When choosing an analysis resolution for your trail planning project in TRAILS, consider the size and location of your Area of Interest (AOI). Learn more about resolution options and how to create a strategy for working with large project areas.
Slope analysis in TRAILS is derived using elevation data from USGS 3DEP. Elevation resolution is based on cell size. For instance, 1 m, or 1-meter, resolution means your Area of Interest (AOI) is divided into 1-meter by 1-meter cells each with an elevation value. Why is this important? The smaller or finer the elevation resolution the more accurate your slope analysis will be. But there is a trade-off…because there are many more cells in the same size AOI, the analysis takes longer to complete. As a result, we’ve had to limit the size of the AOI based on resolution. The table below provides a summary of available elevation resolutions and the maximum allowable AOI dimensions in TRAILS.
When you are working on Step 2 in TRAILS (create AOI), you will notice black and orange lines outlined along the edge of your sketched area, as shown in the image below. The black lines help to guide the size of your AOI in order to have the “best” available resolution.
The application also provides a notification popup, as shown below. This is just a warning saying that if you want to have an option for finer resolution, you must make your AOI smaller. You may ignore this message and close it if you wish to continue with your current AOI. Alternatively, you can reduce the AOI boundary to fit within the black lines outlined along the edge of your sketched area.
What do you do if you have a large study area?
We suggest running your preliminary analysis for the entire study area at 30-meter resolution. This resolution should be available for all study areas. After identifying potential trail corridors at 30-meter resolution, you can create several new projects to focus on specific smaller areas within your AOI to get to the finer resolution options.
Keep in mind that if your AOI is larger than 150 square miles, it will take some time to complete the analysis at 30-meter resolution. The National Digital Trails team tested several sizes; a 1,000 square-mile area took about 30 minutes to complete the analysis, and a 4,500 square-mile area took over six hours to complete.