5 Part 5

5.1 Examine data for making a clutter mask for the basegrid

In addition to identifying regions of topograhic blockage, we need to identify sample volumes for exclusion from analyses due to 1) persistent ground clutter contamination (i.e., radar echoes caused by highway overpasses, wind farms, tall buildings), 2) partial radar beam blockage caused by human infrastructure (e.g., tall buildings), or 3) chronic data filtering that is part of the radar’s intrinsic clutter suppression algorithm. We can identify these sample volumes by analyzing reflectivity measures across thousands of daytime volume scans collected when birds are not migrating. Data can be excluded from sample volumes where reflectivity is detected at a high frequency above 30 dBZ in association with persistent returns from ground targets, or where reflectivity is detected at extremely low frequency (< 2 standard errors below the mean) in association with beam blockage or chronic clutter suppression. Sample volumes with persistent ground clutter contamination are usually relatively few and typically within 25 km of the radar antenna. I’ve previously summarized reflectivity for a large number of daytime volume scans for you to consider.

Let’s join the summary data table to the basegrid in QGIS.

5.1.1 Join table to layer in QGIS

1.  Load the "lwxcluttersummary.dbf" table into QGIS
2.  In the Layers Panel, right-click on the basegrid shapefile to which you'd like to join the table and click Properties
3.  A new window will open.  Click Joins in the left-side menu.  
4.  At the bottom of the window, click the green plus sign. 
5.  A new window will open.  In the Join Layer dropdown, find the table you want to join, select the Join Field "Id" and the Target field "Id".  Click OK.

Now let’s display data. We will start with the “zpropz”. This the Z-score (i.e., standardized value) of the proportion of volumes scans when reflectivity was detected within the sample volume. Make a color ramp that distinguishes values < -2 as unique colors. The regions of low probability of detection (POD) of reflectivity are likely clutter regions. Do you notice any patterns about where these clutter regions tend to occur?

Rays where the POD is noticeably lower along the entire length of the ray denote rays where there is likely partial blockage from an obstruction close to the radar. Reflectivity data from these rays should be masked from animal mapping analyses. What are the azimuths of some of the rays that appear to be partially blocked?

Now display the variable “propclutte”. This variable represents the proportion of volume scans when the reflectivity factor is above 30 dBZ. Try denoting values above 0.03 as a unique color.