Geodetic Vertical Research Articles

Record Low Water Surface Elevations at Great Salt Lake, Utah, 2021-2022

The United States Geological Survey (USGS) operates two long-term water-surface elevation (WSE) gages on Great Salt Lake, Utah, one north of the Union Pacific Railroad causeway in the historic Little Valley Boat Harbor (Saline gage), and one south of the causeway in the harbor at Great Salt Lake State Park (Saltair gage). From September 28 to December 15, 2022, lake levels were too low in the harbor for the Saltair gage to operate and WSE data was measured at the South Causeway gage, a relatively new gaging station (installed in 2020) located immediately south of the causeway. Data collected at the South Causeway gage were used to estimate the daily mean WSE record for the Saltair gage for the period it was shut down, preserving the continuity of the 175-year WSE record that is associated with this gage. The long-standing historic low daily mean WSE measured at the Saltair gage on October 15, 1963 (4,191.35 feet, relative to the National Geodetic Vertical Datum of 1929 (NGVD29)) was broken on July 21, 2021. Seasonal lake-level declines Geodetic Vertical Datum of 1929 (NGVD29)) was broken on July 21, 2021. Seasonal lake-level declines from July 2021 to October 2021 and April 2022 to early November 2022 resulted in a new historic low daily mean WSE of 4,188.5 feet NGVD29, measured during several days during November 2022 at the South Causeway gage. The same value is also the new historic low daily mean WSE for the Saline gage and was measured during several days in November and December 2022 (the previous historic low of 4,188.98 feet NGVD29 was measured in September and October 2016 and was related to closure of two railroad causeway culverts). USGS also operates streamgages on major surface-water inflows including the Bear River, Weber River, Jordan River, and Surplus Canal. The combined annual discharge measured at these gages in water years 2021 and 2022 was 0.704 and 0.743 million acre-feet, respectively, which is less than half of the combined median annual discharge (1.57 million acre-feet) based on the period of record for each gage.

ASSESSMENT OF SEA LEVEL RISE IMPACT ON PENINSULAR MALAYSIA GEODETIC VERTICAL DATUM

Abstract. Sea level is the height of the ocean surface in relation to a benchmark or vertical control point. The rising of sea level has become a crucial topic, which frequently discussed among professionals, government, non-government, and researchers upon their devastating impacts on daily human life and human survival. Nevertheless, there is a great deal of ambiguity around the rates, ranges, and time frame of sea levels rise. It is importance to study the variation of present-day sea level rise to understand its impact, particularly on our National Geodetic Vertical Datum (NGVD). This study focuses on quantifying the rate and magnitude of sea level rise over Peninsular Malaysia using in-situ tide gauge and multi-mission satellite altimeter. The time period for tide gauge data used is 35 years (1984–2018) consisting 12 tide gauges in Peninsular Malaysia. Meanwhile, the period of satellite altimetry data is 26 years (1993–2018). This study uses robust fit regression (Iterative Re-weighted Least-Squares) for both tide gauge and satellite altimetry data to calculate the rate of sea level rise. The outcomes of this study show that the rate of sea level rise at Pelabuhan Kelang tide gauge station is the lowest among 12 other tide gauges with a rate of 2.36 ± 0.35 mm/year and slightly affected by sea level rise. Besides, the rate of sea level rise in Peninsular Malaysia is on an upward trend with an average of 3.20 ± 0.27 mm/year from tide gauge data and 4.14 ± 0.32 mm/year from satellite altimetry data. The sea level rise study provides insights to the rate and magnitude of sea level rise at tide gauge stations, particularly at Pelabuhan Kelang station for further study. Last but not least, this study would also benefit authorities in preparing mitigation plans to secure and improve our Peninsular Malaysia Geodetic Vertical Datum (PMGVD) for various applications.

