He persuaded the United States Geological Survey to incorporate the photographic topography of the United States Forest Service on the Columbia National Forest for the preparation of the Hood River and the Steamboat Mountain sheets. It is said this was the first time this method of topographic mapping had been used in the West and the first time the results were incorporated on the United States Geological Survey maps.
He obtained much new material and read proof on the general land office map of Oregon, issued in 1922, and he read proof on every map of the state issued by the Oregon State Highway Department.
In the field of history Tam was outstanding. He contributed ten articles to the Oregon Historical Quarterly, not including those on place names. Among these were "Pacific Coast Survey of 1849 and 1850" and "The Lakes of Oregon." He also wrote thirty-one book reviews for the Quarterly.
Upon the retirement of his mother, Harriet Nesmith McArthur, in 1924, after twenty-six years of service on the board of directors of the Oregon Historical Society, he succeeded her on the board and served until 1948, or a total period of twenty-four years, thus completing a period of fifty years as a board member for mother and son. Tam filled various offices in the society, and was its president from 1937 to 1945.
Tam McArthur's greatest work and the one that overshadowed all else in bringing him generous and widespread recognition was Oregon Geographic Names. This collection, presented first in the Oregon Historical Quarterly, appeared in two editions in his lifetime and after his death in a third.
This third revised and enlarged edition, prepared for the press by his wife, Nellie Pipes McArthur, included an introduction by Tam's friend, Robert W. Sawyer. What follows is largely quoted or paraphrased from that introduction.
Just when it was that Tam became interested in the geographic names of his native state seems not to be known. Perhaps an editorial by Harvey W. Scott in the Oregonian of December 18, 1904, gave the stimulus. This editorial, under the caption "A General Review of Local Titles," opened with these sentences:
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