Evaluating VDatum in coastal National Parks: Cape Cod National Seashore
Ullman D and Babson A. 2017. Evaluating VDatum in coastal National Parks: Cape Cod National Seashore. Natural Resource Report. NPS/NCBN/NRR—2017/1460. National Park Service. Fort Collins, Colorado
The accuracy of tidal datum elevations provided by NOAA's VDatum software package (version 3.2) is investigated for Cape Cod National Seashore (CACO). All available water level observations that could be referenced to the orthometric datum NAVD88, from NOAA, USGS, and the National Park Service (NPS) in the vicinity of the park, were used in the comparison. For observational sites lacking published NOAA values, datum elevations were computed and corrected to the 1983-2001 National Tidal Datum Epoch (NTDE), the Epoch used for VDatum, using standard NOAA methodologies. The estimate of the uncertainties in computed datum elevations, arising from uncertainties in the observations, instrument elevations, and the computation of datums from short (less than 19 years) records is approximately 0.079 m. The corresponding uncertainty in datum elevations from VDatum, estimated by NOAA, is approximately 0.132 m in this region. Comparisons between observed and VDatum elevations were performed for mean high and low water (MHW and MLW), mean higher high and lower low waters (MHHW and MLLW), as well as mean sea level (MSL). The performance of VDatum was extremely variable. At open water sites, where there is a relatively deep (deeper than the oceanic tidal range) connection to the ocean or Cape Cod Bay, VDatum elevations were accurate to within 0.07 m. However, at sites lacking a deep connection to open water VDatum was highly inaccurate, especially for the low water datums (MLW and MLLW) and MSL. Averaged over all sites without a deep connection, low water datums were in error by approximately 1.6 m and the root-mean-square error in MSL was about 0.75 m. These uncertainties far exceed the VDatum uncertainty estimates. Since the NPS requires datum accuracy of 0.05-0.10 m for their management activities, the results of this study indicate that VDatum is inadequate in at least some of the shallow, constricted areas of CACO. At the present time, therefore, it appears that for accurate datum elevations in these areas water level observations (such as those used in this report) and subsequent calculation of datum elevations will be needed.
- Type
- Published Report
- Authors
- Ullman, David; Babson, Amanda
- Date of Issue
- 2017-06
- Publisher
- National Park Service
- Units
- CACO , NCBN , NRSS
- Keywords
- CACO, Cape Cod Bay, Cape Cod National Seashore, CESU Storm Evaluation and Modeling, datum elevations, mean high and low water, NCBN, NOAA, North Atlantic Coast Cooperative Ecosystem Studies Unit, Northeast Coastal And Barrier Network, tidal datum, VDatum, water level observations