Epoch-Based Height Reference System for Sea Level Rise Impact Assessment on the Coast of Peninsular Malaysia

The Peninsular Malaysia Geodetic Vertical Datum 2000 (PMGVD2000) inherited several deficiencies due to offsets between local datums used, levelling error propagations, land subsidence, sea level rise, and sea level slopes along the southern half of the Malacca Strait on the west coast and the South China Sea in the east coast of the Peninsular relative to the Port Klang (PTK) datum point. To cater for a more reliable elevation-based assessment of both sea level rise and coastal flooding exposure, a new epoch-based height reference system PMGVD2022 has been developed. We have undertaken the processing of more than 30 years of sea level data from twelve tide gauge (TG) stations along the Peninsular Malaysia coast for the determination of the relative mean sea level (RMSL) at epoch 2022.0 with their respective trends and incorporates the quantification of the local vertical land motion (VLM) impact. PMGVD2022 is based on a new gravimetric geoid (PMGeoid2022) fitted to the RMSL at PTK. The orthometric height is realised through the GNSS levelling concept H = hGNSS–Nfit_PTK–NRMDT, where NRMDT is a constant offset due to the relative mean dynamic ocean topography (RMDT) between the fitted geoid at PTK and the local MSL datums along the Peninsular Malaysia coast. PMGVD2022 will become a single height reference system with absolute accuracies of better than ±3 cm and ±10 cm across most of the land/coastal area and the continental shelf of Peninsular Malaysia, respectively.

A New Redefinition of Geodetic and Plane Coordinates on UTM Geodetic Markers

The entire Peninsular Malaysia is situated on Sunda Tectonic Plate, which is subjected to motion at a prominent rate. All the geodetic infrastructures, including geodetic markers, benchmarks, Continuous Operating Reference Stations and Cadastral Reference Marks have moved away from their original position, and their existing coordinates are no longer reliable and secure to use. There are 10 geodetic markers around UTM that are subjected to the issue above. This study aims to redefine new geodetic and plane coordinates on UTM geodetic markers. Two units of Trimble NetR9 Geodetic Type Receiver are used to execute 3D GNSS Control Network on geodetic markers as well as Standard Benchmarks. Receiver Independent Exchange data of Continuous Operating Reference Stations and gravimetric geoid of MyGeoid model are retrieved from Department of Survey and Mapping Malaysia. Trimble Business Center, Golden Surfer 8 and StarNet are used as post-processing, geoid interpolation and one-dimensional network adjustment software, respectively. New sets of geodetic and plane coordinates along with orthometric heights are produced for these 10 geodetic markers. Eventually, UTM geodetic markers are tied to Geocentric Datum Malaysia 2000 (2016) and National Geodetic Vertical Datum, providing reliable horizontal and vertical reference for land surveying work to fulfil both industrial and educational purposes.Keywords: Coordinate redefinition, geodetic coordinate, plane coordinate, geodetic marker

Vertical datum transformation grids for Mexico

Mexico has used two vertical datums—the U.S. National Geodetic Vertical Datum of 1929 (NGVD29) and the North American Vertical Datum of 1988 (NAVD88). Because Mexico started using the NAVD88 as its vertical datum in 2015, most of Mexico’s data is referenced to the NGVD29 and recent high resolution data are referenced to the NAVD88. Compounding this situation, satellite-derived Digital Elevation Models (DEMs) are referenced to the Earth Gravimetric Model 96 (EGM96), and no tools are currently available in Mexico to transform elevation data between the aforementioned vertical datums. To overcome this problem, this work presents the development of two surfaces to transform orthometric heights between the NGVD29 and NAVD88 or between the NGVD29 and EGM96 in Mexico. These surfaces can be used by any interested user to transform orthometric heights referenced to the aforementioned datums.

THE IMPACT OF SEA LEVEL RISE ON GEODETIC VERTICAL DATUM OF PENINSULAR MALAYSIA

Abstract. Sea level rise is rapidly turning into major issues among our community and all levels of the government are working to develop responses to ensure these matters are given the uttermost attention in all facets of planning. It is more interesting to understand and investigate the present day sea level variation due its potential impact, particularly on our national geodetic vertical datum. To determine present day sea level variation, it is vital to consider both in-situ tide gauge and remote sensing measurements. This study presents an effort to quantify the sea level rise rate and magnitude over Peninsular Malaysia using tide gauge and multi-mission satellite altimeter. The time periods taken for both techniques are 32 years (from 1984 to 2015) for tidal data and 23 years (from 1993 to 2015) for altimetry data. Subsequently, the impact of sea level rise on Peninsular Malaysia Geodetic Vertical Datum (PMGVD) is evaluated in this study. the difference between MSL computed from 10 years (1984 – 1993) and 32 years (1984 – 2015) tidal data at Port Kelang showed that the increment of sea level is about 27mm. The computed magnitude showed an estimate of the long-term effect a change in MSL has on the geodetic vertical datum of Port Kelang tide gauge station. This will help give a new insight on the establishment of national geodetic vertical datum based on mean sea level data. Besides, this information can be used for a wide variety of climatic applications to study environmental issues related to flood and global warming in Malaysia.

