One goal of modern geodesy is the global unification of vertical datums so that height data from them can be properly integrated. This thesis studies the unification of the 13 disparate levelling- and tide-gauge-based vertical datums in New Zealand (NZ). It proposes a new NZ-wide single vertical datum based on a gravimetric quasigeoid model to unify the existing local vertical datums. This will also include methods to transform height data in terms of the existing datums to the new datum and vice versa. After defining and comparing the main types of height system and vertical datum used around to world, the system of heights used in NZ was shown to be normal-orthometric. Consequently, datum unification was achieved using a quasigeoid model, as opposed to a geoid model. The quasigeoid was computed by combining the GRACE-based GGM02 and EGM96 global geopotential models with land gravity data (40,737 observations) and a 56-m resolution digital elevation model (DEM). Marine gravity data came from a least-squares collocation combination of 1,300,266 crossover-adjusted ship track observations and gravity anomalies derived from multi-mission satellite altimetry. / To ensure that the best quasigeoid was computed for the NZ datasets, a number of computation processes were compared and contrasted. The Hammer chart, fast Fourier transform (FFT) and prism integration methods of computing terrain corrections (TCs) were compared. This showed that the prism integration TC is the most realistic in NZ. The mean Helmert gravity anomalies, required for numerical integration of Stokes’s formula, were computed via refined Bouguer anomalies with the prism TCs, and reconstruction with heights from the DEM used to ‘reconstruct’ more representative mean anomalies. In addition, five deterministic modifications to Stokes’s formula were compared. There was little difference between three of them, so the Featherstone et al. (1998) modification ( 0 y = 1.5°, M = 40) was chosen because it is theoretically better than its predecessors. The global geopotential, gravimetric geoid, sea surface topography and geodetic boundary-value problem approaches to vertical datum unification were then contrasted. As none was likely to be effective in NZ, a new iterative quasigeoid approach was adopted. This procedure computes an initial quasigeoid from existing data on the various local vertical datums to estimate the vertical datum offsets from co-located GPS-levelling data. These offsets were then subsequently applied to the gravity observations by way of additional reductions to the initially computed quasigeoid to reduce gravity anomaly biases caused by the vertically offset datums. These adjusted gravity anomalies were then used to compute a new quasigeoid model, and the process repeated until the computed offsets between the local vertical datums (or equivalently two quasigeoid solutions) converged, which took only two iterations. / The computed offsets were then compared with spirit-levelled height differences among adjoining datums, where these were available, giving an average agreement of 7 cm. Since the local vertical datums are effectively unified, the new national vertical datum for NZ will comprise the iteratively computed gravimetric quasigeoid model, accompanied by local vertical datums. This approach is implemented to give a new national vertical datum for NZ. When used with the appropriate offset, this enables the transformation of heights in terms of the national vertical datum to the 13 precise-levelling datums and the ellipsoidal national geodetic datum, NZ Geodetic Datum 2000.

