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Portland Bureau of Transportation

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History of the City of Portland's datum

The City of Portland datum is based upon a survey done in 1896 by The Northern Pacific Railway and has remained unchanged for over one hundred years. Despite the many adjustments made to the US Geological Survey’s datum during the same period, the Portland City Council found that it lacked the resources to keep up with the changes and chose to disregard their findings.  Ironically, the adjustments to the US Geological Survey’s datum incrementally approached the City’s datum so that today there is only a difference of 1.375’.

 

Below is a timeline of events affecting the City of Portland’s datum. A more complete historical background of the subject is available in a book by H. B. Schminky written in 1956. It’s a single, handmade copy that has been scanned into a 65 MB Adobe Acrobat PDF file and is available as a download upon request.

 

1854 Ordinance 9 Provided for the election of a City Surveyor and made the establishment of grades part of the office holder’s duties.

 

1854 Ordinance 21 Established grade of Front St by designating "the center of the intersection of Ash and Front Streets at a height of two feet and six inches above the high water line of 1852 and 1853." This, in effect, became the City’s the first datum. Note: to make established grades agree with current City of Portland datum, 29.5’ must be added.

 

1864 Ordinance 154 Lowered the base for street grades by an even 20’ and set the City’s first benchmark, "the Northeast corner of the stone water table of J.L. Parish’s brick building at the Southwest corner of Front and Washington Streets," at 21.5’. Note: to make established grades agree with current City of Portland datum, 9.5’ must be added.

 

1885 Ordinance 517 Established the datum for City of East Portland by assigning the base as, "the stone monument at the intersection of Fourth and H Streets" (SE MLK Blvd and Ash St) and that, "to avoid the use of minus numbers, the elevation of said stone monument . . . shall be called one-hundred and eighteen feet." Note: to make East Portland grades agree with current City of Portland datum, 69.21’ must be subtracted.

 

1896 Ordinance 9667 Establish a datum based upon relation to "mean sea level or mean low tide of the Pacific Ocean" at tide gage at Astoria through a survey done by the Northern Pacific Railway. The elevation of 31.00 feet was assigned to the first benchmark created earlier by Ordinance 154, "the stone water table of J.L. Parish’s brick building at the Southwest corner of Front and Washington Streets." This ordinance also repealed earlier ordinances that set conflicting datums for The City of East Portland, Albina, and The City of Sellwood and made them all conform to the new datum. No correction is necessary.

 

1914 US Geological Survey published Bulletin 556 indicating the previous railroad survey to be in error. To make published elevations conform to current City of Portland datum, 2.45’ must be added.

 

1914 Ordinance 28356 Repealed Ordinance 9667 and declared an emergency to change the City of Portland datum to conform with the US Geological Survey. Stated that, "preliminary surveys to reestablish the elevations of present benchmarks within the City limits" were underway. To make published elevations conform to current City of Portland datum, 2.45’ must be added.

 

1914 Survey was undertaken to "replace the inferior system of benchmarks, both as regards to permanency and accuracy" with "a system of thoroughly good, permanent, definite, easily located and accessible benchmarks." Estimated "apparently heavy cost" of the project was $6000 but "it is believed that when this survey is finished, Portland will have the most complete, accurate and convenient system of benchmarks of any city in the United States."

 

1917 Ordinance 32924 Declared US Geological Survey "as a base for all City grades" after December 31, 1917. To make published elevations conform to current City of Portland datum, 2.45’ must be added.

 

1917 Ordinance 33814 Due to lack of funds and amount of time needed to change all the old records to the new datum, the council amended Ordinance 32924 by postponing the use of US Geological Survey as City’s datum. Although the City published a booklet named, "Tabulation of Official Benchmarks" in 1915 that used as its base the US Geological Survey Bulletin 556, it was never distributed and the datum of 1896 continued to be used.

 

1926 US Coast and Geodetic Survey adjustment, publication number 122, made corrections to US Geological Survey Bulletin 556. To make published elevations conform to current City of Portland datum, 2.116’ must be added.

 

1932 US Coast and Geodetic Survey adjustment, publication number 177, superseded US Coast and Geodetic Survey publication number 122. To make published elevations conform to current City of Portland datum, 1.772’ must be added.

 

1932 City abandoned any attempt to conform to federal government elevations and the datum of 1896 was officially reinstated for use in establishing all grades. This actually made no change, for by postponements, the City had been using this base since it was first established.

 

1947 US Coast and Geodetic Survey made fourth adjustment to datum. To make published elevations conform to current City of Portland datum, 1.375’ must be added.

1947 Both US Coast and Geodetic Survey and Geodetic Service of Canada adopted the Pacific Northwest Supplementary Adjustment of 1947. To make published elevations conform to current City of Portland datum, 1.375’ must be added.

 

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