The Canadian Geodetic Vertical Datum of 2013 (CGVD2013)

The Canadian Geodetic Vertical Datum of 2013 (CGVD2013) is the first major update to the vertical datum in Canada in almost 100 years. Canada is not only moving to a new vertical datum, but it is also using a modernized approach to realize it. The modernization of the height reference system is necessary to make it compatible with Global Navigation Satellite System (GNSS), which is commonly used for positioning by a growing number of users across Canada and the world. The geodetic levelling technique, which established a nation-wide network of benchmarks with known elevations, is replaced by a geoid model that describes the vertical datum with respect to an ellipsoid, which is the reference surface for GNSS positioning. Technically, height modernization replaces the need for the maintenance of benchmarks, as users can now install their own markers at more convenient locations using GNSS. The current geoid model for CGVD2013 is the Canadian Gravimetric Geoid 2013 (CGG2013).

Evaluation of W0 in Canada using tide gauges and GOCE gravity field models

AbstractThe existing Canadian Geodetic Vertical Datum of 1928 (CGVD28) does not meet the needs of the modern user in terms of accuracy and accessibility. As a result, Canada plans to implement a geoid-based and global navigation satellite system (GNSS)-accessible vertical datum by 2013. One of the primary concerns in realizing this new vertical datum is to determine a W0 value that will represent the potential of the zero height surface. The objective of this study is to evaluate W0 by averaging the potential of points on the mean sea water surface utilizing tide gauge recordings and gravity field and steady-state ocean circulation explorer (GOCE)-based global geopotential models. In order to assess the performance of the GOCE-based models for the computation of W0, the models are extended with the high resolution gravitational model EGM2008. Regional gravimetric geoid models are also used for the estimation of W0. Additionally, local sea surface topography models are utilized in order to validate the W0 results at the tide gauges. Excluding the Arctic coast, the W0 values obtained from both tide gauges and oceanic sea surface topography models are not statistically different from the International Earth Rotation and Reference Systems Service (IERS) 2010 global conventional value 62636856.00 m2/s2.

Referencing regional geoid-based vertical datums to national tide gauge networks

AbstractThe objective of this study is to investigate the best means for referencing a regional geoid-based vertical datum to a network of tide gauges. In this study, a network of 27 tide gauge stations scattered along the coasts of Canada are used in order to assess the replacement of the conventionally derived Canadian Geodetic Vertical Datum of 1928 with a geoid-based datum. This is in-line with the future implementation plan of Canada’s geoid-based vertical height system. A mixed least-squares adjustment was performed for various scenarios, including satellite-only global geoid models, combined global geoid models and regional geoid models. In addition, various sea surface topography and vertical ground motion models were tested for estimating orthometric heights. The resulting approximation of a local equipotential surface is compared to previously published values and considerations for referencing a geoid-based vertical datum to tide gauge networks are emphasized.

Evaluation of the use of paleotsunami deposits to reconstruct inundation distance and runup heights associated with prehistoric inundation events, Crescent City, southern Cascadia margin

ABSTRACTHistoric‐ and prehistoric‐tsunami sand deposits are used to independently establish runup records for tsunami hazard mitigation and modeled runup verification in Crescent City, California, located in the southern Cascadia Subduction Zone. Inundation from historic (1964) farfield tsunami (~5–6 m runup height) left sand sheet deposits (100–200 m width) in wetlands located behind a low beach ridge [3–4 m elevation of the National Geodetic Vertical Datum of 1988 (NAVD88)]. The most landward flooding lines (4·5–5 m elevation) in high‐gradient alluvial wetlands exceed the 1964 sand sheet records of inundation by 1–2 m in elevation. The most landward flooding in low‐gradient alluvial wetlands exceed the corresponding sand sheet record of inundation distance by 1000 m. Nevertheless, the sand sheet record is an important proxy for high‐velocity inundation. Sand sheet deposition from the 1964 historic tsunami closely corresponds to the landward extent of large debris transport and structural damage in the Crescent City waterfront. The sand sheet deposits provide a proxy for maximum hazard or ‘kill zone’ in the study area.Six paleotsunami sand sheets (0·3–3 ka) are recorded in the back‐ridge marshes in Crescent City, yielding a ~450 year mean recurrence interval for nearfield Cascadia tsunami. Two paleotsunami sand deposit records, likely correlated to Cascadia ruptures between 1·0 and 1·5 ka, are traced to 1·2 km distance and 9–10 m elevation, as adjusted for paleo‐sea level. The paleotsunami sand deposits demonstrate at least twice the runup height, and four times the inundation distance of the farfield 1964 tsunami sand sheet in the same marsh system. The preserved paleotsunami deposits in Crescent City are compared to the most landward flooding, as modeled by other investigators from a predicted Cascadia (~ Mw 9) rupture. The short geologic record (~1·5 ka) yields slightly lower runup records than those predicted for the modeled Mw 9 rupture scenario in the same marsh, but it generally verifies predicted maximum tsunami runup for use in the planning of emergency response and rapid evacuation. Copyright © 2011 John Wiley & Sons, Ltd.

