Improvements to Sampling Grid Tools

A neat thing  happened this week.  A while back I created a new tool called Create Regular Sampling Grid and posted it online HERE . The tool is designed to help guide field workers in systematic grid around user-defined points. The tool is useful for validating coarse-resolution imagery, such as Landsat or MODIS, or for sampling systematically within polygons. Earlier this week I got a message from Duncan Hornby, a long time ArcGIS programmer from the UK, with a slew of awesome suggestions for ways to improve this tool.  His suggestions ended up dramatically improving the speed and user interface of the Create Regular Sampling Grid Tool resulting in version 2.  The lesson that I learned is that good things can happen with a second pair of eyes, and posting code online is a great idea.  I also learned a fair bit about improving the user experience and anticipating and troubleshooting potential problems.  Thanks Duncan.

New tool - Create Sampling Grid from Points for ArcGIS v. 1.0

With the advent of high-precision GPS spatially-explicit sampling designs have taken on an increasing importance in ecology and natural resource management. Spatially-explicit sampling regimes are useful for understanding processes such as attraction and repulsion that can be described using point pattern processes. This tool also opens up the possibility of random sampling within a larger grid. For example, users may want to collect field data to scale up to Landsat or MODIS pixels. It may be infeasible to collect data for an entire pixel, so some random sampling of the pixel may be necessary. Similarly there may be vegetation polygons or agricultural fields that the researcher wishes to sample in a random or a systematic manner. Finally, even if the researcher wishes to sample the entire grid having the ability to load center points or corner points onto a GPS and navigate to them may expedite field sampling. The creation of this tool was inspired by the needs of a current ongoing pygmy rabbit research project here in Nevada, Oregon, and Idaho.


This tool allows for the creation of polygons and centroids of polygons based on known points. The known points can be random locations, centroids of features of interest (e.g. polygons of agricultural fields or vegetation polygons), or regular gridded points across a landscape. This tool differs from existing tools, such as the Fishnet tools in ArcGIS, because it does not create a single grid for the entire landscape, but rather creates a local grid centered on each point in the input shapefile. It uses the following formula to achieve this:

(-1*(d/2) - 0.5) + i where d is the dimensions parameter and i is the iteration number. The two images below illustrate a grid with an even number of dimensions (6 on the left) and one with an odd number of dimensions (7 on the right). Both sampling grids are centered on points provided by the user, but the the one on the left has the original point (green) falling on a grid corner. The one on the right has the original point (provided by the user, in green) falling in the center of an individual grid tile.  The maroon points were created using the 'Feature Vertices to Points' tool, a standard ArcGIS tool with the advanced license.