Using Groups

The use of "group-by" fields are ubiquitous in airborne data processing: Line Number is a group-by field – all samples in a line share the same line number. Flight Number is a group-by field – all lines in a flight share the same flight number. Group-by fields are database fields whose values group samples (observations) together.

GammaSpec use three additional group-by fields – each for a specific purpose:

• Encal Group groups lines together to ensure that there are a sufficient number of samples in each group to facilitate accurate energy-calibration of the lines in the group. Some surveys have short lines whose sum spectrum is not smooth enough to accurately identify photopeaks for energy calibration. So these need to be grouped with lines flown around the same time to ensure accurate energy calibration. For 1-s data, 1,000 spectra are typically sufficient.
• Nasvd Group groups flights together to ensure that there are a sufficient number of samples in each group to facilitate accurate NASVD-smoothing of spectra. The minimum number of spectra required in each Nasvd Group is debatable, but for 1-s data, somewhere between 20,000-50,000 should suffice if the survey area is reasonably radioactive.
• Nasvd Cluster groups spectra together based on their spectral shape. This is a special type of group-by field in that there is a value for each sample (the cluster number). NASVD by cluster can deliver improved noise reduction for large surveys (typically 500,000 spectra or larger).

Quick Link: The Create Group-By Field tool (Utilities|Create Group-By Field) can be used to create Encal Group and Nasvd Group group-by fields using the date and time fields. If date and time fields are not available, use the flight number as a proxy for Date, and use Fiducial number as a proxy for Time. The Create Spectra Cluster Field tool (Utilities|Create Spectra Cluster Field) can be used to create Nasvd Cluster group-by fields.