The Second Precise Levelling Network of Peninsular Malaysia

The measurement of Second Precise Levelling Network (PLN) for the Peninsular Malaysia which was completed in 2000 by Department of Surveying and Mapping Malaysia (DSMM) is set to replace the First Order Levelling Network of 1967. The new network consists of 113 levelling lines with more than 5000 bench marks and covers a total distance of over 5000 km. Precise levelling technique is used to establish the network where the allowable misclosure between fore and back levelling is less than 3 mm per root kilometre of length along a line. Its configuration is predominantly dictated by the land transportation pattern. The mean sea level (MSL) at Port Kelang, based upon a 10-year tidal observation (1984-93), was later being adopted as the new Peninsular Malaysia Geodetic Vertical Datum (PMGVD). A consistent and accurate set of adjusted heights of benchmarks has been achieved in the adjustment of the Precise Levelling Network of Peninsular Malaysia on the datum defined by MSL height at Port Klang. These adjusted heights are based on the Helmert orthometric height system. By fixing Port Kelang, the precision of the PLN can be expressed as 1.14 mm√km. This implies that for any of the 5,295 first-order levelling bench mark across the nation, a height precision of better than 3 cm can be expected.

Frequency analysis for predicting 1% annual maximum water levels along Florida coast, US

AbstractIn the Coastal Flood Insurance Study by the Federal Emergency Management Agency (FEMA, 2005), 1% annual maximum coastal water levels are used in coastal flood hazard mitigation and engineering design in coastal areas of USA. In this study, a frequency analysis method has been developed to provide more accurate predictions of 1% annual maximum water levels for the Florida coast waters. Using 82 and 94 years of annual maximum water level data at Pensacola and Fernandina, performances of traditional frequency analysis methods, including advanced method of Generalized Extreme Value distribution method, have been evaluated. Comparison with observations of annual maximum water levels with 83 and 95 years of return periods indicate that traditional methods are unable to provide satisfactory predictions of 1% annual maximum water levels to account for hurricane‐induced extreme water levels. Based on the characteristics of annual maximum water level distribution of Pensacola and Fernandina stations, a new probability distribution method has been developed in this study. Comparison with observations indicates that the method presented in this study significantly improves the accuracy of predictions of 1% annual maximum water levels. For Fernandina station, predictions of extreme water level match well with the general trend of observations. With a correlation coefficient of 0·98, the error for the maximum observed extreme water level of 3·11 m (National Geodetic Vertical Datum) with 95 years of return period is 0·92%. For Pensacola station, the prediction error for the maximum observed extreme water level with a return period of 83 years is 5·5%, with a correlation value of 0·98. The frequency analysis has also been reasonably compared to the more costly Monte Carlo simulation method. Copyright © 2008 John Wiley & Sons, Ltd.

A NEW HEIGHT CONTROL FOR PENINSULAR MALAYSIA

The requirements and methods of establishing a new height control in Peninsular Malaysia are discussed. The implementation of the two projects namely the Tidal Observation Network and Precise Levelling Network are designed to fulfil these requirements. Preliminary results show a high accuracy and the levelling network complies with geodetic standards. After establishing a new height datum namely the Peninsular Malaysia Geodetic Vertical Datum (PMGVD) the height of the benchmarks in the network are defined using the system of Helmert orthometric heights based on actual gravity values. Further investigations are required to maintain the reliability of the Mean Sea Level (MSL) values and to independently verify the apparent sea slope.

