In the past, Lennon said, he sometimes had to squeeze in new housing tracts at the edge of hand-drawn base maps that company founder George Coupland Thomas created in 1935.
"The rule always was: 'Thou shalt not change Mr. Thomas' page numbers,' " Lennon said. "But it had to be done."
Company public relations director Terri M. Hayes said map makers tried to give their biggest customers--those using hundreds of street guides--an early warning about the changes.
She said special tables at the back of the new street guides can be used to convert the new grids to the old numbers. Officials are helping firms add the new grid designations to computer programs that had been set up to use the old map numbers, Hayes said.
But it will take time to change data banks such as the one used by the 9,000-member San Fernando Valley Board of Realtors, said James L. Link, executive vice president of the Van Nuys-based group. The computer uses Thomas Guide grid coordinates to keep track of houses that are for sale.
"Brokers live and die with those books," he said. "Thomas Guides are a lot more than 'How do I get there? Do I turn left or right?' "