Wednesday, January 28, 2015

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.

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