A NEW HEIGHT CONTROL FOR PENINSULAR MALAYSIA

The requirements and methods of establishing a new height control in Peninsular Malaysia are discussed. The implementation of the two projects namely the Tidal Observation Network and Precise Levelling Network are designed to fulfil these requirements. Preliminary results show a high accuracy and the levelling network complies with geodetic standards. After establishing a new height datum namely the Peninsular Malaysia Geodetic Vertical Datum (PMGVD) the height of the benchmarks in the network are defined using the system of Helmert orthometric heights based on actual gravity values. Further investigations are required to maintain the reliability of the Mean Sea Level (MSL) values and to independently verify the apparent sea slope.

Differences between NGVD29 and NAVD88 in Southeastern Wisconsin

The Southeastern Wisconsin Regional Planning Commission (SEWRPC) has, over the past 30 years, established and promoted use of elevations referenced to the National Geodetic Vertical Datum of 1929 (NGVD29). Through international agreement, cooperation, and participation the vertical datums for the entire North American continent were recomputed, readjusted, and published as the North American Vertical Datum of 1988 (NAVD88). This paper summarizes a report prepared to describe differences between those vertical datums throughout the seven-county region served by SEWRPC. The report also includes a brief discussion of the International Great Lakes Datums (IGLD) and describes how IGLD 1985 is related to NAVD88. Conclusions published in the report are that within the seven-county SEWRPC region: (1) third-order datum differences can be interpolated from an isohypsometric map available from SEWRPC; (2) second-order datum differences can be obtained using VERTCON, an interpolation program available from the National Geodetic Survey, Silver Spring, Md; and (3) datum differences at first-order benchmarks should be determined on the basis of field surveys using first-order equipment and observational techniques.

Geodetic Coordinate Systems in Florida: GPS Contribution

Measurements and results derived from the three‐dimensional Global Positioning System (GPS) are more accurate than classical geodetic observations and methods used until recently to define conventional horizontal (geometric) da‐tums, e.g., the North American Datum of 1983, the final readjustment of which was finished in 1986, thus the notation NAD 83 (1986). The situation may not be strictly the same with respect to vertical (physical) datums such as the National Geodetic Vertical Datum of 1929 (NGVD 29) because an accurate geoid is required. In this paper comparisons of standard datums [NAD 83 (1986), NGVD 29] with recent GPS results in the state of Florida are analyzed. Their relationship with a very long baseline interferometry (VLBI) reference frame is also discussed. The investigation shows that overall the relative accuracy of NAD 83 (1986) in Florida is between 1/200,000 and 1/500,000 or from five to two parts per million (ppm), yet still not as good as the two‐dimensional results derived from GPS.

October streamflow

Record lows were set for streamflow in the Pacific northwest and in the southwestern Appalachians in October 1987. The Spokane River at Spokane, Wash., the Columbia River at The Dalles, Oreg., and the Yellowstone River at Corwin Springs, Mont., were among the rivers breaking low‐flow records that had stood for nearly 100 years. Fifty‐year lows were reported for the Tombigbee River near Coatopa, Ala., and the Green River at Munfordville, Ky.The U.S. Geological Survey reported that the combined flow of the Mississippi, St. Lawrence, and Columbia Rivers was 14% below median at 551,400 cubic feet per second (cfs). Mean October elevations of the Great Lakes (provisional National Ocean Service data) was lower in October than it was in September and was lower than it had been in October 1986. The Great Salt Lake fell 0.15 ft (0.04 m) during October, reaching 4209.25 ft (1282.97 m) above National Geodetic Vertical Datum.

Size-Specific Emergence of the Marsh Snail, Littorina irrorata: Effect of Predation by Blue Crabs in a Virginia Salt Marsh

