The premise
Ground truth is a surveying term.
Cartographers use it for the data you walk out and verify on the actual dirt after a survey. The map can lie. The satellite can lie. The records can be migrated, sanitized, redacted, and lost. The ground tells the truth.
Grant Prescott runs his firm on that idea. They take cold cases nobody else will touch. They go to the actual ground. They knock on doors. They read the records the records office still has. They find the witnesses. They prove what the family was told had been lost forever, and they hand the proof back.
The first time he solved one, Grant assumed the missing records were a clerical mess. The second time, he started keeping a list. By the third case, the list had a name on it. The name had a payroll. The payroll had people who didn't appreciate being noticed.