Although one of Seattle's founding fathers, Arthur Denny (1822-1899), was a surveyor, more than two decades passed before the young town had a systematic and accurate survey. Before that time, property owners established their property boundaries and fence lines in a more or less haphazard way, often based on their neighbor's previous, unguided planning. For example, at one location a fence that should have been on a property line was actually located nine feet into the street. Prior to 1875, there were two "official" reference points in the city. One was an "X" cut in a boulder at the intersection of the center lines of Yesler Way and First Avenue S. The other was a nail-centered hub (a surveyor's marker) driven a little below the surface at First Avenue S and Main Street.
Finally, in 1875, the City Council hired the engineering firm of Eastwick, Morris & Co. to lay out an accurate grid of the city, including elevations above sea level. The survey team's first task was to ascertain what is known as the city's datum point, or the zero point from which all vertical measurements are made. Engineers and surveyors would use the datum point primarily for establishing new grades on city streets. In addition, with the information and an accurate map based on the datum point, property owners would be able to build structures at an elevation corresponding to the predescribed grade. In Seattle, a marker of some sort indicating the site of the datum point would be placed on the surface, but the actual datum point used to calculate elevations was a specified distance beneath that.
To determine the datum point, Eastwick's men set up a tide gauge on one of the downtown wharves (identified by at least one source as Marshall's Wharf), measured tides over a 30-day period in June 1875, and used the mean of the high tides as zero, or in this case, sea level. Next, they had to transfer the information to a point where later surveyors could find it easily. In a note to The Seattle Times more than 55 years after the fact, an engineer, Frederick H. Whitworth, wrote:
"at that time the most permanent solid point in the city was the large stone slab in front of the entrance to the small, one-story office of the Dexter Horton Bank" (Seattle Times, December 18, 1932).
Accordingly, through Ordinance 138, the official datum point was established at 8.835 feet below the level of the top of the lower granite step in the doorway of the bank, which was located at the northwest corner of Washington and Commercial streets. Now designated as the corner of S Washington Street and 1st Avenue S, it is the site of the Maynard Building, erected in 1892. (Bill Spiedel writes in Sons of the Profits that the new datum, located on a marker in the middle of Yesler Way, was seven feet above the tide. Neither the location nor the elevation is correct.)
A Moving Target
Six years later, with Ordinance 383 (May 18, 1883), the City revised the datum point to 9.6 feet below the top of the granite step in the bank's doorway; there is no apparent record of why. The City may have re-measured the tides and come up with a more accurate number. Another possibility -- old photos of the bank show that the lower step had been removed, and perhaps the new datum point reflected that change.
In 1889, Seattle's Great Fire wiped out much of downtown Seattle, including the area around Mr. Horton's bank. Made of granite, it was one of the few buildings to survive relatively intact, but apparently it was in bad enough shape that the City Council decided to establish a new datum point. On September 16, 1891, Ordinance 1836 designated that spot as the "lower step at the entrance to the Pioneer block ... at the southwest corner thereof." (That location is no longer the entrance to the building.)
The new datum point is now 18.79 feet below this step. Why the 11-foot difference between the original datum and post-fire datum? If you walk west 30 feet from the Pioneer Building, you can look south one block to the corner where Dexter Horton's bank once stood. There is not a drop of 10 feet between you and that corner. Instead, the difference between the old and the new datum points reflects the variation between the present road surface and the historic road surface. In other words, you are standing 10 feet higher than someone standing on the same point in 1875.
When Seattle's pioneers started putting up homes and businesses, they built directly atop the land surface. But when the city rebuilt after the Great Fire they decided to raise many of the streets in Pioneer Square. They did so to improve the city's sewer system by raising the outflow above the tide line.
On October 9, 2003, Mayor Greg Nickels (b. 1955) changed the city's datum point again, through Ordinance 121291. The reason was not fire but "inconsistent and outdated methods" used to determine the older points. Now the city relies on standards established by the United States National Geodetic Survey and its National American Vertical Datum of 1988. The new datum point is still based on the lower step of the Pioneer Block, but it is now 19.79 feet below the step.