Tidal datum conversion grids for the Eastern Shore of Virginia and surrounding waters (2013)

Data consists of conversion factors that can be used to convert between numerous vertical tidal datums and the North American Vertical Datum of 1988 (NAVD88). The data cover the Eastern Shore of Virginia and parts of southeastern Maryland along with the surrounding coastal waters and are represented as approximately 100m (100.584m) resolution grids. The six included tidal datums are local mean sea level (LMSL), mean tidal level (MTL), mean low water (MLW), mean lower low water (MLLW), mean high water (MHW), and mean higher high water (MHHW). All vertical units are in meters. By combining multiple conversions to and from NAVD88, conversion between the various tidal datums is possible. Two versions of the conversion factor grids are provided for each NAVD88-to-tidal-datum pairing: one that only contains data for areas not masked as nodata by the NOAA VDatum program (original source data) and one that contains both the original and interpolated data (see below for details). Naming conventions used were "cfactor_DDD" for the original VDatum-detrived dataset where "DDD" is the local tidal datum and "cf_nd_DDD" for the dataset that includes interpolated values within the nodata masks (IDW interpolation across masked areas, typically upland regions but also shallow seaside bays and creeks for which no adequate tidal benchmarks were available). By definition, the baseline elevation (sea level or 0.0m elevation) for NAVD88 is referenced to the fixed International Great Lakes Datum of 1985 local mean sea level height value, at Rimouski, Quebec, Canada. Additional tidal bench mark elevations were not used to calculate NAVD88 due to the demonstrated variations in sea surface topography, i.e., the fact that mean sea level is not the same equipotential surface at all tidal benchmarks. The magnitude of the difference between local mean sea level (LMSL) at the tidal benchmarks of the Eastern Shore of Virginia and the NAVD88 defined sea-level varies from 0.039 to 0.149 meters BELOW zero NAVD88. Tidal prisms also vary at each tidal benchmark (in part due to differences in basin configuration and tidal interactions) causing the conversion factors for the other tidal datums to also vary spatially in similar but not identical patterns. The VDatum 3.2 software program from NOAA (http://vdatum.noaa.gov/) was used to convert the x,y,z center points of the 100m gridded data wherein all Z elevations were set equal to zero (0) from NAVD88 to each of the six local tidal datums (the X,Y horizontal WGS84 UTM 18N coordinates remained unchanged). The resulting conversion factors represent the new elevation at which the NAVD88 zero level would lie in reference to the new datum; thus, to convert from NAVD88 and the new tidal datum, one would add this conversion factor to the NAVD88 elevations to get elevations relative to the chosen tidal datum. To convert to NAVD88 from a given tidal datum, one would subtract the conversion factor from the tidal elevation. Data were turned back into gridded data with the same resolution and horizontal extent as the original data grid. The internal data grids used by the VDatum program mask as nodata most land areas (including marshes) plus many of the seaside shallow bays, either in part or in full, for which reliable tidal benchmark data is/was not available. As a result, the program cannot be used in these nodata areas, even if immediately adjacent to data areas. So as to make conversion factors available for these coastal bays and marshes and seaside watersheds of interest to the VCRLTER, conversion factors for gridded regions within the NOAA nodata masks were interpolated from neighboring data values using the inverse distance weighting (IDW) techniques employed by ESRI's ArcGIS 10.1 software. IDW interpolation resulted in conversion factors that varied gradually spatially when adjacent to the NOAA VDatum data grids but that often showed relatively sharp transitions when equidistant between different far-apart basins (such