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Determination of Station Depths Relative to NGVD29

Determination of Station Depths Relative to NGVD29. Methods and results. Jan 24, 2003 Charles Seaton. Some useful terms. NGVD29 : Fixed reference vertical datum (used in CORIE model) MLLW (Mean Lower Low Water): The mean of the daily lower low water over a 19 year Tidal Epoch.

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Determination of Station Depths Relative to NGVD29

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  1. Determination of Station Depths Relative to NGVD29 Methods and results Jan 24, 2003 Charles Seaton

  2. Some useful terms NGVD29: Fixed reference vertical datum (used in CORIE model) MLLW (Mean Lower Low Water): The mean of the daily lower low water over a 19 year Tidal Epoch. MHHW (Mean Higher High water): The mean of the daily higher high water over the tidal epoch MLW (Mean Low Water): The mean of both daily low waters over the tidal epoch MHW (Mean High Water): The mean of both daily high waters over the tidal epoch MTL (Mean Tide Level): The average of the MLW and the MHW Tidal Epoch: 19 year period (currently 1960 to 1978) over which the tides are averaged to determine the tidal datums

  3. HHW LHW HLW LLW Derive properties of tidal record • Determine high waters (HW) and low waters (LW) from data. • For each pair of LW, the lower one is the lower low water (LLW) • For each pair of HW, the higher one is the higher high water (HHW) • Low pass filtered data is determined by running the data through a filter with a cutoff frequency of 0.5 cycles/day

  4. Objective: Derive station depths relative to NGVD29 from pressure record Procedure: • Convert pressure to depth • Derive properties of tidal record LLW, LW, HHW, HW, low pass filtered data • Adjust derived properties to account for incomplete tidal record • Convert tidal datum to fixed vertical datum (NGVD29)

  5. Conversion of Pressure to Depth • Adjust pressure to remove variation in atmospheric pressure • Convert pressure to depth a) pressure is affected by salinity and velocity of water column) b) h=(P/rho*g) - ((v^2)/2*g) rho ~ 999 kg/m^2 + 0.808 * S c) try 4 possible values for S: 35 ppt, actual S, actual S * 0.5, and 0 d) velocity set at 0 m/s (over-estimates depth) v of 2 m/s introduces error of ~20 cm v of 1 m/s introduces error of ~5 cm

  6. Adjust derived properties • To determine the local tidal datums (e.g. MLLW) directly from the data, 19 years of tidal data is required. • Lacking 19 years of data, it is necessary to adjust the tidal properties (e.g. LLW) by the variation in that tidal property at an appropriate tidal station with a known tidal datum.* • This adjustment corrects for the bias that would be introduced by using an atypical time period. • The mean of the adjusted tidal property will give the local tidal datum • MLLWlocal = mean( LLWlocal– LLWknown) * CO-OPS, Tidal datums and their applications, NOAA Special Publications NOS CO-OPS 1, p 41, 2000

  7. NGVD29 MLLW (known) MLLW (unknown) MHHW (known) MHHW(unknown) Relationship of Station Depth and Reference Station Tides Variation in LLW Conversion from reference MLLW to NGVD29 Reference Station depth below MHHW depth below MLLW depth below NGVD29 depth at LLW Offset btwn LLW and MLLW Instrument

  8. Procedure for adjusting tidal properties • Derive tidal properties (LLW, LW, etc.) at Tongue Point from hourly tide data • Derive tidal properties (LLW, LW, etc.) at station from depth data • Find matching points in Tongue Point property for points in station property (accounts for tidal phase shift over estuary) • Subtract Tongue Point values (referenced to MLLW) from station values • For properties other than LLW, it is necessary to correct for the variation in the difference between MLLW and the relevant tidal datum (e.g. MHHW) over the estuary • Mean of remainder gives depth of station relative to MLLW

