances along the line where points will be generated.

The tool was created in ArcGIS ModelBuilder and is fairly simple. The first step joins the tabular data to the point shapefile. Then the Bearing Distance to Line tool (standard tool in ArcMap - ArcToolbox - Data Management - Features - Bearing Distance to Lines) creates the lines. Finally the end points of those lines are converted to polygons. The end result is that you get a lien shapefile representing your transects and a point shapefile representing quadrats. Other GIS tools are also available on our lab's website.