Frequently Asked Questions

Which OS does VDatum run on?

I have the latest Java, however when I double click vdatum.bat, the command prompt window flashes for a split second and the application does not launch.

Running "java -jar vdatum.jar" at the command prompt window gives me "java is not recognized as an internal or external command, operable program or batch file".

I can't select any tidal datum, or NAD 27, NAD83(1986), NGVD29, IGLD85!

I recieved a result of -999999.0. What does that mean?

Why doesn’t VDatum provide tidal datums inland?

Tidal datum transformations in VDatum extend only slightly inland of the mean high water (MHW) shoreline (1 or 2 km), but many applications seek to reference elevations to tidal datums further inland. The main reason that VDatum doesn’t provide tidal datums inland is the fact that tidal datums have no physical meaning inland (until or unless that inland location becomes inundated by tides).

One example is the application of VDatum to lidar data to compute a shoreline referenced to a tidal datum. Lidar data collected near the shoreline during low tide can be processed in VDatum and referenced to MHW. The resulting contours of the MHW-referenced data at a value of zero will then represent the MHW shoreline. If the tidal datum transformations do not extend far enough inland to make this transformation, though, users must make decisions about how to manually readjust the datum transformations to enable the shoreline computation to be made. Another example of a need to extend tidal datum transformations further inland is provided by sea level change studies. Such studies require static assumptions to determine which inland areas of a digital elevation model (DEM) would be influenced if coastal sea level was adjusted by a fixed amount. If the DEM heights are referenced to a tidal datum, then transformations are necessary in inland areas affected by sea level rise scenarios.

Spatial variations in tidal datums in coastal waters near shore can influence the method of approximating their inland extension. Tidal datums along a coastline can vary locally for many reasons, some of which include variable bathymetry, the presence of tidal flats, river interactions, presence of barrier islands, geographic/volumetric changes in the shoreline and associated embayments, and the presence of shoreline engineering structures. If none of these factors affects a given area (e.g. straight coastline, absence of other factors mentioned above), tidal datums should be relatively uniform along the coast.

If tidal datums are relatively spatially uniform, extrapolation of the tidal datums can usually be made by assuming a constant datum difference which can be extended inland to a limited distance. For example, the figure below shows how a constant offset between NAVD 88 and local mean high water (MHW) could be extended inland if the project area is just inland of the VDatum model grid. In this manner, land or marsh elevations referenced to NAVD88 can still be related to the nearby MHW elevation.

Often, however, tidal datums are not relatively spatially uniform, so most scenarios encounter some level of complexity due to the factors affecting local tidal datum variations. Therefore, some assumptions need to be made, while also acknowledging the uncertainty that can arise due to the associated factors. For example, the figure below illustrates several inland locations for which decisions would need to be made on how to extrapolate tidal datums inland. The star symbols in these figures indicate the inland locations in question, with sections of Chesapeake Bay on the left and of the Outer Banks of North Carolina on the right. These locations present a problem because they are close to at least two different bodies of water which are likely to have different tidal datums.

An example of inland locations (masked with a star) that are close to two different water bodies and will likely to have different tidal datums

No matter what decision is made on how to extrapolate the tidal datums in such situations, it is suggested that statistics be computed on the variability of the datum transformations in the user‘s area of interest. For example, the constant offset assumption could be used with data from several separate coastal locations, resulting in several sets of offsets over the land region, each using a different datum value. From these sets, a statistical mean and variation could be computed. This information can then be used to document the uncertainty introduced by assuming a certain interpolation scheme or even by using a constant offset value. In some cases, the variability in transformation values between different regimes (river, bay, ocean, marsh, tidal flats, etc.) may be small enough in relation to the coastal issue being addressed.

There are additional caveats. In extending tidal datums inland for sea level change (SLC) studies, the assumptions of how that change will occur need to be considered. For example, if sea level rises by a constant amount in a region, the tides will also change due to new areas now being flooded or dried. The local tide patterns used to construct VDatum may no longer be completely applicable if significant changes occur as a result of a sea level change scenario. For example, a barrier island may be so low that in certain sea level rise scenarios some parts of the island would be breached. This changes the flow of water significantly, allowing the tides to more easily propagate into the inland waterway. Marshes and tidal flats can likewise be flooded and/or dried in sea level change scenarios, thus changing the local tidal datum patterns accordingly. Aside from the static effects of these SLC scenarios, dynamic influences related to morphological changes to the marshes and shoreline environments would also be present. Sediment erosion and marsh accretion would change in ways that could only be evaluated through detailed studies that link processes across a variety of scales together in a physically realistic manner.

Quantifying how all of these static and dynamic effects could change tidal datums extended inland is a difficult task. In many cases, it may be more appropriate to make an assumption of a constant datum transformation offset to be used for the area of interest, while acknowledging these effects in the documentation of the uncertainty. A constant offset, (e.g. between NAVD 88 and local mean sea level or mean high water ) could be determined by examining an offshore average difference between these two datums, far enough away from local factors affecting the nearshore tidal datum patterns. This offset could then be applied to transform topographic land heights from NAVD 88 to local mean sea level and/or mean high water.

What are the VDatum bounding polygons and why are they utilized?

While trying to convert from NAVD88 to MLLW, MLW, MHW, etc.. I got results showing that the MLLW and MLW are higher than MHW and MHHW. Could it be program bug or something mixed up?