Marsh periwinkles of 5 to 7 mm in shell height were eaten regularly by blue crabs. Fractures marking unsuccessful crab attacks were present in about 25% of medium-sized (10-16 mm) snails and over 60% of larger snails (over 16 mm). Medium-sized snails, subject to predation, leave the water more frequently than larger snails, but only about a quarter of the snail population leaves the water during high tide. We found no evidence that the snails leave the water because they sense blue crabs in the water. INTRODUCTION The marsh periwinkle littorina ivorata (Say) is a grayish gibbous, intertidal snail common in salt marshes from New York to Texas. It is believed to be a grazer, feeding largely on plant detritus and epipelic algae (Odum and Smalley 1959; Stiven and Kuenzler 1979). According to Bingham (1972) and Hamilton (1976, 1978), periwinkles are effectively supratidal; they avoid submergence by crawling upward out of the water when innundated. Bingham (1972) attributed the absence of snails in barren areas to “reluctance of the species to remain submerged.” Bleil and Gunn (1978) demonstrated that this response is not caused by threat of drowning; the snails can survive long periods under water. Hamilton (1976) attributed the avoidance of submergence to predation by the blue crab Callinectes sapidus Rathbun, which enters the marsh with the flood tide and preys on periwinkles. Crist (1979) found that a size gradient exists in periwinkles; larger individuals occur more frequently than smaller snails at lower elevations in the marsh. He attributed this in part to differential predation on smaller snails by blue crabs, which are more abundant at lower elevations. Preliminary observations on a study site at Wallops Island, Virginia, indicated that a substantial majority of specimens of L. irrorata remained under water at high tide, despite an abundance of emergent vegetation. That observation was an obvious contradiction to the behavior of L. irrorata in an aquarium-all snails crawl out of the water immediately (Bingham 1972). We noticed also that smaller snails tended to be more abundant out of water on stalks of vegetation than in the water. We reasoned that there may be a size-specific difference in the response of snails to submergence because of differential predation on smaller snails relative to larger ones. The purpose of this paper is to quantify the size-specific ‘Address reprint requests to Dr. Banta. Manuscript received February 8,1982; accepted July 28,1982. proportions of snails which emerge from the water at high tide in the field, and to test the hypothesis that blue crab predation may influence those proportions. MATERIALS AND METHODS The study site was a juvenile sloping foreshore marsh located on Cow Gut Flat at the north end of Wallops Island, near Chincoteague, Virginia. Drainage at low tide is nearly complete, and freshwater input is limited to rainfall and groundwater discharge. No tidal creeks or primary pans are present. Tall and medium vigor Spartina alterniflora (Loisel) predominate. Salicomia spp. is abundant at some higher elevations; the highest elevations are dominated by marsh elder, Iva fmtescens (Linnaeus) (Reidenbaugh and Banta 1980). Tidal wrack, consisting primarily of dead stalks of S. altemiflora, formed dense mats, which were rafted into the site during abnormally high tides (Reidenbaugh and Banta 1980). Vegetation compressed beneath stranded mats often is partly or completely killed, and in the most severe cases, secondary bare areas formed. Field experiments were conducted within a 17,000-m2 study site marked by wooden stakes placed at 10-m intervals from below mean low water to above mean high water. The study site has been named the Intensive Biometric Intertidal Survey (IBIS) marsh (Reidenbaugh and Banta 1980). To determine the size distribution of Littorina irrorata in the IBIS marsh survey grid, 30 sampling points were located relative to stakes, using computer-generated random numbers. Elevations at each collecting site were determined by linear interpolation among the four surrounding stakes. Stake elevations were determined by transit relative to Bench Mark IBIS, t 1.427 m National Geodetic Vertical Datum (NGVD). A 1-m2 quadrat was centered at each of the sampling points at or near the time of low tide between 16 and 18 July, 1979. The weather during this time was uniformly sunny and warm, with temperature maxima near 30°C. All marsh periwinkles were collected, counted, measured to the nearest 1 mm shell height, and returned to the same quadrat.

Oregon Coastal Salt Marsh Upper Limits and Tidal Datums

Three coastal salt marshes were surveyed independently by two teams of biologists to determine the upper limit of marsh. Distribution of species and species assemblages were related to surveyed elevations for South Beach marsh, Yaquina River estuary; Drift Creek marsh, Alsea River estuary; and Bandon marsh, Coquille River estuary. A transition zone between marsh and upland was identified by strong dominance ofPotentilla pacifica and the presence ofAchillea millefolium, Angelica lucida, Aster subspicatus, Oenanthe sarmentosa, Trifolium wormskjoldii, andVicia gigantea. Mean elevation of the lower boundary of the transition zone was 1.38 m above National Geodetic Vertical Datum (N.G.V.D.) and the upper boundary of the transition zone was 1.54 m above N. G. V. D. Relation of the upper limit of marsh to tidal datums varied with marsh. Mean elevation of the upper and lower limit of the transition zone was 0.58 m above MHW and 0.36 m above MHW, respectively. The two teams of biologists using the same biological criteria for defining the upper limit of marsh, independently agreed on the elevational position of the upper limit of marsh. Additional research and testing of the definition of the upper limit of marsh is warranted.

International Conference on the Redefinition of the North American Geodetic Vertical Control Network

International Conference on the Redefinition of the North American Geodetic Vertical Control Network