as mid-peninsula between the Chesapeake Bay and Atlantic Ocean, or within South Bay bounded by data constructed from tidal datums for the Atlantic Ocean (east), Ship Shoal Inlet (south), Sand Shoal Inlet (north), and Magothy Channel (west)). It is suggested that the appropriate use of this data is to convert elevation datasets referenced to a tidal datum to NAVD88 if integrating multiple datasets together over large areas, such as across the full Eastern Shore or across multiple watersheds or coastal bays, so as to not introduce artificial IDW-related transitions into otherwise vertically-consistent upland elevations or basin-scale bathymetric surveys. When converting elevations of fringing upland marshes, the conversion factors (including interpolated values) can likely be used directly on a cell-by-cell level to adjust the tidal elevations to NAVD88 or to another tidal datum without the involvement of any IDW-related transitions. Likewise for many of the back-barrier seaside marshes and bays of Accomack County in the northern portion of the data coverage where no channel tidal datums intervene between the Atlantic coastline and the Chesapeake Bay. However, when converting bathymetric survey data for individual bays or basins with significant nodata-masked areas that exhibit sharp IDW-related transitions, it is suggested that it would be more appropriate to use a single area-averaged conversion factor calculated across the whole basin (or across the whole basin minus any portion where the conversion factor is closer [more-related] to a channel on the other side of an upland barrier without connecting flow paths). Likewise when converting to tidal datums full upland mainland watersheds. This again should prevent introducing sharp transitions and steps into the bathymetry survey data, which it is assumed is otherwise internally consistent.本数据集包含可用于众多垂直潮汐基准面与1988年北美垂直基准面（North American Vertical Datum of 1988, NAVD88）之间相互转换的转换因子。数据集覆盖弗吉尼亚东海岸、马里兰州东南部部分区域及周边近海水域，以约100米（精确为100.584米）分辨率的网格形式存储。 本次数据集包含6种潮汐基准面，分别为当地平均海平面（local mean sea level, LMSL）、平均潮位（mean tidal level, MTL）、平均低潮位（mean low water, MLW）、平均低低潮位（mean lower low water, MLLW）、平均高潮位（mean high water, MHW）以及平均高高潮位（mean higher high water, MHHW）。所有垂直单位均为米。通过组合多组与NAVD88之间的正反转换，即可实现不同潮汐基准面间的相互转换。 针对每一组NAVD88与潮汐基准面的配对，提供两种转换因子网格版本：一种仅包含未被美国国家海洋和大气管理局（National Oceanic and Atmospheric Administration, NOAA）VDatum程序标记为无数据（nodata）区域的数据（原始源数据）；另一种则同时包含原始数据与插值后的数据（详细说明见下文）。命名规则为：以"cfactor_DDD"表示源自VDatum的原始数据集，其中"DDD"为当地潮汐基准面的缩写；以"cf_nd_DDD"表示包含无数据掩码区域内插值结果的数据集，该数据集采用逆距离加权（inverse distance weighting, IDW）插值法对掩码区域进行插值，此类区域通常为陆地区域，或是缺乏足够潮汐基准点数据的近岸浅海湾与溪流水域。 根据定义，NAVD88的基准高程（海平面或0.0米高程）参考了加拿大魁北克里穆斯基的1985年国际大湖基准面（International Great Lakes Datum of 1985）当地平均海平面高度值。由于已证实海面地形存在差异，即平均海平面并非所有潮汐基准点处的同一等势面，因此未使用额外的潮汐水准点高程数据计算NAVD88。弗吉尼亚东海岸潮汐基准点处的当地平均海平面（LMSL）与NAVD88定义的海平面之间的差值范围为0.039至0.149米，低于NAVD88零点。 各潮汐基准点处的潮汐棱柱体也存在差异，部分原因是流域形态与潮汐相互作用的差异，这导致其余潮汐基准面的转换因子也呈现相似但不完全一致的空间变化规律。 本数据集使用美国NOAA发布的VDatum 3.2软件（http://vdatum.noaa.gov/）对100米网格数据的x、y、z中心点进行转换：将所有Z高程值设为0（来自NAVD88），转换至上述6种当地潮汐基准面，X、Y水平坐标保持WGS84 UTM 18N坐标系不变。所得转换因子代表NAVD88零水平面相对于新基准面的新高程值；因此，若要将NAVD88高程转换为选定潮汐基准面下的高程，只需将该转换因子与NAVD88高程相加即可。若要将给定潮汐基准面下的高程转换为NAVD88高程，则需从潮汐高程中减去该转换因子。随后将转换结果重新生成为与原始网格分辨率、空间范围一致的网格数据。 VDatum程序内部使用的数据网格会将大多数陆地区域（包括湿地）以及众多近岸浅海湾（部分或全部区域）标记为无数据，原因是这些区域缺乏可靠的潮汐基准点数据。因此，即使紧邻有数据区域，该程序也无法在这些无数据区域中运行。为了向VCRLTER关注的近岸海湾、湿地及近岸流域提供转换因子数据，针对NOAA无数据掩码区域内的网格区域，采用ESRI ArcGIS 10.1软件所使用的逆距离加权（IDW）插值法，从邻近的有效数据点中插值得到转换因子。 IDW插值得到的转换因子在紧邻VDatum有效数据网格的区域呈现缓慢的空间变化，但在相距较远的不同流域之间，如切萨皮克湾与大西洋之间的半岛中部，或是被大西洋（东侧）、Ship Shoal水道（南侧）、Sand Shoal水道（北侧）与Magothy水道（西侧）环绕的南湾区域，当不同区域的插值点距离相近时，往往会出现相对尖锐的过渡边界。 建议该数据集的合理使用场景为：在大范围内整合多套高程数据集时，如覆盖整个弗吉尼亚东海岸，或是跨多个流域与近岸海湾，将参考潮汐基准面的高程数据集转换为NAVD88基准面，以避免在原本垂直一致性较好的陆域高程或流域尺度水深测量数据中引入人为的IDW相关过渡边界。当转换沿岸潮间带湿地的高程数据时，可直接逐网格单元使用转换因子（包括插值后的数据），将潮汐高程调整为NAVD88基准面或其他潮汐基准面，无需顾虑IDW相关的过渡边界。对于数据覆盖范围北部阿科马克县的许多后滨近岸湿地与海湾而言，由于大西洋海岸线与切萨皮克湾之间不存在水道型潮汐基准面的阻隔，上述方法同样适用。然而，当对存在显著无数据掩码区域的单个海湾或流域进行水深测量数据转换，且这些区域存在明显的IDW相关过渡边界时，建议采用全流域（或全流域扣除与另一侧陆域屏障水道相关、无连通径流区域的部分）计算得到的单一区域平均转换因子。同理，将高程数据转换为陆域流域的潮汐基准面时也应遵循此原则，以避免在原本内部一致的水深测量数据中引入尖锐的过渡与阶跃特征。