This photo is of a billboard that used to be at the Mather Point parking lot, where millions of people saw it in the 1960's and 70's.
It was maybe 3' high by 5' wide. with a glass front, entitled "GRAND CANYON DIMENSIONS". (I dont know who the "ghost in the glass" is.) For years and years, this display misled Mather Point visitors about the Canyon's length.
The Canyon is not 217 river miles long. It is 277 river miles long.
True, at the time of that map, the boundary was at Nankoweap, excluding "Marble Canyon", which was itself a fiction the dam builders used to mislead people into thinking there would not be any dams in the Grand Canyon. Although in 1939 a GCNP superintendent did argue Marble was not part of the Grand Canyon because of differences in size, shape and appearance. What he did not see were the continuities in rock formation such that the true beginning and development of the Canyon can best be understood by starting at the Paria and continuing down into the depths. Maybe that 1939 error got perpetuated somehow.
However, 217 miles from Nankoweap only gets us near the Bat Cave--indeed close under the point the Hualapai are developing with their bootiewalk.
And those locations are not where the arrows point. The right arrow pointing up is somewhere about mile 30, Shinumo Wash. Adding 217 gets us to 247 miles, but that is the location of the big bend marking Lava Cliff and Spencer, upstream of the left arrow, which seems to be aimed at a point short of Quartermaster, so maybe 253, which would put the right arrow (253 - 217) at 36. An accordion canyon, I guess. Or as the text up in the left corner says, this is an "Artist's attempt"; accuracy is not the point.
At that time, there were no easily available maps of the entire Canyon. In the 1960's, an Alburquerque group, the Save Grand Canyon Committee, did actually draw up a new base map, along with overlays. This did not get into general circulation.
The easiest publicly available maps to manipulate were US Geological Survey sheets, dated 1956, at 1:250.000 scale, that cut the Canyon up into four parts. Perhaps that made it difficult for the Artist to be accurate. These maps were used to create plastic representations showing the ups and downs of the country, but at that time, there was no synthesized version of the Canyon. John McComb formed a composite for our use during the congressional debates of the 1970's. Here is one use we made of it, showing the red boundary proposed by Senator Goldwater, our green expansion, and the new Havasupai reservation in blue:
In any case, there was no reason, even in the Dark Ages of the 1960's-70's, for the Park Service to get it wrong. It made me mad every time I went there. An exchange with Superintendent Stitt in late 1973 did prompt his request to the NPS interpretive facility for rehabilitation.