  9. Variation in adjusted LLW At eliot, shows seasonal variation, std dev = 0.19 m At tansy, shows jump, std dev = 0.16 m At yacht, shows jump, std dev = 0.15 m At dsdma, shows large jump, std dev = 0.31 m dsdma clearly needs to split up into 2 depths At sveni, no jumps or seasonal, std dev = 0.08 m Excluding dsdma, range of std dev is 0.07-0.19 m Concern is not with variability from match Concern is with bias, which cannot be determined from std dev Inaccuracy of tidal variation adjustment method estimated to be 0.02 - 0.04 depending on the length of the record (1-12 months)* * Swanson, RL, Variability of tidal datums and accuracy in determining datums from short series of observations, NOAA Tech Rep. NOS 64,pp 41, 1974 cited in CO-OPS 1

  10. Conversion from local MLLW to NGVD29 in Columbia Estuary • MLLW varies from location to location • In Columbia Estuary, dominant variation in is upwards slope towards upriver direction • While continuous tidal data is only available for Tongue Point, MLLW datums referenced against NGVD29 are available for several benchmark stations in lower estuary • In the region covered by CORIE field stations (Mouth to Skamokawa), the slope of the MLLW against NGVD29 can be approximated by a linear regression of the conversion against the longitude (since the up river direction is primarily eastward). • Cross estuary slope is negligible.

  11. MLLW to NGVD29 within lower Estuary Inclusion of Skamokawa lowers std dev of error , but raises maximum error Maximum error (including Skamokawa) is 0.1 (near eliot) Maximum error (excluding Skamokawa) is 0.06 (near red26) ngvd=7.3158-1.7861e-5*x+mllw X is in ORSPCS-N NAD27 meters

  12. Restatement of variants tested For each station, depth relative to NGVD29 was computed using 4 methods of handling salinity, and 6 methods for determining MLLW The 4 methods for handling salinity were: assume 35 psu salinity, assume column of salinity at instrument, assume column w/ average salinity of half instrument salinity,assume 0 psu salinity The 6 methods for determining MLLW water were:mean adjusted LLW (MLLW)mean adjusted LW (MLW) converted to MLLWmean adjusted HW (MHW) converted to MLLWmean adjusted HHW (MHHW) converted to MLLWmean tide level (MTL) converted to MLLWmean adjusted low pass filter converted to MLLW

  13. Comparison of results of variants: salinity Effect of salinity is much less than expected, particularly for MLLW, and introduces <0.05 m error Results from 35 sal variant excluded (ranged from 0.05-0.45) Results vary with both depth and salinity MLLW is the least affected (consistently <0.05 m uncertainty in depth) Odd behavior at woody, cbnc3, and lght6 is caused by lack of valid salinity for extensive portion of data

  14. Comparison of results of variants: tidal property Use of different tidal properties produces a maximum error of <0.2 m due to variation over estuary of shape of tides Although reference tide data was in MLLW datum, there is no inherent reason to prefer MLLW over any other datum Standard deviation of results for all tidal properties (for 0 sal) is 0.064 Low pass should be excluded due to the lack of an established low pass datum Excluding low pass reduces standard deviation to 0.59

  15. Other Sources of Error • Atmospheric correction: atmospheric pressure not identical over estuary (effect unquatified, but expected to be <<0.2 m) • Velocity effect on pressure (unquantified, but expected to be <<0.05 m) • Inaccuracy of tidal variation adjustment (estimated to be 0.02 - 0.04 depending on the length of the record (1-12 months)*) • Inaccuracy of linear estimate of MLLW-> NGVD29 (3*std err = 0.48 m) • Error in calculation of depth is <<0.2 m • Error in determination of MLLW is

  16. Using HHW referenced to MLLW as a correction for HHW • Since the reference datum is MLLW, then the local datum will also be MLLW • Need to take into account the variation in the conversion between MLLW and MHHW over the estuary • Variation can be approximated as linear with x coordinate over lower estuary • Essentially, converting reference datum from MLLW to MHHW at reference station, adjusting local HHW (which gives depth relative to MHHW), and then converting local MHHW to local MLLW, so all that is needed is the difference between MLLW-> MHHW at the local station versus the reference station (see plot) • Equivalent methods were used for MLW, MHW, and MTL • No equivalent method was possible for low pass filtered data (no reference datum available

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