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The tidal prism, or the volume of water exchanged from the sea to an estuary from mean low to mean high tide, influences system hydrodynamics and ecological functioning. Since 1884, the tidal prism in Morro Bay, California has been estimated to be decreasing over time due to sedimentation from upstream practices. What is the current tidal prism in Morro Bay and how will that change with sea level rise? How will eelgrass respond to rising sea levels? For this study, inexpensive tidal gauges were deployed at four locations in Morro Bay from March to August 2023 to measure spatially varying tidal elevations and datums within the bay. I utilized a Digital Elevation Model (DEM) and tidal information to determine volumes of water in Morro Bay. Estimated sea level rise scenarios were utilized to project the 2022 tidal prism into the years 2050 and 2100. Additionally, I estimated the 2019 and 2022 viable eelgrass habitat area using the vertical growth range. I estimated the future potential viable habitat area in the years 2050 and 2100 using estimated sea level rise scenarios. Future projections were made assuming no change in bathymetry over time. Different instruments used to obtain water levels yielded up to ~4 percent differences in the tidal prism estimate. Measurement uncertainty in the monthly tidal datums produced ~3 percent uncertainty within the tidal prism estimate. Compared to the tidal prism in August 2019, the August 2022 tidal prism was lower by ~2 percent. Compared to the tidal prism in August 2019, the August 2023 tidal prism estimated from two nearly co-located tidal instruments at the mouth of Morro Bay were higher by ~5 and ~7 percent, respectively. Spatially varying tidal datums in Morro Bay were found to affect the tidal prism by up to ~3 percent, compared to tidal prism estimates using only a tidal datum near the estuary mouth. However, the effect of spatially varying tidal datums on the tidal prism is the same order of magnitude as measurement uncertainty and is thus not statistically significant. As sea levels rise, the tidal prism is projected to increase by ~40 percent by 2100 from 2022 under the most extreme scenario, H++. Initially, as sea levels rise, the potential viable eelgrass habitat area will increase from the area in 2022 (1108 acres (4.47E+06 m2)). After sea levels rise to 1.5 m above 2000 levels, the potential viable eelgrass area will have reached a maximum area of 1938 acres (7.82E+06 m2). However, under SLR scenario H++, potential viable habitat area is predicted to decrease by up to 59% by 2100 from 2022. The tidal prism, or the volume of water exchanged from a bay to the seathe sea to an estuary from mean lowhigh to mean highlow tide, influences system hydrodynamics and ecological functioning. Since 1884, the tidal prism in Morro Bay, California has been estimated to be decreasing over time due to sedimentation from upstream practices. What is the current tidal prism in Morro Bay and how will that change with sea level rise? How will eelgrass respond to rising sea levels? For this study, inexpensive tidal gauges were deployed at four locations in Morro Bay from March to August 2023 to obtain measure spatially varying tidal elevations and datums within the bay. I utilized a Digital Elevation Model (DEM) and tidal information from NOAA to determine volumes of water in Morro Bay at the estimated monthly mean tidal datums. Estimated sea level rise scenarios were utilized to project the 2022 tidal prism into the years 2050 and 2100. Additionally, I estimated the 2019 and 2022 viable eelgrass habitat area using the vertical growth range. I estimated the future potential viable habitat area in the years 2050 and 2100 using estimated sea level rise scenariosfor various sea level rise scenarios.Future projections were made assuming no change in bathymetry over time. Different instruments used to obtainwater levels yielded up to ~4 percent differences in the tidal prism estimate. Measurement uncertainty in the monthly tidal datums produced~3 percent uncertainty within the tidal prism estimate. Compared to the tidal prism in August 2019, the August 2022 tidal prism from Stilltek decreasedwas lower by ~2.05 percent. Compared to the tidal prism in August 2019, the August 2023 tidal prism estimated from the Stilltek gauge and the Cal Poly Coast Guard gaugetwo nearly co-located tidal instruments at the mouth of Morro Bay increased were higher by ~5.06 and ~6.777 percent, respectively. Spatially varying tidal datums in Morro Bay were found to affect the tidal prism by up to ~2.883 percent, compared to tidal prism estimates using only a tidal datum near the estuary mouth. However, the effect of spatially varying tidal datums on the tidal prism is the same order of magnitude as measurement uncertainty and is thus not statistically significant. As sea levels rise, the tidal prism is projected to increase by ~to a maximum of 40 percent by 2100 from 2022 under the most extreme scenario, H++. InitiallyInitially, as sea levels rise, the potential viable eelgrass habitat area will increase from the area in 2022 (1108 acres (4.47E+06 m2)). ABut after sea levels rise to 1.5 m above 2000 levels, the potential viable eelgrass areas will have reached the thresholda maximum area of 1938 acres (7.82E+06 m2). However, under SLR scenario H++, potential viable habitat area is predicted to decrease by up to 59% by 2100 from 2022.